Bending-related faulting of the incoming oceanic plate and its effect on lithospheric hydration and seismicity: A passive and active seismological study offshore Maule, Chile

Accepted Manuscript Title: Bending-related faulting of the incoming oceanic plate and its effect on lithospheric hydration and seismicity: A passive and active seismological study offshore Maule, Chile Author: E. Moscoso I. Grevemeyer PII: DOI: Reference:

S0264-3707(15)00067-8 http://dx.doi.org/doi:10.1016/j.jog.2015.06.007 GEOD 1373

To appear in:

Journal of Geodynamics

Received date: Revised date: Accepted date:

22-11-2014 25-6-2015 25-6-2015

Please cite this article as: http://dx.doi.org/10.1016/j.jog.2015.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

Ac ce pt e

d

M

an

us

cr

ip t

We present new active and passive seismological data offshore Maule, Chile >We discuss the outer rise seismicity and Vp, Vs and Poisson's ratio models>We compare our results with published data available in the area and with bathymetric features>We confirm hydration of the upper oceanic lithosphere and partial serpentinization of the upper mantle.

Page 1 of 73

*Manuscript Click here to view linked References

1

Bending-related faulting of the incoming oceanic plate and its effect on

2

lithospheric hydration and seismicity: A passive and active

3

seismological study offshore Maule, Chile.

E. Moscoso1 and I. Grevemeyer

5 6

GEOMAR Helmholtz Centre for Marine Research, Wischhofstraße 1-3, 24148 Kiel, Germany

cr

7

us

8

Abstract

an

9

ip t

4

We have studied the dependency between incoming plate structure, bending-related faulting,

11

lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D

12

Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-

13

outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic

14

lithosphere is highly hydrated due to the water infiltration trough bending-related normal faults

15

outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in

16

the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile

17

trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of

18

the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis

19

running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise

20

processes have yet not affected anisotropy.

Ac ce pt e

d

M

10

21

The first weeks following the megatrust Mw=8.8 Maule earthquake in 2010 were

22

characterized by a sudden increase of the outer rise seismic activity, located between 34°S and

23

35°30'S. We concluded that this phenomenon is a result of an intensification of the water infiltration 1

Now at Erdbeben Engineering-Geoscience, Ruiz Tagle 771, Santiago, Chile. Page 2 of 73

24

process in the outer rise, presumably triggered by the main shock, whose epicenter was located

25

some 100 km to the south east of the cluster.

26

Keywords: Outer rise, Subduction zones, Mantle serpentinization, Maule earthquake, Refraction-

27

and wide-angle seismology, Local seismicity.

Introduction

cr

29

ip t

28

The amount of water stored within the oceanic lithosphere plays a fundamental role in the

31

generation of arc and back-arc magmas, hydration of the mantle wedge, and the global budget of

32

water [Hacker et al., 2008]. The south central Chile subduction zone is characterized by a highly

33

fractured trench outer-rise seafloor, which is generated by the bending of the Nazca plate

34

[Contreras-Reyes and Osses, 2010 and reference therein]. This bending-related faulting may

35

reactivate pre-existing cracks in the oceanic crust, previously created at the spreading center, and it

36

may also generate faults cutting deep into the lithosphere [e.g. Grevemeyer et al. 2005]. This

37

process modifies the porosity and permeability structure of the oceanic crust [Carlson, 2010] and

38

allows the water to infiltrate deep into the lithosphere, producing hydration of the crust and

39

eventually serpentinization of the upper mantle [e.g. Contreras-Reyes et al., 2008a,b; Ranero et al.

40

2003]. Another purposed pathway for fluids through the crust are seamounts and outcrops

41

penetrating through the sediments [e.g. Fisher et al., 2003; Contreras-Reyes et al., 2007; Ivandic et

42

al., 2010].

Ac ce pt e

d

M

an

us

30

43 44

The creation of normal faults in the outer rise, due to plate bending, is responsible for the

45

shallow intraplate seismic activity [e.g. Ranero et al., 2005; Lefeldt et al., 2009] that might produce

46

considerable seismicity seaward of the updip region of the seismogenic zone [Moscoso et al, 2011;

47

Moscoso and Contreras-Reyes, 2012] and may also in rare cases produce devastating tsunamis

Page 3 of 73

48

[Satake and Kanioka, 1999]. Although the magnitude of the offshore intraplate events is in general

49

smaller than the subduction-related earthquakes, they can present considerable magnitudes. For

50

instance, the largest normal-faulting event ever reported is the Sanriku, Japan earthquake of 1933,

51

with a magnitude Mw=8.5, that probably ruptured along the entire oceanic lithosphere [Kanamori,

52

1971].

ip t

53

Previous seismic studies conducted offshore of south-central Chile showed evidence of

55

hydration of the oceanic crust and serpentinization of the upper mantle at the Juan Fernandez ridge

56

[Kopp et al. 2004] in the trench-outer rise area offshore Arauco [Contreras-Reyes et al., 2008a] and

57

offshore Chiloe to the north of the Chile triple junction [Contreras-Reyes et al., 2008b]. Upper

58

lithospheric hydration has also been deduced from seismic studies performed at the erosive margin

59

off Northern Chile [Sallarès and Ranero, 2005; Contreras-Reyes et al., 2012] and offshore of

60

Nicaragua [Ivandic et al., 2008]. Seismological studies suggest that hydration of the oceanic plate

61

and outer rise seismicity are not independent phenomena [Ranero et al., 2005; Tilmann et. al. 2008]

62

but are related to each other. Thus, both processes are probably common features along most

63

subduction zones [Contreras-Reyes and Osses 2010; Grevemeyer et al., 2007].

us

an

M

d

Ac ce pt e

64

cr

54

65

A parameter commonly used to evaluate hydration is the Poisson’s ratio (ν). It is defined as

66

the ratio, when a sample object is stretched, of the contraction or transverse strain (perpendicular to

67

the applied load), to the extension or axial strain (in the direction of the applied load). Its analytical

68

formula expressed in terms of body waves is given by the expression (1): 2

69 70

ν=

Vp/ Vs −2 2 2 [Vp/ Vs ]−1

(1)

and its standard deviation ∆ν is calculated by (2):

Page 4 of 73

2

∆ ν=

71

Vp/ Vs 2[Vp/ Vs2 ]−12



2

2

  

∆ Vp ∆ Vs  Vp Vs

(2)

Where ∆Vp and ∆Vs are the standard deviations of P- and S-wave velocity fields,

73

respectively. Poisson's ratio is very sensitive to the existence of water. In a material with high

74

content of water, Vs tends to decrease faster than Vp, producing an increase of ν.

ip t

72

75

To study the relationship between hydration and seimicity offshore of Maule, Chile, we

77

analyzed jointly (1) seismological data from a temporal outer rise seismic network (ORN) deployed

78

for about one month, (2) swath bathymetric data, and (3) the seismic velocity structure of the Nazca

79

plate prior to its subduction, between ~34°S and ~35°S. Wide-angle seismic data from a 85-km-long

80

trench parallel seismic profile was used to derive the compressional (Vp) and shear wave (Vs)

81

velocity structure. We used travel time tomography to yield the 2-D velocity structure and crustal

82

thickness and Poisson's ratio. Further, we studied model uncertainties of both P- and S-wave

83

velocity by applying a non linear Monte Carlo-like inversion method [Korenaga et al., 2000]. We

84

examine the crustal models to derive an estimation of degree of the upper lithosphere hydration and

85

we discuss the implications of variations of the oceanic upper mantle velocity, comparing our

86

results with published data from a seismic profile which runs perpendicular to the current profile

87

[Moscoso et al., 2011].

89

us

an

M

d

Ac ce pt e

88

cr

76

Tectonic framework

90

The oceanic Nazca plate converges beneath the South American plate at relative velocity of

91

6.6 cm/yr with an azimuth of 78°E [Angermann et al., 1999]. The Nazca plate oceanic crust

92

offshore central Chile between 32°S and 38°S, was generated at the Chile Rise ~30 to ~35 Ma

93

[Müller et al., 1997], these spreading center segments and bending related normal faults can be

94

observed in the high resolution bathymeric map of Figure 1. The seabed is covered by a Page 5 of 73

sedimentary layer ~200 m thick on the location of our profile, with increasing depth trenchward to a

96

maximum of ~2 km at the trench axis [Moscoso et al., 2011]. The incoming oceanic plate in the

97

trench-outer rise area is highly faulted and hydrated by intrusions of cold seawater [Grevemeyer et

98

al., 2005]. These faults likely hydrate the crust and upper mantle[e.g. Contreras-Reyes et al.,

99

2008a]. The Juan Fernandez Ridge (JFR) at ~32°S acts as a barrier for the migration of sediments

100

sourced from the Andes [Blumberg et al., 2008] and carried in the trench from south to north, which

101

changes the regime of the margin from erosive in the north to accretionary in the south [von Huene

102

et al., 1997].

cr

ip t

95

us

103

The central Chilean margin has hosted some of the largest subduction zone earthquakes. A

105

few times in every century large thrust earthquakes broke several hundreds of kilometers in a single

106

shock [e.g. Barrientos, 2005], often producing devastating tsunamis [Cisternas et al., 2005].

107

Previously, the thrust offshore Maule has been reported as fully locked [Ruegg et. al., 2009;

108

Campos et al. 2002]. In fact, the 2010 megathrust Maule earthquake (Mw=8.8) ruptured some 400

109

km along the margin, producing a devastating Tsunami [Madariaga et al., 2010]. The second largest

110

slip of the 2010 Maule earthquake was reported between 34° and 35° [Delouis et al., 2010; Moreno

111

et al., 2010]. This zone coincides with an anomalous high outer-rise seismic activity after the

112

earthquake [Moscoso et al., 2011] in comparison to the rest of the Maule seismic segment.

114

M

d

Ac ce pt e

113

an

104

Seismic experiment and data

115

The data analyzed in this study consist of high resolution multibeam bathymetry, wide-angle

116

seismics and local earthquake measurements made offshore the Maule region (34°S-35°30'S),

117

during March and April 2008 during the cruise JC23 of the British RV James Cook [Flueh and

118

Bialas, 2008].

119 Page 6 of 73

120

Local earthquake data The outer rise network (ORN), deployed between the 1 st of March and 8th of April 2008,

122

consists of 19 ocean bottom seismometers and hydrophones (OBS/H) [Flueh and Bialas, 1996;

123

Bialas and Flueh, 1999]. The instruments were deployed with a spacing between 20 and 40 km,

124

covering an area of approximately 100 by 100 km². The network extended over the outer rise from

125

~200 km offshore to the deformation front, surrounding the Maule seamount as is shown in Figure

126

1.

cr

ip t

121

127

Preprocessing of the OBS/H data included calculation of the clock-drift corrections to

129

adjust the clock in each instrument to the shipboard GPS base time and instrument locations were

130

corrected for drift from the deployment position during their descent to the seafloor using the direct

131

water wave arrival, recorded from the active seismics. A short-term average versus long-term

132

average (STA/LTA) trigger algorithm was applied to the data to detect signal variations that could

133

indicate an event. Earthquake identification from the triggered data was done manually, giving a

134

total of 29 events, 7 of which were inside the network. The data reading and picking of the P and S

135

arrivals was done using the software SEISAN [Havskov and Ottemöller, 1999]. We manually chose

136

a quality factor for each picked phase and this quality factor was used to account for the picking

137

uncertainties.

139

an

M

d

Ac ce pt e

138

us

128

Wide angle seismic data

140

The wide angle seismic profile P04 is a ~84 km long transect that runs parallel to the trench

141

in the outer rise area, some 130 km offshore Constitución (See Figure 1). The seismic source

142

consisted of four tuned arrays of three airguns each plus two single airguns, providing a total

143

volume of 11200 inch³. The airguns were fired at intervals of 60 s, or 150 meters at a ship speed of

144

5 knots . The airgun shots were recorded by 8 OBS and 1 OBH, which makes a total of 9 stations

145

deployed along the transect. A time-gated deconvolution filter was applied to remove predictable Page 7 of 73

146

bubble reverberations.

147 P wave arrivals were recorded with excellent quality and clear S wave arrivals were

149

recorded in 8 of the 9 seismic stations (only station 404 did not present distinguishable S waves

150

arrivals). Data examples from two stations are shown in Figure 2, with their respective seismic

151

phases identified. Picking of the seismic phases was done manually, and picking errors were

152

assigned on the basis of the dominant period of the phase. Typically, errors were assumed to be half

153

a period of one arrival and weighted according to the phase quality. Based on the quality of the data,

154

for S wave arrivals we assigned a higher error than for the P phases and due to the larger

155

uncertainties for larger offsets. We also differentiated picking uncertainties for long offset arrivals.

156

Detailed information regarding pick uncertainties and model fitness are summarized in Table 1.

an

us

cr

ip t

148

M

157

Velocity field modeling procedure

159

P wave travel time tomography

Ac ce pt e

d

158

160

The absence of refracted arrivals from the sedimentary layer and reflections from the

161

basement was overcome by assuming typical Vp=1.8 km/s and Vs=0.25 km/s for sediment and

162

calculating the sediment thickness beneath each station through the time difference between the Pg

163

and PPS phases. While Pg corresponds to crustal refractions, the PPS wave modes travel through

164

the sedimentary layer and oceanic crust as a Pg wave but they are converted to an S wave at the

165

sediment-crust interface when the seismic rays dive up [Spudich and Orcutt, 1980], and finally

166

travels though the sedimentary layer as an S wave, therefore PPS is recorded as an S wave in the

167

OBS with the same apparent velocity of Pg but delayed respect to it due to lower travel velocity in

168

the sediment. Time differences between Pg and PPS phases range between 0.6 [s] and 1.1 [s] in our

169

dataset (see Figure 3b), yielding a sediment thickness of 180-300 meters.

Page 8 of 73

170 The 2D Vp-depth distribution below the sedimentary layer was obtained using the joint

172

refraction and reflection travel time inversion code TOMO 2D, that simultaneously solves for the

173

seismic velocity field and the depth of a floating reflecting interface [Korenaga et al. 2000]. Travel

174

times and ray paths are calculated employing a hybrid ray tracing scheme, based on the graph

175

method [e.g. Dijkstra 1959] and in the local ray bending refinement [e.g. Van Avendonk 1998]. The

176

velocity grid used for representing the velocity field is parametrized as a sheared mesh hanging

177

beneath the seafloor, where the node spacing was fixed constant in 0.5 km for the horizontal and

178

varying from 0.05 Km at the top of the model to 0.5 km at the bottom for the vertical. The smaller

179

spacing on top accounts the higher resolution in shallower parts of the model than at the bottom.

180

The Moho is represented by a floating reflector, which consists of an array of linear segments with a

181

horizontal equidistant spacing of 0.5 km between its nodes, and only one degree of freedom in the

182

vertical direction.

cr

us

an

M d

183

ip t

171

The data set considered for the P-wave tomography consists of 2701 Pg, 1067 PmP and 543

185

Pn phases that were handpicked from 9 OBS/H. The size of the model is ~84 Km long and 14 Km

186

deep. We performed the inversion using the “layer stripping” method: First we inverted the oceanic

187

crust reflections (Pg) and wide angle Moho reflections (PmP) and then the upper mantle refractions

188

(Pn), keeping the model over the Moho fixed. Tomographic inversion was undertaken using the

189

technique of Hole [Hole, 1992]. It consists of inverting in the first iterations only the picks with

190

offsets smaller than 20 km, and this threshold was increased stepwise to 90 km in steps of 10 km for

191

the subsequent iterations. This approach ensures that the shallow portion of the model is inverted

192

before the deep portion. The described procedure is necessary because the ray coverage for the

193

deeper parts is less dense and because the calculated travel times for deeply penetrating rays are

194

also influenced by the upper portions of the model. The inversion is stabilized by using smoothness

195

correlation lengths in the horizontal and vertical directions of the velocity mesh and for the depth

Ac ce pt e

184

Page 9 of 73

nodes. The values used for the correlation lengths vary from 2 km to 10 km for the horizontal and

197

from 0.1 km to 2 km for the vertical, at the top and bottom of the model respectively. Initially, the

198

depth sensitivity parameter kernel w was set to 1, which means that velocity field and reflector

199

nodes are equally weighted during the inversion. For each data set we run 4 iterations which were

200

enough to obtain a good fit between observed and calculated arrivals (See Table 1). Picked and

201

calculated travel times, and ray tracings for two ocean bottom instruments are shown in Figure 2.

ip t

196

S wave travel time tomography

us

203

cr

202

Since airguns generate a purely compressional wavefield, the shear waves observed in the

205

data are generated through mode conversion. The most plausible interface for generation of

206

converted shear arrivals is the interface between the sedimentary layer and the crystalline basement

207

[Spudich and Orcutt, 1980], whether when they are diving down (PSS), up (PPS) or both (PSP).

