Seismic structure along the South American subduction zone using satellite gravity data

CHAPTER

Seismic structure along the South American subduction zone using satellite gravity data

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Orlando Alvarez⁎,†, Stefanie Pechuan⁎,†, Mario Gimenez⁎,†, Andrés Folguera‡ Seismological Geophysical Institute Ing. Volponi (IGSV), FCEFyN, National University of San Juan, San Juan, Argentina⁎ National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina† Department of Geological Sciences, National Scientific and Technical Research Council (CONICET), IDEAN—Institute of Andean Studies "Don Pablo Groeber", FCEN, University of Buenos Aires, Buenos Aires, Argentina‡

1 ­Introduction Western South America is subject to high stress as a consequence of oceanic plate subduction beneath the South American plate (e.g., Barazangi and Isacks, 1976; Jordan et al., 1983; Ramos and Folguera, 2009; Ramos, 2010; Horton, 2018). Although part of the deformation resulting from plate convergence occurs aseismically, large earthquakes also occur and affect coastal areas hundreds of kilometers in length. In the last decade, a large portion of the South American western margin along the Chilean coast has been affected by three large earthquakes: the 2010 Maule Mw = 8.8, the 2014 Pisagua Mw = 8.2, and the 2015 Illapel Mw = 8.3 earthquakes. Due to technological and scientific advances, these recent earthquakes have been studied at an unprecedented detail with different methods and data types, allowing for testing of different hypotheses. The development of large networks of seismological and GPS stations, together with new geodetic methods (e.g., InSAR: Interferometric synthetic aperture radar), has allowed detailed mapping of coseismic and postseismic slippage and the interseismic coupling between plates. In addition, numerous studies of the subduction zone based mainly on wide-angle seismic and gravity profiles have allowed illumination of the structural complexity and compositional heterogeneity of the interplate region and a better understanding of its behavior during earthquakes. Satellite gravimetry has enabled global and homogeneous mapping of the distribution of mass anomalies inside the Earth, and in particular, densities along the entire forearc zone (Song and Simons, 2003; Wells et al., 2003), difficult and expensive through other methods. In this chapter, we build a compilation of the seismogenic structure of the South American active margin (Fig. 1) using data derived from the GOCE satellite mission. From the vertical gravity gradient, we analyze the relationships between negative Tzz lobes, historical ruptures, and slip models for recent events along the margin. Other seismologic parameters are inferred from the density distribution, including the directivity, downdip limit of the seismogenic zone, and location of the main asperities and barriers along the margin. Andean Tectonics. https://doi.org/10.1016/B978-0-12-816009-1.00001-0 © 2019 Elsevier Inc. All rights reserved.

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Chapter 1  Observing rupture areas from satellite gravity data

FIG. 1 Relief of the Nazca-South American plates from ETOPO1 (Amante and Eakins, 2009) with main bathymetric features. We divided the margin into six segments for a detailed analysis. Rectangles indicate locations of Figs. 2–7.

2 ­ Data and method

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2 ­Data and method 2.1  ­Satellite GOCE gravity data The satellite GOCE model GO_CONS_GCF_2_DIR_R5 (Bruinsma et al., 2013) is a full combination of GOCE-SGG (Satellite Gravity Gradiometer), GOCE-SST (Satellite to Satellite Tracking), GRACE (Gravity Recovery and Climatic Experiment), and LAGEOS (Laser GEOdynamics Satellite). This model presents homogeneous precision and an excellent performance of the long as well as of the short wavelengths when compared to previous GOCE models (Pail et al., 2011; Bruinsma et al., 2010). The degree/order of the spherical harmonic coefficients is up to Nmax = 300, being the half-wavelength resolution of approximately 67 km according to λ/2 = πR/Nmax (Li, 2001; Hofmann-Wellenhof and Moritz, 2006; Barthelmes, 2013), with R being the mean Earth radius. We obtained the vertical gravity gradient by direct modeling of the satellite-only GOCE data, from the spherical harmonic coefficients (Janak and Sprlak, 2006) on a regular grid of 0.05° grid cell size. The vertical gravity gradient (Tzz) is obtained as the second radial derivative of the disturbing potential (Tscherning, 1976; Rummel et al., 2011): mGal  ∂ 2T  Tzz = 2 1 Eötvös = 10 −4 (1) m  ∂r  Tzz is expressed in Eötvös and represents a better spatial resolution than the gravity vector itself for detecting shallower crustal density variations (Li, 2001) allowing determining the edges of anomalous masses with better detail and accuracy (Braitenberg et al., 2011; Alvarez et al., 2012).

2.2 ­Reduction by topographic and sediments effect The topographic effect must be removed from the satellite observations (Forsberg and Tscherning, 1997) in order to reduce the correlation of the gravity signal with the topography. To remove the topographic effect from Tzz, we calculated the topographic contribution by discretizing a digital elevation model (ETOPO1, Amante and Eakins, 2009) using spherical prisms of constant density (see Grombein et al., 2013 and references therein). We considered the Earth’s curvature by using a spherical approximation (instead of a planar one) (Uieda et al., 2010), avoiding considerable errors as the region under study is wide (Hofmann-Wellenhof and Moritz, 2006; Alvarez et al., 2012, 2013; Grombein et al., 2013; Bouman et al., 2013). We performed the calculation of the topography contribution over the Tzz using the software Tesseroids (Uieda et al., 2010; Alvarez et al., 2013) densities used are mean standard values of 2670 kg/m3 for masses above sea level and a 1030 kg/m3 for sea water. The calculation height is of 7000 m to ensure that all values are above the topography.

2.3 ­Harmonic decomposition Featherstone (1997) performed a spectral analysis of the geoid and gravity anomalies finding that by decreasing the cut-off degree/order of the harmonic expansion the gravimetric signal is increasingly generated by a causative mass of increasing depth. In a recent work, Alvarez et al. (2017a) derived an equation (Eq. 1) relating the depth (Zl) of a causative mass with a determined degree of the spherical harmonic expansion (N) for the Tzz: ( R + HC )( N − 1) , Zl = E (2) ( N + 2 )( N + 1)

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Chapter 1  Observing rupture areas from satellite gravity data

Table 1  Associated depth (Zl) of a causative mass with a determined degree of the spherical harmonic expansion for Tzz Degree/order N

Spatial resolution λ/2 = πR/Nmax (km)

Zl (km) for Tzz (HC = 7 km)

300 250 200

66.72 80.06 100.07

20.98 25.11 31.26

where RE = 6371 km is the Earth’s radius, HC is the Tzz calculation height, and N is the degree/order of the harmonic expansion. Table 1 shows for different degree/orders the corresponding depth Zl and spatial resolution. Higher orders are associated with shallower sources; on the contrary, low orders are related to deeper mass anomalies. Results from this harmonic decomposition tool by truncating the harmonic expansion allow analyzing Tzz response with increasing depths of the causative masses for the different events under study.

3 ­Results and discussion 3.1  ­The Valdivia 1960 Mw = 9.5 earthquake This earthquake nucleated at the latitudes where the Mocha Fracture Zone (FZ) intersects the Chilean trench and propagated to the south along approximately 1.000 km up to the Chile Rise with the slip distribution being segmented by the incoming Fracture Zones (Contreras-Reyes and Carrizo, 2011; Melnick et al., 2009; Moreno et al., 2009). In this region the trench is almost completely filled, with sediment thicknesses ranging from 2.2 to 3.5 km causing a flat seafloor morphology (Lamb and Davis, 2