208

Under this assumption, the PSP waves recorded were inverted keeping the sediment/basement

209

interface geometry, the Moho reflector and the velocity in the sediment fixed for the inversion of

210

the Sg, Sn and SmS phases. In other words, the geometry is assumed from the Vp tomography and

211

only Vs values are updated during the inversion. For this purpose the kernel weighting factor is set

212

to w = 0.001

M

d

Ac ce pt e

213

an

204

214

Velocity model assessment

215

Model uncertainty

216

In order to estimate the sensitivity of our final model to different starting models and data,

217

we applied a Monte Carlo-like approach by averaging the solutions of 100 realizations [Korenaga et

218

al. 2000]. The degree of dependence of the final solution on the starting model can be assessed by

219

conducting a number of inversions with a variety of randomly generated initial models and initial

Page 10 of 73

data sets [Tarantola, 1987]. To estimate the model uncertainty, 10 initial models were derived from

221

the 1D starting reference model by varying the velocity and the initial reflector ±10% (Figure 3a).

222

These models are in the vicinity of the possible solutions and also cover a wide range of seismic

223

velocities found in Nazca plate, offshore central Chile [e.g. Scherwath et al. 2009, Contreras-Reyes

224

et al. 2010]. In order to include the picking subjectivity in our analysis, each model was inverted

225

with 10 noisy data sets obtained by adding a random value within the picking uncertainty time for

226

each phase. Regarding the Moho reflector, it reaches minimum error at the center of the profile and

227

larger uncertainty at the extremes of the model. The results of this test for Vp and Vs are in Figures

228

4a and 4b, respectively, and final Trms and χ2 of the average final models are summarized in Table 1.

us

cr

ip t

220

an

229

The seismic ray distribution in our model is represented by the derivative weight sum

231

(DWS), which mathematically corresponds to the column vector sum of the velocity kernel. This

232

parameter provides crude information on the linear sensitivity of the inversion describing the

233

relative ray density near a given velocity node [Toomey and Fulger, 1989]. The DWS value of

234

Figures 4c shows excellent ray coverage in the upper crust and good ray coverage in the lower crust

235

and upper mantle. Hence, zones of high and low resolution can be explained by high and low ray

236

coverage respectively (see Figures 4a and 4c).

238

d

Ac ce pt e

237

M

230

Resolution test

239

For checking the resolvability of the obtained velocity models and explore whether our data

240

set can resolve anomalous crustal velocity zones, we have created synthetic models by using the

241

final average velocity models for Vp and Vs and superimposing onto the oceanic crust five

242

Gaussian velocity anomalies (Figure 5). The maximum amplitude of each anomaly is +/-5% of the

243

velocity. In order to estimate how well the data can resolve perturbations of this scale, synthetic

244

travel time data with the same source-receiver geometry as in the real data set were inverted using

Page 11 of 73

the final average velocity model as the initial model. For qualifying the robustness of the test, this

246

procedure was repeated for a second set of perturbations with the same geometry and amplitudes of

247

the initial set, but with opposite sign. The result shows that, for the given anomalies, the recovery of

248

shape and amplitude is maximum between the 20 km and 60 km of distance, while at the extremes

249

the models are still capable to discriminate between positive and negative anomalies and to estimate

250

their amplitude and position, but the shape of the anomalies is not fully recovered. This is observed

251

in both Vp and Vs models. Thus, the resolving power of our data set is good enough to resolve

252

features similar to the one found in the lower crust of the Vp model between 40 km and 60 km.

cr

ip t

245

Velocity-Depth ambiguity

an

254

us

253

255

The Vp values for the lower crust found by our inversion methodology, based in a Monte

257

Carlo-like approach, would not be fully revealed due to the low amount of crossing ray paths on this

258

zone (Figure 4d) and the geometry of the experiment, and can not be attributed to the inversion

259

procedure [Tieman, 1994]. It produces a trade off between Moho depth and Vp, called velocity-

260

depth ambiguity. This implies that our initial choice of w=1, might drive to the calculation of an

261

unconstrained model [Korenaga, 2011].

d

Ac ce pt e

262

M

256

263

In order to evaluate the influence of the kernel weighting factor w into the final model

264

calculation, we recomputed the velocity models in a similar way as described in the previous

265

section, but using a w=0.001 for the Vp inversion with a starting flat Moho reflector at 10.5 km

266

depth. For the Vs modelling, we used the same inversion procedure previously described.

267 268

The results obtained by applying this procedure are presented Figure 7, showing a

269

systematic decrement of Vp and Vs in the lower crust between 25 km and 60 km, in comparison to

Page 12 of 73

average Monte Carlo models, yielding a lower υ of 0.2 in the same area. We attribute this velocity

271

reduction of Vp and Vs to a compensation of the traveltimes due to the imposed restriction of

272

pondering during the inversion 1000 times more the velocity variations than the Moho reflector

273

geometry changes. From the geologic point of view, reduced velocities in the outer rise can be

274

attributed to fracturing and hydration [e.g. Grevemeyer et al., 2007]. Thus, according to the

275

bathymetric map of Figure 1, one would expect such low velocity zone in the trench perpendicular

276

direction towards the trench, were bending related faults are observed. For this reason we consider

277

the models of Figure 6 more likely, due to its moderate horizontal variation.

cr

ip t

270

us

278

Discussion

280

Seismic structure of the oceanic crust in the outer rise offshore Maule Sedimentary layer

d

282

M

281

an

279

Across the profile, we have modeled a thin sedimentary layer of fairly uniform 180-300 m

284

depth and its basement tends to mimic the seafloor. The assumed velocities of Vp=1.8 km/s and

285

Vs=0.25 km/s correspond to a υ=0.49. These values are in agreement with in situ measurements of

286

υ for depth shallower than 100 m below the seafloor, which present υ ranging from 0.46 to 0.49

287

[Hamilton, 1976] and is also consistent with the value of υ=0.46, found for sediment near the Chile

288

triple junction [Contreras-Reyes et al., 2008a].

289

Ac ce pt e

283

290

The sedimentary layer gets thicker towards the trench, resulting in a maximum thickness of

291

some 2000 m at the trench axis [Moscoso et al. 2011]. In the trench fill sediments, Vp tends to

292

increase gradually from 1.8 km/s on top to 3.5 km/s at the bottom, principally due to compaction

293

and the increase of the sediment size from top to bottom, associated to successive sedimentary

294

deposit events [Contreras-Reyes et al., 2008a].

Page 13 of 73

295 296

Upper crust The oceanic crust has a thickness of about 6 km velocities increase from 4.5 km/sto ~7.0

298

km/s. Within the crust at ~6 km depth we can identify for both seismic models a Vp and Vs change

299

from high to low velocity gradient. This transition zone characterizes the change between the upper

300

and lower crust.

ip t

297

cr

301

The upper crust is product of a sequence of extrusive basalts on top of a sheeted dike

303

complex characterized by a high velocity gradient [e.g. Vera et al., 1990]. Our results show Vp

304

ranging from 4.0-4.5 km/s on top to 6.5-6.6 km/s at the bottom of a ~2 km thick layer, producing a

305

high velocity gradient of 1.3 s-1. The Vs model includes velocities ranging from 2 km/s on top to 3.5

306

km/s at the bottom yielding a gradient of 0.75 s -1. The values of υ decrease from 0.4 to 0.3, at the

307

top and bottom respectively, the highest value of the whole crust imaged. Carlson (2010) based on

308

sonic velocity logs, concluded that the upper oceanic crust is highly fractured, creating secondary

309

porosity. Therefore, fracturing and hydro-alteration might explain the high υ in the upper crust.

311

an

M

d

Ac ce pt e

310

us

302

Lower crust

312

The lower crust is characterized by velocity gradients less steep than in the upper crust,

313

presenting Vp ranging from 6.6-6.7 km/s at the uppermost part to values near to 7.0 km/s at the

314

lowermost crust and Vs values close to 3.5 km/s below the upper-lower crust interface to 4 km/s at

315

the Moho interface. In turn, υ decreases from 0.3 on top to 0.26-0.27 at the bottom lower than the

316

Poisson's ratio in the upper crust.

317 318

The lithology of the lower crust is dominated by gabbro overlying layered gabbro rocks [e.g.

319

Vera et al., 1990]. Large scale in situ values for the uppermost part of the lower crust are Vp=6.7

Page 14 of 73

km/s and υ=0.28 while for its lowermost part are Vp=6.9 km/s and υ=0.31 [Hyndman, 1979]. In

321

some areas that present reduced lower mantle velocity, it has been suggested the presence of

322

hydrous minerals such as chlorite and amphibole [e.g. Christensen and Salisbury, 1975]. A factor

323

that might affect the estimation of the seismic parameters is the anisotropy of metamorphic rocks as

324

amphybolites [Siegesmund et al., 1989] that can constitute 5 to 15 % of gabbros [Carlson and

325

Miller, 2004]. According to laboratory measurements made at 200 MPa, unaltered dry gabbro might

326

change its velocities of Vp = 7.138 km/s and Vs = 3.862 km/s to Vp = 6.866 km/s and Vs =3.909

327

km/s when it metamorphoses to amphybolite, while its Poisson's ratio is reduced from 0.293 to

328

0.260 [Christensen, 1996]. Comparing these results with the seismic models of Figure 6, we observe

329

a coincidence between Vp, Vs and Poisson's ratio for the lower crust and the values reported for

330

metamorphic amphybolite.

an

us

cr

ip t

320

M

331

Although the seismic velocities obtained on our preferred models show little lateral change

333

for the lower crust, the Poisson's ratio reveals lateral heterogeneity. In the uppermost part there are

334

two clear zones of high υ, between 20 km to 40 km and for distances larger than 65 km (Figure 6c),

335

which might indicate a high degree of hydration. According to our model, the sedimentary layer has

336

reduced thickness at the same distances of the high anomalies detected, this perhaps facilitates the

337

infiltration of sea water into the crust. The upper crust is presumably constituted by a highly

338

fractured extrusive layeroverlying brecciated dykes, also called “cracked zone” [Lister, 1974; Vera

339

et al., 1990]. These fractures likely constitute pathways for seawater to penetrate deeper into the

340

crust. An anomalous low υ is observed in the lowermost part of the crust at a distance between 35

341

km and 60 km, Carlson and Miller (2004) showed that although the compressional velocity does not

342

change, the gabbro might be highly altered. Similarly we observe that υ drops near the Moho to

343

values close to 0.27 while Vp is almost unaltered. This observationindicates that the gabbro

344

presents a high degree of metamorphism, likely due to the presence of sea water infiltration.

Ac ce pt e

d

332

345 Page 15 of 73

346

Crustal thickness

347 348

The Moho geometry is well constrained by PmP reflections, with an error estimated below

349

500 m at the center of the profile, due to the higher ray coverage on this zone (Figure 4c), yielding a

350

minimum thickness of ~6 km and a maximum of ~8 km.

ip t

351 The variation in crustal thickness observed along P04 (Figure 6a), might be a local feature that is

353

not directly related to any isostatic regional process. The presence of a seamount's root product of

354

isostatic compensation is unlikely, due to Maule seamount's modest altitude (~1000[m] high from

355

its base) and the evidence of root's absence beneath the more prominent O'Higgins seamount,

356

located at 32ºS on an area of the Nazca plate of similar age and seismic structure [Kopp et al.,

357

2004]. A likely mechanism that might have produced the prominent change of crustal thickness is at

358

the genesis of the Nazca Plate in central Chile, the Chile Rise. The oceanic crust's thickness is

359

independent of its age and spreading velocity, but strongly dependent on the thermal conditions of

360

the mantle upwelling. Different extents of partial melting of the oceanic upper mantle at its

361

generating ridge, can produce small scale variations of the thickness in the axial direction along its

362

generating ridge [Canales et al., 2003; Holmes et al., 2008; Mutter and Mutter, 1993].

364 365

us

an

M

d

Ac ce pt e

363

cr

352

Hydration of the oceanic lithosphere and upper mantle serpentinization

366

The Vp velocity structure along P04, located about 90 km seaward of the trench axis,

367

remains almost constant. However, the most striking feature of the trench perpendicular profile P03

368

(Figure 8a) [Moscoso et al., 2011], is the reduction of crustal and upper mantle velocity

369

approaching to the trench (Figure 8b), indicating changes in the physical properties of the incoming

370

plate. This feature has also been reported along strike in the Chilean margin [Sallarès and Ranero,

371

2005; Contreras Reyes et al., 2008a,b; Scherwath et al., 2009], in the highly hydrated subduction

Page 16 of 73

372

zone offshore Nicaragua [Grevemeyer et al., 2007; Ivandic et al., 2008], and in the Tonga

373

subduction [Contreras-Reyes et al., 2011].

374 A proposed mechanism for velocity reduction in the trench-outer rise is the creation of

376

bending related faulting in the near-trench region and penetration of sea water into the crust along

377

these faults [Ranero et al., 2003, 2005]. The reported depth down to which the faults cut into the

378

crust or mantle is around 20 km below the sea floor [Ranero et al., 2003]. These large seafloor

379

cutting faults might act as pathways that possibility migration of seawater along the fault to reach

380

and hydrate the uppermost mantle. In south central Chile, heat flow decreasing has been observed

381

toward the trench, indicating that the bend faulting facilitates hydrothermal circulation [Grevemeyer

382

et al., 2005; Contreras-Reyes et al., 2007].

cr

us

an M

383

ip t

375

According to the bathymetric data of Figure 1 and the seismic models of Figures 6a and 8a,

385

there is a thick blanket of relative impermeable sediment that might produce the blocking of water

386

infiltration into the fault system through the underlying oceanic crust [Contreras-Reyes et al., 2007].

387

However, reflection seismic data on this zone [Grevemeyer et al., 2005; Diaz-Naveas, 1999] show

388

evidence that some of the fissures are capable of reaching the seafloor and likely creates pathways

389

for seawater into the lithosphere. The reduction of seismic velocities is coincident with an

390

increment in the seafloor roughness that might be an indicator of faults in the basement exposed to

391

the sea water (see Figure 8a). Another effective link between the basement and the sea water is

392

through outcrops and seamounts, as the Maule seamount observed in Figure 1. Seamounts can guide

393

hydrothermal recharge and discharge between sites separated by large distances, due to percolation

394

through their flanks in direct contact with seawater [Fisher et al., 2003]. Although both mechanisms,

395

fracturing and hydration, produce a reduction in seismic velocities, they are related to each other

396

making it difficult to discriminate between them based on seismic properties. Offshore Nicaragua,

397

Ivandic et al. (2010) show a profound correlation between the occurrence of velocity anomalies and

Ac ce pt e

d

384

Page 17 of 73

398

seamounts, suggesting a close link between seamounts and hydratation.

399 Laboratory seismic measurements made on ophiolite dry samples of ultramafic rocks at 200

401

MPa as dunite, present an average Vp of 8.299 km/s [Christensen, 1996], for samples of peridotite

402

the average values estimated are Vp of 8.4 [Hyndman, 1979]; for serpentinite samples at 200 MPa,

403

the seismic velocities reduce drastically to Vp of 5.308 [Christensen, 1996]. Along P04 we obtained

404

average mantle velocities Vp of 8.17 km/s, Vp values present a reduced velocity in comparison with

405

the values correspondent to dry and unaltered mantle. This suggests that we are in presence of

406

hydration of the upper mantle and partial serpentinization. According to Christensen (2004)

407

lizardite and chysotile are abundant in regions where sea water percolates into the upper mantle.

408

This is in close agreement with isotherm computations for an altered portion of the Nazca plate at

409

the outer rise offshore Arauco (38ºS) [Contreras-Reyes et al., 2008a], with similar upper mantle

410

velocities in comparison to offshore Maule [Moscoso et al., 2011], the calculations are based on

411

heat flow measurements and yield temperatures below 300ºC for the upper mantle. At that

412

temperature lizardite and chrysotile serpentines are stable while antigorite is unstable [Christensen,

413

2004]. On the other hand by analyzing only compressional velocities it is hard to discriminate

414

whether lizardite or chysotile is predominant, as their elastic properties are similar, nevertheless

415

under the described conditions lizardite-chrysotile–bearing serpentinites are stable [Christensen,

416

2004] and therefore they are the most probable serpentine constituent of the upper mantle at the

417

trench-outer rise in central Chile.

419

cr

us

an

M

d

Ac ce pt e

418

ip t

400

Upper mantle anisotropy

420

Azimuthal P-wave anisotropy in the upper mantle is well established by seismic refraction

421

experiments [e.g. Shearer and Orcutt, 1986; Beé and Bibee, 1989; Contreras-Reyes et al., 2008b]

422

and also suggested by surface wave studies [Park and Levin, 2002]. The velocity anisotropy can be

Page 18 of 73

described by a sinusoidal function of azimuth with the fast direction, generally parallel to the

424

original spreading direction, as result of the preferred orientation of olivine crystals [Hess, 1964].

425

For dry olivine, the a axis presents the fastest and b axis the slowest Vp [Verma, 1960]. Hess (1964)

426

proposed that mantle anisotropy originated from preferred olivine orientation based on evidence of

427

Pn velocities anisotropy in the north-east Pacific. Plots of velocities versus azimuth on this region

428

showed that upper mantle compressional-wave velocities were fast for propagation directions

429

approximately parallel to fracture zones. Hence, the direction of the fastest P-wave velocity is

430

usually assumed to indicate the flow direction in the mantle [e.g. Zhang and Karato, 1995, Park and

431

Levin, 2002]. The described behavior might not be valid for environments highly hydrated and/or

432

under a high stress field [e.g. Katayama et al., 2009], as the trench-outer rise here under study. Due

433

to the effect of water in the anisotropic deformation of olivine, the high speed axis is reoriented with

434

the low speed axis nearly parallel to the shear direction [Jung and Karato, 2001], it implies that the

435

high speed axis becomes perpendicular to plate motion [Ando et al., 1983]. Additionally, high

436

pressure conditions produce in the olivine the same crystallographic preferred orientation that is

437

produced by high water activity at lower pressure [Jung et al., 2009].

cr

us

an

M

d

Ac ce pt e

438

ip t

423

439

To estimate the degree of anisotropy of Pn velocities, we extracted from the trench parallel

440

velocity model P04 a track 500 m beneath the Moho, it yielded a media velocity of Vp=8.17 km/s.

441

For the velocity model of P03, perpendicular to P04 and to the trench, we also extracted a similar

442

velocity profile in the vicinity of the crossing point between the profiles, which yielded a media

443

velocity of Vp=7.5 km/s. Therefore the Pn anisotropy is near 8%, with a preferential higher velocity

444

direction parallel to the trench axis. The seafloor fabric generated at the spreading axis is well

445

preserved in the trench-outer rise, striking roughly NW-SE (see Figure 1). Thus, P04 runs roughly

446

in spreading direction and hence samples the fast direction as defined by Hess (1964).

447 448

Contreras-Reyes et al. (2008b) found in a younger section of the Nazca plate located at Page 19 of 73

449

~43°S, near the Chile triple junction (CTJ), a lower anisotropy around 2% and the direction of the

450

high velocity axis rotated in comparison to the results presented here. However, in southern Chile

451

the spreading axis runs in N-S direction. Thus, in southern Chile the fast direction faces towards the

452

trench while offshore Maule the slow direction faces the trench.

Outer rise seismic activity

cr

454

ip t

453

For the hypocentral determination we employed 1-D Vp and Vs models, derived from the

456

seismic refraction tomography of Figures 6a and 6b, and the location of the 29 events was done

457

using the linear location program HYP, included in SEISAN [Havskov and Ottemöller, 1999].

an

us

455

458

The set of events analyzed, shown in Figure 9, suggests the existence of a zone of high

460

seismic activity beneath the seamount to a depth of ~20 km. This intraplate seismicity is consistent

461

with the faults imaged in the bathymetry shown in Figure 1, and seems to be product of the

462

reactivation of normal faults formed by plate bending near the trench [Ranero et al., 2005]. Outer

463

rise seismicity also coincides with Vp reduction of the oceanic lithosphere when approaches the

464

trench (see Figures 8 and 9). In southern Chile, a younger section of the Nazca plate is highly

465

hydrated [Contreras-Reyes et al., 2007], it is thought that the shallow outer rise seismicity (<30 km)

466

is triggered by the increment of pore pressure within the fault system produced by infiltration of sea

467

water into the lithosphere [Tilmann et al., 2008], facilitated by the lack of a thick sedimentary layer

468

at seamounts [e.g. Ivandic et al., 2010]. However, bending related faulting itself might be an

469

important mechanism of recurrence of earthquakes and may act as a fault valve, causing seismic

470

pumping [Grevemeyer et al., 2007]. Therefore, the high seismicity nearby the seamount might be

471

explained by a higher fracturing and hydration of the crust. This result suggests that the seamount

472

plays an important role for hydrothermal circulation in the area. In contrast, seaward from the

473

seamount no background seismicity was found, this supports our interpretation that normal

Ac ce pt e

d

M

459

Page 20 of 73

474

anisotropy at the crossing point of the seismic profiles, coincident little faulting in the bathymetry

475

and low Vp variation along P04 are evidences of an oceanic lithosphere not yet affected by faulting

476

and hydration.

477 The shallow outer rise seismic activity might also be related to large interplate earthquakes

479

in the subduction zone. Kato and Hirasawa (2000) showed through a numerical simulation, that a

480

large tensional outer rise earthquake tend to advance the occurrence time and reduce the magnitude

481

of the next interplate earthquake, while a compressional one tends to delay the occurrence of a large

482

interplate earthquake. On the other hand, large underthrusting events might transfer tensional

483

stresses along the slab and subsequently trigger intraplate earthquakes in the outer rise [Christensen

484

and Ruff, 1983 and 1988].

M

485

an

us

cr

ip t

478

The Maule area presented little background seismic activity in comparison to the rest of the

487

Chilean margin [Campos et al., 2002]. This is probably due to the high locking in the interplate

488

between Constitución and Concepción [Ruegg et al., 2009]. However, the months following the

489

mainshock, several events occurred in the outer rise area between ~34°S and ~35°30'S, presenting 6

490

events with magnitude >5.0 over the year following the main shock, as it is shown in Figure 9. It is

491

likely that the large slip reported after the earthquake , between 34°S and 35°30'S [Delouis et al.

492

2010; Moreno et al., 2010], led a transport of slab pull stress to the outer rise, causing a reactivation

493

of bending related normal faults and perhaps producing new fissures in the outer rise, that might

494

trigger new seismic events in the neighboring area. In addition to plate bending, the fracturing and

495

hydration weakens the oceanic plate when approaching the trench [Contreras-Reyes and Osses,

496

2010; Chapple and Forsith, 1979; Kao and Chen, 1996] intensifying the outer rise earthquake

497

genesis process [Lefeldt et al., 2009].

Ac ce pt e

d

486

498

Page 21 of 73

499

Conclusions

500 We analyzed high resolution bathymetric, seimological and active seismic data to investigate

502

the structure of the incoming plate prior to its subduction, in the trench outer rise area offshore

503

Maule, Chile, between 34°S and 35°S. In particular, wide angle seismic data was used to obtain the

504

high resolution 2D velocity structure and derive the Poisson's ratio distribution of the Nazca plate

505

on this area. From this study we have concluded the following:

cr

506

ip t

501

(1)

As the incoming plate approaches the trench, Vp velocity tends to decrease in both, oceanic

508

crust and upper mantle. Those anomalies reported in the compressional velocity are likely produced

509

by a combination of progressive bending related faulting, lithospheric hydration by water

510

percolation and subsequent mantle serpentinization. Thus, the reduction of the upper mantle

511

velocities near the trench, might reflect partial serpentinization of the mantle peridotites.

M

an

us

507

d

512 (2)

514

probably due to mantle upwelling and not by isostatic compensation of the neighboring Maule

515

seamount.

516

The possible differences of the crustal thickness observed along strike in the profile P04 are

Ac ce pt e

513

517

(3)

Unaltered upper mantle presents a Vp anisotropy of ~8%, with the faster velocity axis

518

trending in SW-NE direction and hence in spreading direction, roughly paralleling the trench axis.

519

Therefore, hydration may have affected the lithospheric structure; however, little evidence is found

520

that the anisotropic structure inherited at the spreading axis has been altered by bending-related

521

faulting, water intrusion or serpentinization.

522 523

(4)

We found shallow seismic activity in the outer rise area near the seamount, we conclude that

Page 22 of 73

524

this seismicity is produced due to generation and reactivation of outer rise faults. To the bending-

525

related faulting we have to add the presence of the Maule seamount that probably intensifies the

526

percolation of seawater into the deep structures, producing an intense hydrothermal activity and

527

likely an increment of the pore pressure.

ip t

528 (5)

The main shock of the 2010 Maule earthquake triggered an anomalous high seismic activity

530

in the trench outer rise area, likely due to the stress transmission along the incoming plate, that

531

might have produced a massive crack opening of the bending faults and subsequently water

532

intrusion into the lithosphere.

us

cr

529

an

533 (6)

All the previous conclusions indicate presence of sea water in the upper lithosphere that

535

produces changes on its seismic properties and likely in the petrology.

536

Acknowledgements

Ac ce pt e

537

d

M

534

538

We are grateful to the participants of the JC23 cruise and specially the crew of RV James

539

Cook for their excellent performance on board. We thank E. Contreras-Reyes and E. R. Flueh for

540

critically reading earlier versions of the manuscript and I. Arroyo for her support during the

541

processing of earthquake data. We finally thank to the journal's editor W. Schellart and two

542

anonymous reviewers for constructive criticism and editing. Eduardo Moscoso acknowledges a

543

scholarship granted by the Chilean Comisión Nacional de Investigación Científica y Tecnológica

544

(CONICYT) and the German Academic Exchange Service (DAAD).

545

546

Figure captions

547 Page 23 of 73

Figure 1: (Top) High resolution bathymetric map offshore Maule region in south-central Chile with

549

the identification of its main features. The white arrow indicates the relative convergence velocity

550

between Nazca and South American plates. Transects P03 [Moscoso et al., 2011] and P04 (this

551

study) are represented by solid black lines, the green dots show the stations' locations for the wide

552

angle experiment and the white triangles show the positions of the local seismic network sensors.

553

Station 229, represented by a green triangle, was used for both experiments. The profile P04

554

presented here runs parallel to the Chile-Peru trench, and some 25 km landward its location we

555

identify a seamount. (Bottom) Locations of the OBS/H projected on the bathymetry.

cr

ip t

548

us

556

Figure 2: (Top) Examples of wide-angle seismic data. (Bottom) Manually picked arrivals (pick

558

uncertainty is represented by color bars). Predicted traveltimes using the average 2D final models

559

are superimposed on the seismic sections. Solid lines represent the calculations for refractions (red)

560

and reflections from Moho (black).

Ac ce pt e

d

561

M

an

557

562

Figure 3: a) Initial models used for the Monte Carlo inversion procedure. Green bands in the

563

velocity profile show the 1D initial velocity models for Vs (left) and Vp (right). Gray band

564

represents the initial depth range used for Moho initial reflectors, b) Example of the delay between

565

Pg and PPS phases.

566 567

Figure 4: a) Error for the Vp model, b) Error for the Vs model, c) DWS for Vp model, d) Poisson's

568

ratio error calculated from equation (2).

569 570

Figure 5: Resolution tests for a) Vp model, b) Vs model.

571

Page 24 of 73

572

Figure 6: Final velocity model derived from averaging 100 Monte Carlo ensembles for Vp (Top)

573

and Vs (Center). (Bottom) Poisson’s ratio masked by the intersection of rays fromthe P and S wave

574

velocity models.

575 Figure 7: Final velocity models for Vp (Top). Vs (Center) and Poisson's ratio (Bottom) using a flat

577

Moho and a kernel w=0,01, this test shows a velocity-depth trade off for the lower crust. The

578

overall error for the models is 95 ms for Vp and 92 ms for Vs.

cr

579

ip t

576

Figure 8: a) Velocity model of P03 from Moscoso et al. (2011), the segmented line CP denotes the

581

crossing point with P04 (this study). b) Velocity profiles extracted from the locations A1 and A2 in

582

a). The red profile was extracted beneath the crossing point (CP) in Profile P04 (Figure 6a).

an

us

580

M

583

Figure 9: a) Map of seismicity and bathymetry along the Chile-Peru trench offshore Maule where

585

the 2010 megathrust earthquake, followed by a tsunami, hit central Chile. Its NEIC location is

586

indicated by a large red star. The black solid lines stand for the locations of the profiles P03 and

587

P04; The deformation front is indicated by a thick black line. The yellow dots denote the seismicity

588

over a 3 months period after the main shock, extracted from the NEIC catalog. The white stars

589

represent the outer rise events with Mw> 5.0 over a period of 1 year after the main shock, with its

590

respective Harvard GCMT fault plane solutions. The local earthquakes recorded by our outer rise

591

network (ORN) operative between early March and the first week of April 2008, are represented by

592

red dots. Their projection over P03 and P04 are in b) and c), respectively.

Ac ce pt e

d

584

593 594

Table 1: Summary of data picking information and statistics of the fitness between the final average

595

models and picks.

596

Page 25 of 73

597

Supplementary material

598 599

Figures S1 and S2: Data examples of two outer rise seismic events recorded during the seismic

600

experiment.

601

603

ip t

602

References

604

Ando, M., Y. Ishikawa, and F. Yamazaki (1983), Shear Wave Polarization Anisotropy in the Upper Mantle Beneath Honshu, Japan, J. Geophys. Res., 88(B7), 5850-5864.

cr

605 606 608 609

us

607

Angermann, D., J. Koltz, and C. Rigber (1999), Space-Geodetic estimation of the Nazca-South America Euler vector, Earth Planet. Sci. Lett., 171(3), 329-334.

611

an

610

Barrientos, S., Giant returns in time (2005), Nature, 737, 329, doi:10.1038/437329a.

612

617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632

M

616

Bialas, J., and E. R. Flueh (1999), Ocean bottom seismometers, Sea Technol., 40(4), 41-46.

d

615

Bée, M. and Bibee, L. D. (1989), A seismic refraction study of cretaceous oceanic lithosphere in the Northwest Pacific Basin, Marine Geophysical Researches, Volume 11, Issue 4, pp.239-261, doi:10.1007/BF00282578.

Blumberg, S., F. Lamy, H.W. Arz, H.P. Echtler, M. Wiedicke, G.H. Haug and O. Oncken (2008), Turbiditic trench deposits at the South-Chilean active margin: A Pleistocene–Holocene record of climate and tectonics, Earth Planet. Sci. Lett., 268, 526-539, doi:10.1016/j.epsl.2008.02.007.

Ac ce pt e

613 614

Campos J., Hatzfeld, R. Madariaga, G. Lopez, E. Kausel, A. Zollo, G. Iannacone, R. Fromm, S. Barrientos and H. Lyon-Caen (2002), A seismological study of the 1835 seismic gap in south-central Chile, Physics of The Earth and Planetary Interiors, Volume 132, Issues 1-3, 30 September 2002, Pages 177-195 , doi:10.1016/S0031-9201(02)00051-1. Canales, J. P., R. S. Detrick, D. R. Toomey, and W. S. D. Wilcock (2003), Segment‐scale variations in the crustal structure of 150–300 kyr old fast spreading oceanic crust (East Pacific Rise, 8°15′N–10°5′N) from wide‐angle seismic refraction profiles, Geophys. J. Int., 152(3), 766–794, doi:10.1046/j.1365-246X.2003.01885.x. Carlson, R. L. and D. Jay Miller (2004), Influence of pressure and mineralogy on seismic velocities in oceanic gabbros: Implications for the composition and state of the lower oceanic crust, J. Geophys. Res., 109, B09205, doi:10.1029/2003JB002699.

633 634 635

Carlson, R. L. (2010), How crack porosity and shape control seismic velocities in the upper oceanic crust: Modeling downhole logs from Holes 504B and 1256D, Geochem. Geophys. Geosyst., 11, Q04007, doi:10.1029/2009GC002955.

636 637 638

Chapple, W., and D. Forsyth (1979), Earthquakes and Bending of Plates at Trenches, J. Geophys. Res., 84(B12), 67296749.

639

Page 26 of 73

640

Christensen, N. (1996), Poisson's ratio and crustal seismology, J. Geophys. Res., 101(B2), 3139-3156.

641 642 643

Christensen, N. (2004), Serpentinites, Peridotites, and Seismology, International Geology Review, 46:9, 795-816, doi:10.2747/0020-6814.46.9.795

644 645 646

Christensen, D. H., and L. J. Ruff (1983), Outer‐rise earthquakes and seismic coupling, Geophys. Res. Lett., 10(8), 697700.

647

Christensen, D., and L. Ruff (1988), SEISMIC COUPLING AND OUTER RISE EARTHQUAKES, J. Geophys. Res., 93(B11), 13421-13444.

ip t

648 649 650

Christensen, N. I., and M. H. Salisbury (1975), Structure and constitution of the lower oceanic crust, Reviews of Geophysics, 13, 57–86.

cr

651 652 653

Cisternas, M., et al. (2005), Predecessors of the giant 1960 Chile earthquake, Nature, 437, 404–407, doi:10.1038/nature03943.

us

654 655 656

Contreras-Reyes, E., and A. Osses (2010), Lithospheric flexure modeling seaward of the Chile trench: Implications for oceanic plate weakening in the Trench Outer Rise region. Geophysical Journal International 182(1), 97-112.

an

657 658 659

664 665 666 667 668 669 670 671 672 673 674 675 676 677 678

M

Contreras‐Reyes, E., I. Grevemeyer, E. R. Flueh, and C. Reichert (2008a), Upper lithospheric structure of the subduction zone offshore of southern Arauco peninsula, Chile, at ∼38°S, J. Geophys. Res., 113, B07303, doi:10.1029/2007JB005569.

d

663

Contreras‐Reyes, E., I. Grevemeyer, E. R. Flueh, M. Scherwath, and M. Heesemann (2007), Alteration of the subducting oceanic lithosphere at the southern central Chile trench–outer rise, Geochem. Geophys. Geosyst. , 8 , Q07003, doi:10.1029/2007GC001632.

Ac ce pt e

660 661 662

Contreras-Reyes E., I. Grevemeyer, E. R. Flueh, M. Scherwath, and J. Bialas (2008b), Effect of trench-outer rise bending-related faulting on seismic Poisson's ratio and mantle anisotrophy: a case study offshore of Southern Central Chile. Geophys. J. Int., 173, 142-156, doi:10.1111/j.1365-246X.2008.03716.x Contreras‐Reyes, E., E. R. Flueh, and I. Grevemeyer (2010), Tectonic control on sediment accretion and subduction off south central Chile: Implications for coseismic rupture processes of the 1960 and 2010 megathrust earthquakes, Tectonics, 29, TC6018, doi:10.1029/2010TC002734. Contreras-Reyes, E., I. Grevemeyer, A. Watts, E. R. Flueh, C. Peirce, S. Moeller, and C. Papenberg (2011), Deep seismic structure of the Tonga subduction zone: implications for mantle hydration, tectonic erosion, and arc magmatism J. Geophys. Res., doi:10.1029/2011JB008434.

679 680 681

Contreras-Reyes, E., J. Jara, I. Grevemeyer, S. Ruiz, and D. Carrizo (2012), Abrupt change in the dip of the subducting plate beneath north Chile, Nature Geoscience., 5, 342-345, doi:10.1038/ngeo1447

682 683 684 685

Delouis, B., J.-M. Nocquet, and M. Vallee (2010), Slip distribution of the February 27, 2010 Mw = 8.8 Maule Earthquake, central Chile, from static and high-rate GPS, InSAR, and broadband teleseismic data, Geophys. Res. Lett., doi:10.1029/2010GL043899, in press.

686 687

Diaz-Naveas, J. L (1999), Sediment subduction and accretion at the Chilean convergent margin between 35° and 40°S,

Page 27 of 73

688

Ph.D. Thesis, Ch.-A.-Universität, Kiel, Germany.

689 690

Dijkstra, E. W. (1959). "A note on two problems in connexion with graphs". Numerische Mathematik 1: 269–271.

691 692 693 694

Fisher A. T., E. E. Davis, M. Hutnak, V. Spiess, L. Zuehlsdorff, A. Cherkaoul, L. Christiansen, K. Edwards, R. Macdonald, H. Villinger, M. J. Mottl, C. G. Wheat, and K. Becker (2003), Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank, Nature, 421, 618-621, doi:10.1038/nature01352.

695

Flueh, E. R., and J. Bialas (1996), A digital, high data capacity ocean bottom recorder for seismic investigations, Int. Underwater Syst. Design, 18(3), 18-20.

698 699

Flueh, E. R., and J. Bialas (Eds.) 2008. Cruise report JC23, IFM-Geomar rep. nº 20.

Grevemeyer, I., N. Kaul, J. L. Diaz-Naveas, H. Villinger, C. R. Ranero, and C. Reichert (2005), Heat flow and bendingrelated faulting at subduction trenches: case studies offshore of Nicaragua and central Chile, Earth Planet. Sci. Lett., 236, 238–248, doi:10.1016/j.epsl.2005.04.048.

us

701 702 703

cr

700

704

Grevemeyer, I., C. R. Ranero, E. R. Flueh, D. Klaeschen, and J. Bialas (2007), Passive and active seismological study of bending-related faulting and mantle serpentinization at the Middle America trench, Earth Planet. Sci. Lett., doi:10.1016/j.epsl.2007.04.013.

708

712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734

beyond

arcs,

M

subduction

Geochem.

Geophys.

Geosyst.,

9,

Q03001,

Hamilton, E. L. (1976), Shear-wave velocity versus depth in marine sediments: a review, Geophysics, 41(5), 985-996.

d

711

Hacker, B. R. (2008), H2O doi:10.1029/2007GC001707.

Havskov, J and L. Ottemoller (1999), SeisAn Earthquake analysis software, Seis. Res. Lett., 70, 532-534.

Ac ce pt e

709 710

an

705 706 707

ip t

696 697

Hess, H. H. (1964), Seismic Anisotropy of the Uppermost Mantle under Oceans, Nature, 203, 629-631, doi:10.1038/203629a0. Hole, J. A. (1992), Nonlinear High-Resolution Three-Dimensional Seismic Travel Time Tomography, J. Geophys. Res., 97(B5), 6553–6562. Holmes, R. C., M. Tolstoy, J. R. Cochran, and J. S. Floyd (2008), Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography, Geochem. Geophys. Geosyst., 9, Q12020, doi:10.1029/2008GC002152. Hyndman, R. D. (1979), Poisson's ratio in the oceanic crust: a rewiew, Tectonophysics, 59, 321-333. Ivandic, M., I. Grevemeyer, A. Berhorst, E. R. Flueh, and K. McIntosh (2008), Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua, J. Geophys. Res., 113, B05410, doi:10.1029/2007JB005291. Ivandic, M., I. Grevemeyer, J. Bialas, and C. J. Petersen (2010), Serpentinization in the trench‐outer rise region offshore of Nicaragua: Constraints from seismic refraction and wide‐angle data, Geophys. J. Int., 180, 1253–1264, doi:10.1111/j.1365-246X.2009.04474.x.

735 736 737

Jung, H., M. Wo & H. W. Green (2009), Upper mantle seismic anisotropy resulting from pressure-induced slip transition in olivine, Nature Geoscience, 2, 73 – 77, doi:10.1038/ngeo389.

738

Page 28 of 73

739 740

Jung, H. and S. Karato (2001), Water-Induced Fabric Transitions in Olivine, Science, Vol. 293, no. 5534, pp. 14601463, doi: 10.1126/science.1062235.

741 742 743

Kanamori H. (1971), Seismological evidence for a lithospheric normal faulting - the Sanriku earthquake of 1933 , Physics of The Earth and Planetary Interiors, vol. 4, no. 4, pp. 289-300, doi: 10.1016/0031-9201(71)90013-6.

744 745 746

Kao, H., and W.‐P. Chen (1996), Seismicity in the outer rise–forearc region and configuration of the subducting lithosphere with special reference to the Japan Trench, J. Geophys. Res., 101(B12), 27811-27831.

747

Katayama, I., K. Hirauchi, K. Michibayashi and J. Ando (2009), Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge, Nature, 461, 1114-1117, doi:10.1038/nature08513.

ip t

748 749 750

Kato, N., and T. Hirasawa (2000), Effect of a large outer rise earthquake on seismic cycles of interplate earthquakes: A model study, J. Geophys. Res., 105(B1), 653-662.

cr

751 752 754 755 756

us

753

Kopp, H., E. R. Flueh, C. Papenberg, and D. Klaeschen (2004), Seismic investigations of the O'Higgins Seamount Group and Juan Fernández Ridge: Aseismic ridge emplacement and lithosphere hydration, Tectonics, 23, TC2009, doi:10.1029/2003TC001590.

764 765 766 767 768 769 770 771 772 773 774 775 776 777 778

M

762 763

Korenaga (2011), Velocity–depth ambiguity and the seismic structure of large igneous provinces: a case study from the Ontong Java Plateau, Geophys. J. Int., 185, 2, 1022–1036, 10.1111/j.1365-246X.2011.04999.x

d

761

Korenaga, J., W. Holbrook, G. Kent, P. Kelemen, R. Detrick, H. ‐C. Larsen, J. Hopper, and T. Dahl ‐Jensen (2000), Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography, J. Geophys. Res., 105(B9), 21591-21614.

Lefeldt, M., I. Grevemeyer, J. Goßler and J. Bialas (2009), Intraplate seismicity and related mantle hydration at the Nicaraguan trench outer rise, Geophys. J. Int., 178(2), 742-752, doi:10.1111/j.1365-246X.2009.04167.x.

Ac ce pt e

758 759 760

an

757

Lister, C. R. B. (1974), On the penetration of water into hot rock, Geophys. J. R. Astronom. Soc., 39, 465-509. Madariaga, R., M. Métois, Ch. Vigny, and J. Campos (2010), Central Chile Finally Breaks, Science (in Perspectives), 328, 181-182, doi:10.1126/science.1189197. Moreno, M., M. Rosenau and Onno Oncken (2010), 2010 Maule earthquake slip correlates with pre-seismic locking of andean subduction zone, Nature, 467, 198-202 ,doi:10.1038/nature09349. Moscoso, E., I. Grevemeyer, E. Contreras-Reyes and E. Flueh (2011), Revealing the deep structure and rupture plane of the 2010 Maule, Chile Earthquake (Mw=8.8) using wide angle seismic data, Earth Planet. Sci. Lett., 307, 147-155, doi:10.1016/j.epsl.2011.04.025.

779 780 781

Moscoso, E., and E. Contreras-Reyes (2012), Outer rise seismicity related to the maule, Chile 2010 megathrust earthquake and hydration of the incoming oceanic lithosphere, Andean Geology, 39 (3), 563-571.

782 783 784

Mutter C. Z. and J. C. Mutter (1993), Variations in thickness of layer 3 dominate oceanic crustal structure, Earth Planet. Sci. Lett., 117(1993), 295-317.

785

Page 29 of 73

786 787

Müller, R. D., W. R. Roest, J.-Y. Royer, L. M. Gahagan, and J. G. Sclater (1997), Digital isochrons of the world's ocean floor, J. Geophys. Res., 102, 3211-3214, doi:10.1029/96JB01781.

788 789 790

Park, J. and V. Levin (2002), Seismic Anisotropy: Tracing Plate Dynamics in the Mantle, Science, Vol. 296 no. 5567 pp. 485-489, doi:10.1126/science.1067319.

791 792 793

Ranero, C. R., J. Phipps Morgan, K. McIntosh, and C. Reichert (2003), Bending, faulting, and mantle serpentinization at the Middle America Trench, Nature, 425, 367–373, doi:10.1038/nature01961.

794

Ranero, C. R., A. Villaseñor, J. Phipps Morgan, and W. Weinrebe (2005), Relationship between bend-faulting at trenches and intermediate-depth seismicity, Geochem. Geophys. Geosyst., 6, Q12002, doi:10.1029/2005GC000997.

ip t

795 796 797

cr

Ruegg J. C., A. Rudloff, C. Vigny, R. Madariaga, J.B. de Chabalier, J. Campos, E. Kausel, S. Barrientos and D. Dimitrov (2009), Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile, Physics of the Earth and Planetary Interiors, Volume 175, Issues 1-2, June 2009, Pages 78-85, doi:10.1016/j.pepi.2008.02.015.

us

798 799 800 801 802

Sallarès, V. and C. R. Ranero (2005), Structure and tectonics of the erosional convergent margin off Antofagasta, north Chile (23°30′S), J. Geophys. Res., 110, B06101, doi:10.1029/2004JB003418.

an

803 804 805

811 812 813 814 815 816 817 818 819 820 821 822 823 824

M

809 810

Siegesmund, S., T. Takeshita, and H. Kern (1989), Anisotropy of Vp and Vs in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural, and textural characteristics of the rock, Tectonophysics, 157, 25-38.

d

808

Satake, K, and Y. Tanioka (1999), Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones, Pure Appl. Geophys., Volume 154, Numbers 3-4, 467-483, DOI: 10.1007/s000240050240

Shearer, P. M. and J. A. Orcutt (1986), Compressional and shear wave anisotropy in the oceanic lithosphere - the Ngendei seismic refraction experiment, Geophys. J. R. astr. Soc., Volume 87, Issue 3, pages 967–1003, doi:10.1111/j.1365-246X.1986.tb01979.x.

Ac ce pt e

806 807

Scherwath, M., E. Contreras-Reyes, E. R. Flueh, I. Grevemeyer, A. Krabbenhoeft, C. Papenberg, C. J. Petersen, and R. W. Weinrebe (2009), Deep lithospheric structures along the southern central Chile margin from wide-angle P-wave modelling, Geophys. J. Int., 179(1), 579-600, doi:10.1111/j.1365-246X.2009.04298.x. Spudich, P., and J. A. Orcutt (1980), Petrology and porosity of an oceanic crust site: results from waveform modeling of seismic refraction data. J. Geophys. Res., 85:1409-1433. Tarantola, A. (1987), Inverse Problem Theory: Methods for Data Fitting and Model Parameter Estimation, 613 pp., Elsevier, New York.

825 826

Tieman, H. J. (1994), Investigating the velocity-depth ambiguity of reflection traveltimes, Geophysics, 59 , 1763–1773.

827 828 829 830

Tilmann, F. J., I. Grevemeyer, E. Flueh, T. Dahm, J. Goßler (2008), Seimicity in the outer rise offshore southern chile: Indication of fluid effects in crust and mantle, Earth Planet. Sci. Lett., 269(2008), 41-55, doi:10.1016/j.epsl.2008.01.044.

831 832 833

Toomey, D.R., and G.R. Foulger (1989), Tomographic inversion of local earthquaked data from the Hengill-Grensdalur central volcano complex, Iceland, J. Geophys. Res., 94, 17.497-17.510.

Page 30 of 73

834 835 836

Van Avendonk, H., A. Harding, J. Orcutt, and J. McClain (1998), A two ‐dimensional tomographic study of the Clipperton transform fault, J. Geophys. Res., 103(B8), 17885-17899.

837 838 839 840

Vera, E., J. Mutter, P. Buhl, J. Orcutt, A. Harding, M. Kappus, R. Detrick, and T. Brocher (1990), The Structure of 0 ‐ to 0.2‐m.y.‐Old Oceanic Crust at 9°N on the East Pacific Rise From Expanded Spread Profiles, J. Geophys. Res., 95(B10), 15529-15556.

841 842

Verma, R. (1960), Elasticity of Some High‐Density Crystals, J. Geophys. Res., 65(2), 757-766.

844 845

ip t

843

Watts, A. B. (1994), Crustal structure, gravity anomalies and flexure of the lithosphere in the vivinity of the Canary Isalands, Geophys. J. Int., 119, 648-666. von Huene, R., J. Corvalán, E. Flueh, K. Hinz, J. Korstgard, C. Ranero, W. Weinrebe, and (1997), Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile, Tectonics, 16(3), 474-488.

us

847 848

cr

846

849

d

M

an

Zhang, S. and S. Karato (1995), Lattice preferred orientation of olivine aggregates deformed in simple shear, Nature, 375, 774-777, doi:10.1038/375774a0.

Ac ce pt e

850 851

Page 31 of 73

*Manuscript with marked corrections Click here to view linked References

1

Bending-related faulting of the incoming oceanic plate and its effect on

2

lithospheric hydration and seismicity: A passive and active

3

seismological study offshore Maule, Chile.

E. Moscoso1 and I. Grevemeyer

5 6

GEOMAR Helmholtz Centre for Marine Research, Wischhofstraße 1-3, 24148 Kiel, Germany

cr

7

us

8

Abstract

an

9

ip t

4

We have studied the dependency between incoming plate structure, bending-related faulting,

11

lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D

12

Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-

13

outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic

14

lithosphere is highly hydrated due to the water infiltration trough bending-related normal faults

15

outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in

16

the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile

17

trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of

18

the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis

19

running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise

20

processes have yet not affected anisotropy.

Ac ce pt e

d

M

10

21

The first weeks following the megatrust Mw=8.8 Maule earthquake in 2010 were

22

characterized by a sudden increase of the outer rise seismic activity, located between 34°S and

23

35°30'S. We concluded that this phenomenon is a result of an intensification of the water infiltration 1

Now at Erdbeben Engineering-Geoscience, Ruiz Tagle 771, Santiago, Chile. Page 32 of 73

24

process in the outer rise, presumably triggered by the main shock, whose epicenter was located

25

some 100 km to the south east of the cluster.

26

Keywords: Outer rise, Subduction zones, Mantle serpentinization, Maule earthquake, Refraction-

27

and wide-angle seismology, Local seismicity.

Introduction

cr

29

ip t

28

The amount of water stored within the oceanic lithosphere plays a fundamental role in the

31

generation of arc and back-arc magmas, hydration of the mantle wedge, and the global budget of

32

water [Hacker et al., 2008]. The south central Chile subduction zone is characterized by a highly

33

fractured trench outer-rise seafloor, which is generated by the bending of the Nazca plate

34

[Contreras-Reyes and Osses, 2010 and reference therein]. This bending-related faulting may

35

reactivate pre-existing cracks in the oceanic crust, previously created at the spreading center, and it

36

may also generate faults cutting deep into the lithosphere [e.g. Grevemeyer et al. 2005]. This

37

process modifies the porosity and permeability structure of the oceanic crust [Carlson, 2010] and

38

allows the water to infiltrate deep into the lithosphere, producing hydration of the crust and

39

eventually serpentinization of the upper mantle [e.g. Contreras-Reyes et al., 2008a,b; Ranero et al.

40

2003]. Another purposed pathway for fluids through the crust are seamounts and outcrops

41

penetrating through the sediments [e.g. Fisher et al., 2003; Contreras-Reyes et al., 2007; Ivandic et

42

al., 2010].

Ac ce pt e

d

M

an

us

30

43 44

The creation of normal faults in the outer rise, due to plate bending, is responsible for the

45

shallow intraplate seismic activity [e.g. Ranero et al., 2005; Lefeldt et al., 2009] that might produce

46

considerable seismicity seaward of the updip region of the seismogenic zone [Moscoso et al, 2011;

47

Moscoso and Contreras-Reyes, 2012] and may also in rare cases produce devastating tsunamis

Page 33 of 73

48

[Satake and Kanioka, 1999]. Although the magnitude of the offshore intraplate events is in general

49

smaller than the subduction-related earthquakes, they can present considerable magnitudes. For

50

instance, the largest normal-faulting event ever reported is the Sanriku, Japan earthquake of 1933,

51

with a magnitude Mw=8.5, that probably ruptured along the entire oceanic lithosphere [Kanamori,

52

1971].

ip t

53

Previous seismic studies conducted offshore of south-central Chile showed evidence of

55

hydration of the oceanic crust and serpentinization of the upper mantle at the Juan Fernandez ridge

56

[Kopp et al. 2004] in the trench-outer rise area offshore Arauco [Contreras-Reyes et al., 2008a] and

57

offshore Chiloe to the north of the Chile triple junction [Contreras-Reyes et al., 2008b]. Upper

58

lithospheric hydration has also been deduced from seismic studies performed at the erosive margin

59

off Northern Chile [Sallarès and Ranero, 2005; Contreras-Reyes et al., 2012] and offshore of

60

Nicaragua [Ivandic et al., 2008]. Seismological studies suggest that hydration of the oceanic plate

61

and outer rise seismicity are not independent phenomena [Ranero et al., 2005; Tilmann et. al. 2008]

62

but are related to each other. Thus, both processes are probably common features along most

63

subduction zones [Contreras-Reyes and Osses 2010; Grevemeyer et al., 2007].

us

an

M

d

Ac ce pt e

64

cr

54

65

A parameter commonly used to evaluate hydration is the Poisson’s ratio (ν). It is defined as

66

the ratio, when a sample object is stretched, of the contraction or transverse strain (perpendicular to

67

the applied load), to the extension or axial strain (in the direction of the applied load). Its analytical

68

formula expressed in terms of body waves is given by the expression (1): 2

69 70

ν=

Vp/ Vs −2 2 2 [Vp/ Vs ]−1

(1)

and its standard deviation ∆ν is calculated by (2):

Page 34 of 73

2

∆ ν=

71

Vp/ Vs 2[Vp/ Vs2 ]−12



2

2

  

∆ Vp ∆ Vs  Vp Vs

(2)

Where ∆Vp and ∆Vs are the standard deviations of P- and S-wave velocity fields,

73

respectively. Poisson's ratio is very sensitive to the existence of water. In a material with high

74

content of water, Vs tends to decrease faster than Vp, producing an increase of ν.

ip t

72

75

To study the relationship between hydration and seimicity offshore of Maule, Chile, we

77

analyzed jointly (1) seismological data from a temporal outer rise seismic network (ORN) deployed

78

for about one month, (2) swath bathymetric data, and (3) the seismic velocity structure of the Nazca

79

plate prior to its subduction, between ~34°S and ~35°S. Wide-angle seismic data from a 85-km-long

80

trench parallel seismic profile was used to derive the compressional (Vp) and shear wave (Vs)

81

velocity structure. We used travel time tomography to yield the 2-D velocity structure and crustal

82

thickness and Poisson's ratio. Further, we studied model uncertainties of both P- and S-wave

83

velocity by applying a non linear Monte Carlo-like inversion method [Korenaga et al., 2000]. We

84

examine the crustal models to derive an estimation of degree of the upper lithosphere hydration and

85

we discuss the implications of variations of the oceanic upper mantle velocity, comparing our

86

results with published data from a seismic profile which runs perpendicular to the current profile

87

[Moscoso et al., 2011].

89

us

an

M

d

Ac ce pt e

88

cr

76

Tectonic framework

90

The oceanic Nazca plate converges beneath the South American plate at relative velocity of

91

6.6 cm/yr with an azimuth of 78°E [Angermann et al., 1999]. The Nazca plate oceanic crust

92

offshore central Chile between 32°S and 38°S, was generated at the Chile Rise ~30 to ~35 Ma

93

[Müller et al., 1997], these spreading center segments and bending related normal faults can be

94

observed in the high resolution bathymeric map of Figure 1. The seabed is covered by a Page 35 of 73

sedimentary layer ~200 m thick on the location of our profile, with increasing depth trenchward to a

96

maximum of ~2 km at the trench axis [Moscoso et al., 2011]. The incoming oceanic plate in the

97

trench-outer rise area is highly faulted and hydrated by intrusions of cold seawater [Grevemeyer et

98

al., 2005]. These faults likely hydrate the crust and upper mantle[e.g. Contreras-Reyes et al.,

99

2008a]. The Juan Fernandez Ridge (JFR) at ~32°S acts as a barrier for the migration of sediments

100

sourced from the Andes [Blumberg et al., 2008] and carried in the trench from south to north, which

101

changes the regime of the margin from erosive in the north to accretionary in the south [von Huene

102

et al., 1997].

cr

ip t

95

us

103

The central Chilean margin has hosted some of the largest subduction zone earthquakes. A

105

few times in every century large thrust earthquakes broke several hundreds of kilometers in a single

106

shock [e.g. Barrientos, 2005], often producing devastating tsunamis [Cisternas et al., 2005].

107

Previously, the thrust offshore Maule has been reported as fully locked [Ruegg et. al., 2009;

108

Campos et al. 2002]. In fact, the 2010 megathrust Maule earthquake (Mw=8.8) ruptured some 400

109

km along the margin, producing a devastating Tsunami [Madariaga et al., 2010]. The second largest

110

slip of the 2010 Maule earthquake was reported between 34° and 35° [Delouis et al., 2010; Moreno

111

et al., 2010]. This zone coincides with an anomalous high outer-rise seismic activity after the

112

earthquake [Moscoso et al., 2011] in comparison to the rest of the Maule seismic segment.

114

M

d

Ac ce pt e

113

an

104

Seismic experiment and data

115

The data analyzed in this study consist of high resolution multibeam bathymetry, wide-angle

116

seismics and local earthquake measurements made offshore the Maule region (34°S-35°30'S),

117

during March and April 2008 during the cruise JC23 of the British RV James Cook [Flueh and

118

Bialas, 2008].

119 Page 36 of 73

120

Local earthquake data The outer rise network (ORN), deployed between the 1 st of March and 8th of April 2008,

122

consists of 19 ocean bottom seismometers and hydrophones (OBS/H) [Flueh and Bialas, 1996;

123

Bialas and Flueh, 1999]. The instruments were deployed with a spacing between 20 and 40 km,

124

covering an area of approximately 100 by 100 km². The network extended over the outer rise from

125

~200 km offshore to the deformation front, surrounding the Maule seamount as is shown in Figure

126

1.

cr

ip t

121

127

Preprocessing of the OBS/H data included calculation of the clock-drift corrections to

129

adjust the clock in each instrument to the shipboard GPS base time and instrument locations were

130

corrected for drift from the deployment position during their descent to the seafloor using the direct

131

water wave arrival, recorded from the active seismics. A short-term average versus long-term

132

average (STA/LTA) trigger algorithm was applied to the data to detect signal variations that could

133

indicate an event. Earthquake identification from the triggered data was done manually, giving a

134

total of 29 events, 7 of which were inside the network. The data reading and picking of the P and S

135

arrivals was done using the software SEISAN [Havskov and Ottemöller, 1999]. We manually chose

136

a quality factor for each picked phase and this quality factor was used to account for the picking

137

uncertainties.

139

an

M

d

Ac ce pt e

138

us

128

Wide angle seismic data

140

The wide angle seismic profile P04 is a ~84 km long transect that runs parallel to the trench

141

in the outer rise area, some 130 km offshore Constitución (See Figure 1). The seismic source

142

consisted of four tuned arrays of three airguns each plus two single airguns, providing a total

143

volume of 11200 inch³. The airguns were fired at intervals of 60 s, or 150 meters at a ship speed of

144

5 knots . The airgun shots were recorded by 8 OBS and 1 OBH, which makes a total of 9 stations

145

deployed along the transect. A time-gated deconvolution filter was applied to remove predictable Page 37 of 73

146

bubble reverberations.

147 P wave arrivals were recorded with excellent quality and clear S wave arrivals were

149

recorded in 8 of the 9 seismic stations (only station 404 did not present distinguishable S waves

150

arrivals). Data examples from two stations are shown in Figure 2, with their respective seismic

151

phases identified. Picking of the seismic phases was done manually, and picking errors were

152

assigned on the basis of the dominant period of the phase. Typically, errors were assumed to be half

153

a period of one arrival and weighted according to the phase quality. Based on the quality of the data,

154

for S wave arrivals we assigned a higher error than for the P phases and due to the larger

155

uncertainties for larger offsets. We also differentiated picking uncertainties for long offset arrivals.

156

Detailed information regarding pick uncertainties and model fitness are summarized in Table 1.

an

us

cr

ip t

148

M

157

Velocity field modeling procedure

159

P wave travel time tomography

Ac ce pt e

d

158

160

The absence of refracted arrivals from the sedimentary layer and reflections from the

161

basement was overcome by assuming typical Vp=1.8 km/s and Vs=0.25 km/s for sediment and

162

calculating the sediment thickness beneath each station through the time difference between the Pg

163

and PPS phases. While Pg corresponds to crustal refractions, the PPS wave modes travel through

164

the sedimentary layer and oceanic crust as a Pg wave but they are converted to an S wave at the

165

sediment-crust interface when the seismic rays dive up [Spudich and Orcutt, 1980], and finally

166

travels though the sedimentary layer as an S wave, therefore PPS is recorded as an S wave in the

167

OBS with the same apparent velocity of Pg but delayed respect to it due to lower travel velocity in

168

the sediment. Time differences between Pg and PPS phases range between 0.6 [s] and 1.1 [s] in our

169

dataset (see Figure 3b), yielding a sediment thickness of 180-300 meters.

Page 38 of 73

170 The 2D Vp-depth distribution below the sedimentary layer was obtained using the joint

172

refraction and reflection travel time inversion code TOMO 2D, that simultaneously solves for the

173

seismic velocity field and the depth of a floating reflecting interface [Korenaga et al. 2000]. Travel

174

times and ray paths are calculated employing a hybrid ray tracing scheme, based on the graph

175

method [e.g. Dijkstra 1959] and in the local ray bending refinement [e.g. Van Avendonk 1998]. The

176

velocity grid used for representing the velocity field is parametrized as a sheared mesh hanging

177

beneath the seafloor, where the node spacing was fixed constant in 0.5 km for the horizontal and

178

varying from 0.05 Km at the top of the model to 0.5 km at the bottom for the vertical. The smaller

179

spacing on top accounts the higher resolution in shallower parts of the model than at the bottom.

180

The Moho is represented by a floating reflector, which consists of an array of linear segments with a

181

horizontal equidistant spacing of 0.5 km between its nodes, and only one degree of freedom in the

182

vertical direction.

cr

us

an

M d

183

ip t

171

The data set considered for the P-wave tomography consists of 2701 Pg, 1067 PmP and 543

185

Pn phases that were handpicked from 9 OBS/H. The size of the model is ~84 Km long and 14 Km

186

deep. We performed the inversion using the “layer stripping” method: First we inverted the oceanic

187

crust reflections (Pg) and wide angle Moho reflections (PmP) and then the upper mantle refractions

188

(Pn), keeping the model over the Moho fixed. Tomographic inversion was undertaken using the

189

technique of Hole [Hole, 1992]. It consists of inverting in the first iterations only the picks with

190

offsets smaller than 20 km, and this threshold was increased stepwise to 90 km in steps of 10 km for

191

the subsequent iterations. This approach ensures that the shallow portion of the model is inverted

192

before the deep portion. The described procedure is necessary because the ray coverage for the

193

deeper parts is less dense and because the calculated travel times for deeply penetrating rays are

194

also influenced by the upper portions of the model. The inversion is stabilized by using smoothness

195

correlation lengths in the horizontal and vertical directions of the velocity mesh and for the depth

Ac ce pt e

184

Page 39 of 73

nodes. The values used for the correlation lengths vary from 2 km to 10 km for the horizontal and

197

from 0.1 km to 2 km for the vertical, at the top and bottom of the model respectively. Initially, the

198

depth sensitivity parameter kernel w was set to 1, which means that velocity field and reflector

199

nodes are equally weighted during the inversion. For each data set we run 4 iterations which were

200

enough to obtain a good fit between observed and calculated arrivals (See Table 1). Picked and

201

calculated travel times, and ray tracings for two ocean bottom instruments are shown in Figure 2.

ip t

196

S wave travel time tomography

us

203

cr

202

Since airguns generate a purely compressional wavefield, the shear waves observed in the

205

data are generated through mode conversion. The most plausible interface for generation of

206

converted shear arrivals is the interface between the sedimentary layer and the crystalline basement

207

[Spudich and Orcutt, 1980], whether when they are diving down (PSS), up (PPS) or both (PSP).

208

Under this assumption, the PSP waves recorded were inverted keeping the sediment/basement

209

interface geometry, the Moho reflector and the velocity in the sediment fixed for the inversion of

210

the Sg, Sn and SmS phases. In other words, the geometry is assumed from the Vp tomography and

211

only Vs values are updated during the inversion. For this purpose the kernel weighting factor is set

212

to w = 0.001

M

d

Ac ce pt e

213

an

204

214

Velocity model assessment

215

Model uncertainty

216

In order to estimate the sensitivity of our final model to different starting models and data,

217

we applied a Monte Carlo-like approach by averaging the solutions of 100 realizations [Korenaga et

218

al. 2000]. The degree of dependence of the final solution on the starting model can be assessed by

219

conducting a number of inversions with a variety of randomly generated initial models and initial

Page 40 of 73

data sets [Tarantola, 1987]. To estimate the model uncertainty, 10 initial models were derived from

221

the 1D starting reference model by varying the velocity and the initial reflector ±10% (Figure 3a).

222

These models are in the vicinity of the possible solutions and also cover a wide range of seismic

223

velocities found in Nazca plate, offshore central Chile [e.g. Scherwath et al. 2009, Contreras-Reyes

224

et al. 2010]. In order to include the picking subjectivity in our analysis, each model was inverted

225

with 10 noisy data sets obtained by adding a random value within the picking uncertainty time for

226

each phase. Regarding the Moho reflector, it reaches minimum error at the center of the profile and

227

larger uncertainty at the extremes of the model. The results of this test for Vp and Vs are in Figures

228

4a and 4b, respectively, and final Trms and χ2 of the average final models are summarized in Table 1.

us

cr

ip t

220

an

229

The seismic ray distribution in our model is represented by the derivative weight sum

231

(DWS), which mathematically corresponds to the column vector sum of the velocity kernel. This

232

parameter provides crude information on the linear sensitivity of the inversion describing the

233

relative ray density near a given velocity node [Toomey and Fulger, 1989]. The DWS value of

234

Figures 4c shows excellent ray coverage in the upper crust and good ray coverage in the lower crust

235

and upper mantle. Hence, zones of high and low resolution can be explained by high and low ray

236

coverage respectively (see Figures 4a and 4c).

238

d

Ac ce pt e

237

M

230

Resolution test

239

For checking the resolvability of the obtained velocity models and explore whether our data

240

set can resolve anomalous crustal velocity zones, we have created synthetic models by using the

241

final average velocity models for Vp and Vs and superimposing onto the oceanic crust five

242

Gaussian velocity anomalies (Figure 5). The maximum amplitude of each anomaly is +/-5% of the

243

velocity. In order to estimate how well the data can resolve perturbations of this scale, synthetic

244

travel time data with the same source-receiver geometry as in the real data set were inverted using

Page 41 of 73

the final average velocity model as the initial model. For qualifying the robustness of the test, this

246

procedure was repeated for a second set of perturbations with the same geometry and amplitudes of

247

the initial set, but with opposite sign. The result shows that, for the given anomalies, the recovery of

248

shape and amplitude is maximum between the 20 km and 60 km of distance, while at the extremes

249

the models are still capable to discriminate between positive and negative anomalies and to estimate

250

their amplitude and position, but the shape of the anomalies is not fully recovered. This is observed

251

in both Vp and Vs models. Thus, the resolving power of our data set is good enough to resolve

252

features similar to the one found in the lower crust of the Vp model between 40 km and 60 km.

cr

ip t

245

Velocity-Depth ambiguity

an

254

us

253

255

The Vp values for the lower crust found by our inversion methodology, based in a Monte

257

Carlo-like approach, would not be fully revealed due to the low amount of crossing ray paths on this

258

zone (Figure 4d) and the geometry of the experiment, and can not be attributed to the inversion

259

procedure [Tieman, 1994]. It produces a trade off between Moho depth and Vp, called velocity-

260

depth ambiguity. This implies that our initial choice of w=1, might drive to the calculation of an

261

unconstrained model [Korenaga, 2011].

d

Ac ce pt e

262

M

256

263

In order to evaluate the influence of the kernel weighting factor w into the final model

264

calculation, we recomputed the velocity models in a similar way as described in the previous

265

section, but using a w=0.001 for the Vp inversion with a starting flat Moho reflector at 10.5 km

266

depth. For the Vs modelling, we used the same inversion procedure previously described.

267 268

The results obtained by applying this procedure are presented Figure 7, showing a

269

systematic decrement of Vp and Vs in the lower crust between 25 km and 60 km, in comparison to

Page 42 of 73

average Monte Carlo models, yielding a lower υ of 0.2 in the same area. We attribute this velocity

271

reduction of Vp and Vs to a compensation of the traveltimes due to the imposed restriction of

272

pondering during the inversion 1000 times more the velocity variations than the Moho reflector

273

geometry changes. From the geologic point of view, reduced velocities in the outer rise can be

274

attributed to fracturing and hydration [e.g. Grevemeyer et al., 2007]. Thus, according to the

275

bathymetric map of Figure 1, one would expect such low velocity zone in the trench perpendicular

276

direction towards the trench, were bending related faults are observed. For this reason we consider

277

the models of Figure 6 more likely, due to its moderate horizontal variation.

cr

ip t

270

us

278

Discussion

280

Seismic structure of the oceanic crust in the outer rise offshore Maule Sedimentary layer

d

282

M

281

an

279

Across the profile, we have modeled a thin sedimentary layer of fairly uniform 180-300 m

284

depth and its basement tends to mimic the seafloor. The assumed velocities of Vp=1.8 km/s and

285

Vs=0.25 km/s correspond to a υ=0.49. These values are in agreement with in situ measurements of

286

υ for depth shallower than 100 m below the seafloor, which present υ ranging from 0.46 to 0.49

287

[Hamilton, 1976] and is also consistent with the value of υ=0.46, found for sediment near the Chile

288

triple junction [Contreras-Reyes et al., 2008a].

289

Ac ce pt e

283

290

The sedimentary layer gets thicker towards the trench, resulting in a maximum thickness of

291

some 2000 m at the trench axis [Moscoso et al. 2011]. In the trench fill sediments, Vp tends to

292

increase gradually from 1.8 km/s on top to 3.5 km/s at the bottom, principally due to compaction

293

and the increase of the sediment size from top to bottom, associated to successive sedimentary

294

deposit events [Contreras-Reyes et al., 2008a].

Page 43 of 73

295 296

Upper crust The oceanic crust has a thickness of about 6 km velocities increase from 4.5 km/sto ~7.0

298

km/s. Within the crust at ~6 km depth we can identify for both seismic models a Vp and Vs change

299

from high to low velocity gradient. This transition zone characterizes the change between the upper

300

and lower crust.

ip t

297

cr

301

The upper crust is product of a sequence of extrusive basalts on top of a sheeted dike

303

complex characterized by a high velocity gradient [e.g. Vera et al., 1990]. Our results show Vp

304

ranging from 4.0-4.5 km/s on top to 6.5-6.6 km/s at the bottom of a ~2 km thick layer, producing a

305

high velocity gradient of 1.3 s-1. The Vs model includes velocities ranging from 2 km/s on top to 3.5

306

km/s at the bottom yielding a gradient of 0.75 s -1. The values of υ decrease from 0.4 to 0.3, at the

307

top and bottom respectively, the highest value of the whole crust imaged. Carlson (2010) based on

308

sonic velocity logs, concluded that the upper oceanic crust is highly fractured, creating secondary

309

porosity. Therefore, fracturing and hydro-alteration might explain the high υ in the upper crust.

311

an

M

d

Ac ce pt e

310

us

302

Lower crust

312

The lower crust is characterized by velocity gradients less steep than in the upper crust,

313

presenting Vp ranging from 6.6-6.7 km/s at the uppermost part to values near to 7.0 km/s at the

314

lowermost crust and Vs values close to 3.5 km/s below the upper-lower crust interface to 4 km/s at

315

the Moho interface. In turn, υ decreases from 0.3 on top to 0.26-0.27 at the bottom lower than the

316

Poisson's ratio in the upper crust.

317 318

The lithology of the lower crust is dominated by gabbro overlying layered gabbro rocks [e.g.

319

Vera et al., 1990]. Large scale in situ values for the uppermost part of the lower crust are Vp=6.7

Page 44 of 73

km/s and υ=0.28 while for its lowermost part are Vp=6.9 km/s and υ=0.31 [Hyndman, 1979]. In

321

some areas that present reduced lower mantle velocity, it has been suggested the presence of

322

hydrous minerals such as chlorite and amphibole [e.g. Christensen and Salisbury, 1975]. A factor

323

that might affect the estimation of the seismic parameters is the anisotropy of metamorphic rocks as

324

amphybolites [Siegesmund et al., 1989] that can constitute 5 to 15 % of gabbros [Carlson and

325

Miller, 2004]. According to laboratory measurements made at 200 MPa, unaltered dry gabbro might

326

change its velocities of Vp = 7.138 km/s and Vs = 3.862 km/s to Vp = 6.866 km/s and Vs =3.909

327

km/s when it metamorphoses to amphybolite, while its Poisson's ratio is reduced from 0.293 to

328

0.260 [Christensen, 1996]. Comparing these results with the seismic models of Figure 6, we observe

329

a coincidence between Vp, Vs and Poisson's ratio for the lower crust and the values reported for

330

metamorphic amphybolite.

an

us

cr

ip t

320

M

331

Although the seismic velocities obtained on our preferred models show little lateral change

333

for the lower crust, the Poisson's ratio reveals lateral heterogeneity. In the uppermost part there are

334

two clear zones of high υ, between 20 km to 40 km and for distances larger than 65 km (Figure 6c),

335

which might indicate a high degree of hydration. According to our model, the sedimentary layer has

336

reduced thickness at the same distances of the high anomalies detected, this perhaps facilitates the

337

infiltration of sea water into the crust. The upper crust is presumably constituted by a highly

338

fractured extrusive layeroverlying brecciated dykes, also called “cracked zone” [Lister, 1974; Vera

339

et al., 1990]. These fractures likely constitute pathways for seawater to penetrate deeper into the

340

crust. An anomalous low υ is observed in the lowermost part of the crust at a distance between 35

341

km and 60 km, Carlson and Miller (2004) showed that although the compressional velocity does not

342

change, the gabbro might be highly altered. Similarly we observe that υ drops near the Moho to

343

values close to 0.27 while Vp is almost unaltered. This observationindicates that the gabbro

344

presents a high degree of metamorphism, likely due to the presence of sea water infiltration.

Ac ce pt e

d

332

345 Page 45 of 73

346

Crustal thickness

347 348

The Moho geometry is well constrained by PmP reflections, with an error estimated below

349

500 m at the center of the profile, due to the higher ray coverage on this zone (Figure 4c), yielding a

350

minimum thickness of ~6 km and a maximum of ~8 km.

ip t

351 The variation in crustal thickness observed along P04 (Figure 6a), might be a local feature that is

353

not directly related to any isostatic regional process. The presence of a seamount's root product of

354

isostatic compensation is unlikely, due to Maule seamount's modest altitude (~1000[m] high from

355

its base) and the evidence of root's absence beneath the more prominent O'Higgins seamount,

356

located at 32ºS on an area of the Nazca plate of similar age and seismic structure [Kopp et al.,

357

2004]. A likely mechanism that might have produced the prominent change of crustal thickness is at

358

the genesis of the Nazca Plate in central Chile, the Chile Rise. The oceanic crust's thickness is

359

independent of its age and spreading velocity, but strongly dependent on the thermal conditions of

360

the mantle upwelling. Different extents of partial melting of the oceanic upper mantle at its

361

generating ridge, can produce small scale variations of the thickness in the axial direction along its

362

generating ridge [Canales et al., 2003; Holmes et al., 2008; Mutter and Mutter, 1993].

364 365

us

an

M

d

Ac ce pt e

363

cr

352

Hydration of the oceanic lithosphere and upper mantle serpentinization

366

The Vp velocity structure along P04, located about 90 km seaward of the trench axis,

367

remains almost constant. However, the most striking feature of the trench perpendicular profile P03

368

(Figure 8a) [Moscoso et al., 2011], is the reduction of crustal and upper mantle velocity

369

approaching to the trench (Figure 8b), indicating changes in the physical properties of the incoming

370

plate. This feature has also been reported along strike in the Chilean margin [Sallarès and Ranero,

371

2005; Contreras Reyes et al., 2008a,b; Scherwath et al., 2009], in the highly hydrated subduction

Page 46 of 73

372

zone offshore Nicaragua [Grevemeyer et al., 2007; Ivandic et al., 2008], and in the Tonga

373

subduction [Contreras-Reyes et al., 2011].

374 A proposed mechanism for velocity reduction in the trench-outer rise is the creation of

376

bending related faulting in the near-trench region and penetration of sea water into the crust along

377

these faults [Ranero et al., 2003, 2005]. The reported depth down to which the faults cut into the

378

crust or mantle is around 20 km below the sea floor [Ranero et al., 2003]. These large seafloor

379

cutting faults might act as pathways that possibility migration of seawater along the fault to reach

380

and hydrate the uppermost mantle. In south central Chile, heat flow decreasing has been observed

381

toward the trench, indicating that the bend faulting facilitates hydrothermal circulation [Grevemeyer

382

et al., 2005; Contreras-Reyes et al., 2007].

cr

us

an M

383

ip t

375

According to the bathymetric data of Figure 1 and the seismic models of Figures 6a and 8a,

385

there is a thick blanket of relative impermeable sediment that might produce the blocking of water

386

infiltration into the fault system through the underlying oceanic crust [Contreras-Reyes et al., 2007].

387

However, reflection seismic data on this zone [Grevemeyer et al., 2005; Diaz-Naveas, 1999] show

388

evidence that some of the fissures are capable of reaching the seafloor and likely creates pathways

389

for seawater into the lithosphere. The reduction of seismic velocities is coincident with an

390

increment in the seafloor roughness that might be an indicator of faults in the basement exposed to

391

the sea water (see Figure 8a). Another effective link between the basement and the sea water is

392

through outcrops and seamounts, as the Maule seamount observed in Figure 1. Seamounts can guide

393

hydrothermal recharge and discharge between sites separated by large distances, due to percolation

394

through their flanks in direct contact with seawater [Fisher et al., 2003]. Although both mechanisms,

395

fracturing and hydration, produce a reduction in seismic velocities, they are related to each other

396

making it difficult to discriminate between them based on seismic properties. Offshore Nicaragua,

397

Ivandic et al. (2010) show a profound correlation between the occurrence of velocity anomalies and

Ac ce pt e

d

384

Page 47 of 73

398

seamounts, suggesting a close link between seamounts and hydratation.

399 Laboratory seismic measurements made on ophiolite dry samples of ultramafic rocks at 200

401

MPa as dunite, present an average Vp of 8.299 km/s [Christensen, 1996], for samples of peridotite

402

the average values estimated are Vp of 8.4 [Hyndman, 1979]; for serpentinite samples at 200 MPa,

403

the seismic velocities reduce drastically to Vp of 5.308 [Christensen, 1996]. Along P04 we obtained

404

average mantle velocities Vp of 8.17 km/s, Vp values present a reduced velocity in comparison with

405

the values correspondent to dry and unaltered mantle. This suggests that we are in presence of

406

hydration of the upper mantle and partial serpentinization. According to Christensen (2004)

407

lizardite and chysotile are abundant in regions where sea water percolates into the upper mantle.

408

This is in close agreement with isotherm computations for an altered portion of the Nazca plate at

409

the outer rise offshore Arauco (38ºS) [Contreras-Reyes et al., 2008a], with similar upper mantle

410

velocities in comparison to offshore Maule [Moscoso et al., 2011], the calculations are based on

411

heat flow measurements and yield temperatures below 300ºC for the upper mantle. At that

412

temperature lizardite and chrysotile serpentines are stable while antigorite is unstable [Christensen,

413

2004]. On the other hand by analyzing only compressional velocities it is hard to discriminate

414

whether lizardite or chysotile is predominant, as their elastic properties are similar, nevertheless

415

under the described conditions lizardite-chrysotile–bearing serpentinites are stable [Christensen,

416

2004] and therefore they are the most probable serpentine constituent of the upper mantle at the

417

trench-outer rise in central Chile.

419

cr

us

an

M

d

Ac ce pt e

418

ip t

400

Upper mantle anisotropy

420

Azimuthal P-wave anisotropy in the upper mantle is well established by seismic refraction

421

experiments [e.g. Shearer and Orcutt, 1986; Beé and Bibee, 1989; Contreras-Reyes et al., 2008b]

422

and also suggested by surface wave studies [Park and Levin, 2002]. The velocity anisotropy can be

Page 48 of 73

described by a sinusoidal function of azimuth with the fast direction, generally parallel to the

424

original spreading direction, as result of the preferred orientation of olivine crystals [Hess, 1964].

425

For dry olivine, the a axis presents the fastest and b axis the slowest Vp [Verma, 1960]. Hess (1964)

426

proposed that mantle anisotropy originated from preferred olivine orientation based on evidence of

427

Pn velocities anisotropy in the north-east Pacific. Plots of velocities versus azimuth on this region

428

showed that upper mantle compressional-wave velocities were fast for propagation directions

429

approximately parallel to fracture zones. Hence, the direction of the fastest P-wave velocity is

430

usually assumed to indicate the flow direction in the mantle [e.g. Zhang and Karato, 1995, Park and

431

Levin, 2002]. The described behavior might not be valid for environments highly hydrated and/or

432

under a high stress field [e.g. Katayama et al., 2009], as the trench-outer rise here under study. Due

433

to the effect of water in the anisotropic deformation of olivine, the high speed axis is reoriented with

434

the low speed axis nearly parallel to the shear direction [Jung and Karato, 2001], it implies that the

435

high speed axis becomes perpendicular to plate motion [Ando et al., 1983]. Additionally, high

436

pressure conditions produce in the olivine the same crystallographic preferred orientation that is

437

produced by high water activity at lower pressure [Jung et al., 2009].

cr

us

an

M

d

Ac ce pt e

438

ip t

423

439

To estimate the degree of anisotropy of Pn velocities, we extracted from the trench parallel

440

velocity model P04 a track 500 m beneath the Moho, it yielded a media velocity of Vp=8.17 km/s.

441

For the velocity model of P03, perpendicular to P04 and to the trench, we also extracted a similar

442

velocity profile in the vicinity of the crossing point between the profiles, which yielded a media

443

velocity of Vp=7.5 km/s. Therefore the Pn anisotropy is near 8%, with a preferential higher velocity

444

direction parallel to the trench axis. The seafloor fabric generated at the spreading axis is well

445

preserved in the trench-outer rise, striking roughly NW-SE (see Figure 1). Thus, P04 runs roughly

446

in spreading direction and hence samples the fast direction as defined by Hess (1964).

447 448

Contreras-Reyes et al. (2008b) found in a younger section of the Nazca plate located at Page 49 of 73

449

~43°S, near the Chile triple junction (CTJ), a lower anisotropy around 2% and the direction of the

450

high velocity axis rotated in comparison to the results presented here. However, in southern Chile

451

the spreading axis runs in N-S direction. Thus, in southern Chile the fast direction faces towards the

452

trench while offshore Maule the slow direction faces the trench.

Outer rise seismic activity

cr

454

ip t

453

For the hypocentral determination we employed 1-D Vp and Vs models, derived from the

456

seismic refraction tomography of Figures 6a and 6b, and the location of the 29 events was done

457

using the linear location program HYP, included in SEISAN [Havskov and Ottemöller, 1999].

an

us

455

458

The set of events analyzed, shown in Figure 9, suggests the existence of a zone of high

460

seismic activity beneath the seamount to a depth of ~20 km. This intraplate seismicity is consistent

461

with the faults imaged in the bathymetry shown in Figure 1, and seems to be product of the

462

reactivation of normal faults formed by plate bending near the trench [Ranero et al., 2005]. Outer

463

rise seismicity also coincides with Vp reduction of the oceanic lithosphere when approaches the

464

trench (see Figures 8 and 9). In southern Chile, a younger section of the Nazca plate is highly

465

hydrated [Contreras-Reyes et al., 2007], it is thought that the shallow outer rise seismicity (<30 km)

466

is triggered by the increment of pore pressure within the fault system produced by infiltration of sea

467

water into the lithosphere [Tilmann et al., 2008], facilitated by the lack of a thick sedimentary layer

468

at seamounts [e.g. Ivandic et al., 2010]. However, bending related faulting itself might be an

469

important mechanism of recurrence of earthquakes and may act as a fault valve, causing seismic

470

pumping [Grevemeyer et al., 2007]. Therefore, the high seismicity nearby the seamount might be

471

explained by a higher fracturing and hydration of the crust. This result suggests that the seamount

472

plays an important role for hydrothermal circulation in the area. In contrast, seaward from the

473

seamount no background seismicity was found, this supports our interpretation that normal

Ac ce pt e

d

M

459

Page 50 of 73

474

anisotropy at the crossing point of the seismic profiles, coincident little faulting in the bathymetry

475

and low Vp variation along P04 are evidences of an oceanic lithosphere not yet affected by faulting

476

and hydration.

477 The shallow outer rise seismic activity might also be related to large interplate earthquakes

479

in the subduction zone. Kato and Hirasawa (2000) showed through a numerical simulation, that a

480

large tensional outer rise earthquake tend to advance the occurrence time and reduce the magnitude

481

of the next interplate earthquake, while a compressional one tends to delay the occurrence of a large

482

interplate earthquake. On the other hand, large underthrusting events might transfer tensional

483

stresses along the slab and subsequently trigger intraplate earthquakes in the outer rise [Christensen

484

and Ruff, 1983 and 1988].

M

485

an

us

cr

ip t

478

The Maule area presented little background seismic activity in comparison to the rest of the

487

Chilean margin [Campos et al., 2002]. This is probably due to the high locking in the interplate

488

between Constitución and Concepción [Ruegg et al., 2009]. However, the months following the

489

mainshock, several events occurred in the outer rise area between ~34°S and ~35°30'S, presenting 6

490

events with magnitude >5.0 over the year following the main shock, as it is shown in Figure 9. It is

491

likely that the large slip reported after the earthquake , between 34°S and 35°30'S [Delouis et al.

492

2010; Moreno et al., 2010], led a transport of slab pull stress to the outer rise, causing a reactivation

493

of bending related normal faults and perhaps producing new fissures in the outer rise, that might

494

trigger new seismic events in the neighboring area. In addition to plate bending, the fracturing and

495

hydration weakens the oceanic plate when approaching the trench [Contreras-Reyes and Osses,

496

2010; Chapple and Forsith, 1979; Kao and Chen, 1996] intensifying the outer rise earthquake

497

genesis process [Lefeldt et al., 2009].

Ac ce pt e

d

486

498

Page 51 of 73

499

Conclusions

500 We analyzed high resolution bathymetric, seimological and active seismic data to investigate

502

the structure of the incoming plate prior to its subduction, in the trench outer rise area offshore

503

Maule, Chile, between 34°S and 35°S. In particular, wide angle seismic data was used to obtain the

504

high resolution 2D velocity structure and derive the Poisson's ratio distribution of the Nazca plate

505

on this area. From this study we have concluded the following:

cr

506

ip t

501

(1)

As the incoming plate approaches the trench, Vp velocity tends to decrease in both, oceanic

508

crust and upper mantle. Those anomalies reported in the compressional velocity are likely produced

509

by a combination of progressive bending related faulting, lithospheric hydration by water

510

percolation and subsequent mantle serpentinization. Thus, the reduction of the upper mantle

511

velocities near the trench, might reflect partial serpentinization of the mantle peridotites.

M

an

us

507

d

512 (2)

514

probably due to mantle upwelling and not by isostatic compensation of the neighboring Maule

515

seamount.

516

The possible differences of the crustal thickness observed along strike in the profile P04 are

Ac ce pt e

513

517

(3)

Unaltered upper mantle presents a Vp anisotropy of ~8%, with the faster velocity axis

518

trending in SW-NE direction and hence in spreading direction, roughly paralleling the trench axis.

519

Therefore, hydration may have affected the lithospheric structure; however, little evidence is found

520

that the anisotropic structure inherited at the spreading axis has been altered by bending-related

521

faulting, water intrusion or serpentinization.

522 523

(4)

We found shallow seismic activity in the outer rise area near the seamount, we conclude that

Page 52 of 73

524

this seismicity is produced due to generation and reactivation of outer rise faults. To the bending-

525

related faulting we have to add the presence of the Maule seamount that probably intensifies the

526

percolation of seawater into the deep structures, producing an intense hydrothermal activity and

527

likely an increment of the pore pressure.

ip t

528 (5)

The main shock of the 2010 Maule earthquake triggered an anomalous high seismic activity

530

in the trench outer rise area, likely due to the stress transmission along the incoming plate, that

531

might have produced a massive crack opening of the bending faults and subsequently water

532

intrusion into the lithosphere.

us

cr

529

an

533 (6)

All the previous conclusions indicate presence of sea water in the upper lithosphere that

535

produces changes on its seismic properties and likely in the petrology.

536

Acknowledgements

Ac ce pt e

537

d

M

534

538

We are grateful to the participants of the JC23 cruise and specially the crew of RV James

539

Cook for their excellent performance on board. We thank E. Contreras-Reyes and E. R. Flueh for

540

critically reading earlier versions of the manuscript and I. Arroyo for her support during the

541

processing of earthquake data. We finally thank to the journal's editor W. Schellart and two

542

anonymous reviewers for constructive criticism and editing. Eduardo Moscoso acknowledges a

543

scholarship granted by the Chilean Comisión Nacional de Investigación Científica y Tecnológica

544

(CONICYT) and the German Academic Exchange Service (DAAD).

545

546

Figure captions

547 Page 53 of 73

Figure 1: (Top) High resolution bathymetric map offshore Maule region in south-central Chile with

549

the identification of its main features. The white arrow indicates the relative convergence velocity

550

between Nazca and South American plates. Transects P03 [Moscoso et al., 2011] and P04 (this

551

study) are represented by solid black lines, the green dots show the stations' locations for the wide

552

angle experiment and the white triangles show the positions of the local seismic network sensors.

553

Station 229, represented by a green triangle, was used for both experiments. The profile P04

554

presented here runs parallel to the Chile-Peru trench, and some 25 km landward its location we

555

identify a seamount. (Bottom) Locations of the OBS/H projected on the bathymetry.

cr

ip t

548

us

556

Figure 2: (Top) Examples of wide-angle seismic data. (Bottom) Manually picked arrivals (pick

558

uncertainty is represented by color bars). Predicted traveltimes using the average 2D final models

559

are superimposed on the seismic sections. Solid lines represent the calculations for refractions (red)

560

and reflections from Moho (black).

Ac ce pt e

d

561

M

an

557

562

Figure 3: a) Initial models used for the Monte Carlo inversion procedure. Green bands in the

563

velocity profile show the 1D initial velocity models for Vs (left) and Vp (right). Gray band

564

represents the initial depth range used for Moho initial reflectors, b) Example of the delay between

565

Pg and PPS phases.

566 567

Figure 4: a) Error for the Vp model, b) Error for the Vs model, c) DWS for Vp model, d) Poisson's

568

ratio error calculated from equation (2).

569 570

Figure 5: Resolution tests for a) Vp model, b) Vs model.

571

Page 54 of 73

572

Figure 6: Final velocity model derived from averaging 100 Monte Carlo ensembles for Vp (Top)

573

and Vs (Center). (Bottom) Poisson’s ratio masked by the intersection of rays fromthe P and S wave

574

velocity models.

575 Figure 7: Final velocity models for Vp (Top). Vs (Center) and Poisson's ratio (Bottom) using a flat

577

Moho and a kernel w=0,01, this test shows a velocity-depth trade off for the lower crust. The

578

overall error for the models is 95 ms for Vp and 92 ms for Vs.

cr

579

ip t

576

Figure 8: a) Velocity model of P03 from Moscoso et al. (2011), the segmented line CP denotes the

581

crossing point with P04 (this study). b) Velocity profiles extracted from the locations A1 and A2 in

582

a). The red profile was extracted beneath the crossing point (CP) in Profile P04 (Figure 6a).

an

us

580

M

583

Figure 9: a) Map of seismicity and bathymetry along the Chile-Peru trench offshore Maule where

585

the 2010 megathrust earthquake, followed by a tsunami, hit central Chile. Its NEIC location is

586

indicated by a large red star. The black solid lines stand for the locations of the profiles P03 and

587

P04; The deformation front is indicated by a thick black line. The yellow dots denote the seismicity

588

over a 3 months period after the main shock, extracted from the NEIC catalog. The white stars

589

represent the outer rise events with Mw> 5.0 over a period of 1 year after the main shock, with its

590

respective Harvard GCMT fault plane solutions. The local earthquakes recorded by our outer rise

591

network (ORN) operative between early March and the first week of April 2008, are represented by

592

red dots. Their projection over P03 and P04 are in b) and c), respectively.

Ac ce pt e

d

584

593 594

Table 1: Summary of data picking information and statistics of the fitness between the final average

595

models and picks.

596

Page 55 of 73

597

Supplementary material

598 599

Figures S1 and S2: Data examples of two outer rise seismic events recorded during the seismic

600

experiment.

601

603

ip t

602

References

604

Ando, M., Y. Ishikawa, and F. Yamazaki (1983), Shear Wave Polarization Anisotropy in the Upper Mantle Beneath Honshu, Japan, J. Geophys. Res., 88(B7), 5850-5864.

cr

605 606 608 609

us

607

Angermann, D., J. Koltz, and C. Rigber (1999), Space-Geodetic estimation of the Nazca-South America Euler vector, Earth Planet. Sci. Lett., 171(3), 329-334.

611

an

610

Barrientos, S., Giant returns in time (2005), Nature, 737, 329, doi:10.1038/437329a.

612

617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632

M

616

Bialas, J., and E. R. Flueh (1999), Ocean bottom seismometers, Sea Technol., 40(4), 41-46.

d

615

Bée, M. and Bibee, L. D. (1989), A seismic refraction study of cretaceous oceanic lithosphere in the Northwest Pacific Basin, Marine Geophysical Researches, Volume 11, Issue 4, pp.239-261, doi:10.1007/BF00282578.

Blumberg, S., F. Lamy, H.W. Arz, H.P. Echtler, M. Wiedicke, G.H. Haug and O. Oncken (2008), Turbiditic trench deposits at the South-Chilean active margin: A Pleistocene–Holocene record of climate and tectonics, Earth Planet. Sci. Lett., 268, 526-539, doi:10.1016/j.epsl.2008.02.007.

Ac ce pt e

613 614

Campos J., Hatzfeld, R. Madariaga, G. Lopez, E. Kausel, A. Zollo, G. Iannacone, R. Fromm, S. Barrientos and H. Lyon-Caen (2002), A seismological study of the 1835 seismic gap in south-central Chile, Physics of The Earth and Planetary Interiors, Volume 132, Issues 1-3, 30 September 2002, Pages 177-195 , doi:10.1016/S0031-9201(02)00051-1. Canales, J. P., R. S. Detrick, D. R. Toomey, and W. S. D. Wilcock (2003), Segment‐scale variations in the crustal structure of 150–300 kyr old fast spreading oceanic crust (East Pacific Rise, 8°15′N–10°5′N) from wide‐angle seismic refraction profiles, Geophys. J. Int., 152(3), 766–794, doi:10.1046/j.1365-246X.2003.01885.x. Carlson, R. L. and D. Jay Miller (2004), Influence of pressure and mineralogy on seismic velocities in oceanic gabbros: Implications for the composition and state of the lower oceanic crust, J. Geophys. Res., 109, B09205, doi:10.1029/2003JB002699.

633 634 635

Carlson, R. L. (2010), How crack porosity and shape control seismic velocities in the upper oceanic crust: Modeling downhole logs from Holes 504B and 1256D, Geochem. Geophys. Geosyst., 11, Q04007, doi:10.1029/2009GC002955.

636 637 638

Chapple, W., and D. Forsyth (1979), Earthquakes and Bending of Plates at Trenches, J. Geophys. Res., 84(B12), 67296749.

639

Page 56 of 73

640

Christensen, N. (1996), Poisson's ratio and crustal seismology, J. Geophys. Res., 101(B2), 3139-3156.

641 642 643

Christensen, N. (2004), Serpentinites, Peridotites, and Seismology, International Geology Review, 46:9, 795-816, doi:10.2747/0020-6814.46.9.795

644 645 646

Christensen, D. H., and L. J. Ruff (1983), Outer‐rise earthquakes and seismic coupling, Geophys. Res. Lett., 10(8), 697700.

647

Christensen, D., and L. Ruff (1988), SEISMIC COUPLING AND OUTER RISE EARTHQUAKES, J. Geophys. Res., 93(B11), 13421-13444.

ip t

648 649 650

Christensen, N. I., and M. H. Salisbury (1975), Structure and constitution of the lower oceanic crust, Reviews of Geophysics, 13, 57–86.

cr

651 652 653

Cisternas, M., et al. (2005), Predecessors of the giant 1960 Chile earthquake, Nature, 437, 404–407, doi:10.1038/nature03943.

us

654 655 656

Contreras-Reyes, E., and A. Osses (2010), Lithospheric flexure modeling seaward of the Chile trench: Implications for oceanic plate weakening in the Trench Outer Rise region. Geophysical Journal International 182(1), 97-112.

an

657 658 659

664 665 666 667 668 669 670 671 672 673 674 675 676 677 678

M

Contreras‐Reyes, E., I. Grevemeyer, E. R. Flueh, and C. Reichert (2008a), Upper lithospheric structure of the subduction zone offshore of southern Arauco peninsula, Chile, at ∼38°S, J. Geophys. Res., 113, B07303, doi:10.1029/2007JB005569.

d

663

Contreras‐Reyes, E., I. Grevemeyer, E. R. Flueh, M. Scherwath, and M. Heesemann (2007), Alteration of the subducting oceanic lithosphere at the southern central Chile trench–outer rise, Geochem. Geophys. Geosyst. , 8 , Q07003, doi:10.1029/2007GC001632.

Ac ce pt e

660 661 662

Contreras-Reyes E., I. Grevemeyer, E. R. Flueh, M. Scherwath, and J. Bialas (2008b), Effect of trench-outer rise bending-related faulting on seismic Poisson's ratio and mantle anisotrophy: a case study offshore of Southern Central Chile. Geophys. J. Int., 173, 142-156, doi:10.1111/j.1365-246X.2008.03716.x Contreras‐Reyes, E., E. R. Flueh, and I. Grevemeyer (2010), Tectonic control on sediment accretion and subduction off south central Chile: Implications for coseismic rupture processes of the 1960 and 2010 megathrust earthquakes, Tectonics, 29, TC6018, doi:10.1029/2010TC002734. Contreras-Reyes, E., I. Grevemeyer, A. Watts, E. R. Flueh, C. Peirce, S. Moeller, and C. Papenberg (2011), Deep seismic structure of the Tonga subduction zone: implications for mantle hydration, tectonic erosion, and arc magmatism J. Geophys. Res., doi:10.1029/2011JB008434.

679 680 681

Contreras-Reyes, E., J. Jara, I. Grevemeyer, S. Ruiz, and D. Carrizo (2012), Abrupt change in the dip of the subducting plate beneath north Chile, Nature Geoscience., 5, 342-345, doi:10.1038/ngeo1447

682 683 684 685

Delouis, B., J.-M. Nocquet, and M. Vallee (2010), Slip distribution of the February 27, 2010 Mw = 8.8 Maule Earthquake, central Chile, from static and high-rate GPS, InSAR, and broadband teleseismic data, Geophys. Res. Lett., doi:10.1029/2010GL043899, in press.

686 687

Diaz-Naveas, J. L (1999), Sediment subduction and accretion at the Chilean convergent margin between 35° and 40°S,

Page 57 of 73

688

Ph.D. Thesis, Ch.-A.-Universität, Kiel, Germany.

689 690

Dijkstra, E. W. (1959). "A note on two problems in connexion with graphs". Numerische Mathematik 1: 269–271.

691 692 693 694

Fisher A. T., E. E. Davis, M. Hutnak, V. Spiess, L. Zuehlsdorff, A. Cherkaoul, L. Christiansen, K. Edwards, R. Macdonald, H. Villinger, M. J. Mottl, C. G. Wheat, and K. Becker (2003), Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank, Nature, 421, 618-621, doi:10.1038/nature01352.

695

Flueh, E. R., and J. Bialas (1996), A digital, high data capacity ocean bottom recorder for seismic investigations, Int. Underwater Syst. Design, 18(3), 18-20.

698 699

Flueh, E. R., and J. Bialas (Eds.) 2008. Cruise report JC23, IFM-Geomar rep. nº 20.

Grevemeyer, I., N. Kaul, J. L. Diaz-Naveas, H. Villinger, C. R. Ranero, and C. Reichert (2005), Heat flow and bendingrelated faulting at subduction trenches: case studies offshore of Nicaragua and central Chile, Earth Planet. Sci. Lett., 236, 238–248, doi:10.1016/j.epsl.2005.04.048.

us

701 702 703

cr

700

704

Grevemeyer, I., C. R. Ranero, E. R. Flueh, D. Klaeschen, and J. Bialas (2007), Passive and active seismological study of bending-related faulting and mantle serpentinization at the Middle America trench, Earth Planet. Sci. Lett., doi:10.1016/j.epsl.2007.04.013.

708

712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734

beyond

arcs,

M

subduction

Geochem.

Geophys.

Geosyst.,

9,

Q03001,

Hamilton, E. L. (1976), Shear-wave velocity versus depth in marine sediments: a review, Geophysics, 41(5), 985-996.

d

711

Hacker, B. R. (2008), H2O doi:10.1029/2007GC001707.

Havskov, J and L. Ottemoller (1999), SeisAn Earthquake analysis software, Seis. Res. Lett., 70, 532-534.

Ac ce pt e

709 710

an

705 706 707

ip t

696 697

Hess, H. H. (1964), Seismic Anisotropy of the Uppermost Mantle under Oceans, Nature, 203, 629-631, doi:10.1038/203629a0. Hole, J. A. (1992), Nonlinear High-Resolution Three-Dimensional Seismic Travel Time Tomography, J. Geophys. Res., 97(B5), 6553–6562. Holmes, R. C., M. Tolstoy, J. R. Cochran, and J. S. Floyd (2008), Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography, Geochem. Geophys. Geosyst., 9, Q12020, doi:10.1029/2008GC002152. Hyndman, R. D. (1979), Poisson's ratio in the oceanic crust: a rewiew, Tectonophysics, 59, 321-333. Ivandic, M., I. Grevemeyer, A. Berhorst, E. R. Flueh, and K. McIntosh (2008), Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua, J. Geophys. Res., 113, B05410, doi:10.1029/2007JB005291. Ivandic, M., I. Grevemeyer, J. Bialas, and C. J. Petersen (2010), Serpentinization in the trench‐outer rise region offshore of Nicaragua: Constraints from seismic refraction and wide‐angle data, Geophys. J. Int., 180, 1253–1264, doi:10.1111/j.1365-246X.2009.04474.x.

735 736 737

Jung, H., M. Wo & H. W. Green (2009), Upper mantle seismic anisotropy resulting from pressure-induced slip transition in olivine, Nature Geoscience, 2, 73 – 77, doi:10.1038/ngeo389.

738

Page 58 of 73

739 740

Jung, H. and S. Karato (2001), Water-Induced Fabric Transitions in Olivine, Science, Vol. 293, no. 5534, pp. 14601463, doi: 10.1126/science.1062235.

741 742 743

Kanamori H. (1971), Seismological evidence for a lithospheric normal faulting - the Sanriku earthquake of 1933 , Physics of The Earth and Planetary Interiors, vol. 4, no. 4, pp. 289-300, doi: 10.1016/0031-9201(71)90013-6.

744 745 746

Kao, H., and W.‐P. Chen (1996), Seismicity in the outer rise–forearc region and configuration of the subducting lithosphere with special reference to the Japan Trench, J. Geophys. Res., 101(B12), 27811-27831.

747

Katayama, I., K. Hirauchi, K. Michibayashi and J. Ando (2009), Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge, Nature, 461, 1114-1117, doi:10.1038/nature08513.

ip t

748 749 750

Kato, N., and T. Hirasawa (2000), Effect of a large outer rise earthquake on seismic cycles of interplate earthquakes: A model study, J. Geophys. Res., 105(B1), 653-662.

cr

751 752 754 755 756

us

753

Kopp, H., E. R. Flueh, C. Papenberg, and D. Klaeschen (2004), Seismic investigations of the O'Higgins Seamount Group and Juan Fernández Ridge: Aseismic ridge emplacement and lithosphere hydration, Tectonics, 23, TC2009, doi:10.1029/2003TC001590.

764 765 766 767 768 769 770 771 772 773 774 775 776 777 778

M

762 763

Korenaga (2011), Velocity–depth ambiguity and the seismic structure of large igneous provinces: a case study from the Ontong Java Plateau, Geophys. J. Int., 185, 2, 1022–1036, 10.1111/j.1365-246X.2011.04999.x

d

761

Korenaga, J., W. Holbrook, G. Kent, P. Kelemen, R. Detrick, H.‐C. Larsen, J. Hopper, and T. Dahl ‐Jensen (2000), Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography, J. Geophys. Res., 105(B9), 21591-21614.

Lefeldt, M., I. Grevemeyer, J. Goßler and J. Bialas (2009), Intraplate seismicity and related mantle hydration at the Nicaraguan trench outer rise, Geophys. J. Int., 178(2), 742-752, doi:10.1111/j.1365-246X.2009.04167.x.

Ac ce pt e

758 759 760

an

757

Lister, C. R. B. (1974), On the penetration of water into hot rock, Geophys. J. R. Astronom. Soc., 39, 465-509. Madariaga, R., M. Métois, Ch. Vigny, and J. Campos (2010), Central Chile Finally Breaks, Science (in Perspectives), 328, 181-182, doi:10.1126/science.1189197. Moreno, M., M. Rosenau and Onno Oncken (2010), 2010 Maule earthquake slip correlates with pre-seismic locking of andean subduction zone, Nature, 467, 198-202 ,doi:10.1038/nature09349. Moscoso, E., I. Grevemeyer, E. Contreras-Reyes and E. Flueh (2011), Revealing the deep structure and rupture plane of the 2010 Maule, Chile Earthquake (Mw=8.8) using wide angle seismic data, Earth Planet. Sci. Lett., 307, 147-155, doi:10.1016/j.epsl.2011.04.025.

779 780 781

Moscoso, E., and E. Contreras-Reyes (2012), Outer rise seismicity related to the maule, Chile 2010 megathrust earthquake and hydration of the incoming oceanic lithosphere, Andean Geology, 39 (3), 563-571.

782 783 784

Mutter C. Z. and J. C. Mutter (1993), Variations in thickness of layer 3 dominate oceanic crustal structure, Earth Planet. Sci. Lett., 117(1993), 295-317.

785

Page 59 of 73

786 787

Müller, R. D., W. R. Roest, J.-Y. Royer, L. M. Gahagan, and J. G. Sclater (1997), Digital isochrons of the world's ocean floor, J. Geophys. Res., 102, 3211-3214, doi:10.1029/96JB01781.

788 789 790

Park, J. and V. Levin (2002), Seismic Anisotropy: Tracing Plate Dynamics in the Mantle, Science, Vol. 296 no. 5567 pp. 485-489, doi:10.1126/science.1067319.

791 792 793

Ranero, C. R., J. Phipps Morgan, K. McIntosh, and C. Reichert (2003), Bending, faulting, and mantle serpentinization at the Middle America Trench, Nature, 425, 367–373, doi:10.1038/nature01961.

794

Ranero, C. R., A. Villaseñor, J. Phipps Morgan, and W. Weinrebe (2005), Relationship between bend-faulting at trenches and intermediate-depth seismicity, Geochem. Geophys. Geosyst., 6, Q12002, doi:10.1029/2005GC000997.

ip t

795 796 797

cr

Ruegg J. C., A. Rudloff, C. Vigny, R. Madariaga, J.B. de Chabalier, J. Campos, E. Kausel, S. Barrientos and D. Dimitrov (2009), Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile, Physics of the Earth and Planetary Interiors, Volume 175, Issues 1-2, June 2009, Pages 78-85, doi:10.1016/j.pepi.2008.02.015.

us

798 799 800 801 802

Sallarès, V. and C. R. Ranero (2005), Structure and tectonics of the erosional convergent margin off Antofagasta, north Chile (23°30′S), J. Geophys. Res., 110, B06101, doi:10.1029/2004JB003418.

an

803 804 805

811 812 813 814 815 816 817 818 819 820 821 822 823 824

M

809 810

Siegesmund, S., T. Takeshita, and H. Kern (1989), Anisotropy of Vp and Vs in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural, and textural characteristics of the rock, Tectonophysics, 157, 25-38.

d

808

Satake, K, and Y. Tanioka (1999), Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones, Pure Appl. Geophys., Volume 154, Numbers 3-4, 467-483, DOI: 10.1007/s000240050240

Shearer, P. M. and J. A. Orcutt (1986), Compressional and shear wave anisotropy in the oceanic lithosphere - the Ngendei seismic refraction experiment, Geophys. J. R. astr. Soc., Volume 87, Issue 3, pages 967–1003, doi:10.1111/j.1365-246X.1986.tb01979.x.

Ac ce pt e

806 807

Scherwath, M., E. Contreras-Reyes, E. R. Flueh, I. Grevemeyer, A. Krabbenhoeft, C. Papenberg, C. J. Petersen, and R. W. Weinrebe (2009), Deep lithospheric structures along the southern central Chile margin from wide-angle P-wave modelling, Geophys. J. Int., 179(1), 579-600, doi:10.1111/j.1365-246X.2009.04298.x. Spudich, P., and J. A. Orcutt (1980), Petrology and porosity of an oceanic crust site: results from waveform modeling of seismic refraction data. J. Geophys. Res., 85:1409-1433. Tarantola, A. (1987), Inverse Problem Theory: Methods for Data Fitting and Model Parameter Estimation, 613 pp., Elsevier, New York.

825 826

Tieman, H. J. (1994), Investigating the velocity-depth ambiguity of reflection traveltimes, Geophysics, 59 , 1763–1773.

827 828 829 830

Tilmann, F. J., I. Grevemeyer, E. Flueh, T. Dahm, J. Goßler (2008), Seimicity in the outer rise offshore southern chile: Indication of fluid effects in crust and mantle, Earth Planet. Sci. Lett., 269(2008), 41-55, doi:10.1016/j.epsl.2008.01.044.

831 832 833

Toomey, D.R., and G.R. Foulger (1989), Tomographic inversion of local earthquaked data from the Hengill-Grensdalur central volcano complex, Iceland, J. Geophys. Res., 94, 17.497-17.510.

Page 60 of 73

834 835 836

Van Avendonk, H., A. Harding, J. Orcutt, and J. McClain (1998), A two ‐dimensional tomographic study of the Clipperton transform fault, J. Geophys. Res., 103(B8), 17885-17899.

837 838 839 840

Vera, E., J. Mutter, P. Buhl, J. Orcutt, A. Harding, M. Kappus, R. Detrick, and T. Brocher (1990), The Structure of 0 ‐ to 0.2‐m.y.‐Old Oceanic Crust at 9°N on the East Pacific Rise From Expanded Spread Profiles, J. Geophys. Res., 95(B10), 15529-15556.

841 842

Verma, R. (1960), Elasticity of Some High‐Density Crystals, J. Geophys. Res., 65(2), 757-766.

844 845

ip t

843

Watts, A. B. (1994), Crustal structure, gravity anomalies and flexure of the lithosphere in the vivinity of the Canary Isalands, Geophys. J. Int., 119, 648-666. von Huene, R., J. Corvalán, E. Flueh, K. Hinz, J. Korstgard, C. Ranero, W. Weinrebe, and (1997), Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile, Tectonics, 16(3), 474-488.

us

847 848

cr

846

849

d

M

an

Zhang, S. and S. Karato (1995), Lattice preferred orientation of olivine aggregates deformed in simple shear, Nature, 375, 774-777, doi:10.1038/375774a0.

Ac ce pt e

850 851

Page 61 of 73

Table 1

Table 1:

ip t

Pg PmP Pn Total Sg SmS Total

Total picks Pick error min. (ms) Pick error max. (ms) [1] Average final Trms (ms) [2] Average final χ2 2701 50 65 59.8 1.18 1850 50 65 60.5 0.95 543 50 65 67.7 1.09 5094 60.9 1.11 984 75 85 74.3 0.87 235 75 85 97.1 1.61 1219 78 0.98

cr

Phase

us

Caption:

an

Summary of data picking information and statistics of the fitness between the final average models and picks.

M

Table's Footnotes:

Ac ce p

te

d

[1] Picking error for arrivals with offset smaller than 20 km for P waves arrivals and 30 km for S waves arrivals. [2] Picking error for arrivals with offset larger than 20 km for P waves arrivals and 30 km for S waves arrivals.

Page 62 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure 1 Click here to download high resolution image

Page 63 of 73

Ac

ce

pt

ed

M

an

us

cr

i

Figure 2 Click here to download high resolution image

Page 64 of 73

Ac

ce

pt

ed

M

an

us

cr

i

Figure 3 Click here to download high resolution image

Page 65 of 73

Ac

ce

pt

ed

M

an

us

cr

i

Figure 4 Click here to download high resolution image

Page 66 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure 5 Click here to download high resolution image

Page 67 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure 6 Click here to download high resolution image

Page 68 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure 7 Click here to download high resolution image

Page 69 of 73

Ac

ce

pt

ed

M

an

us

cr

i

Figure 8 Click here to download high resolution image

Page 70 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure 9 Click here to download high resolution image

Page 71 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure S1

Page 72 of 73

Ac ce p

te

d

M

an

us

cr

ip t

Figure S2

Page 73 of 73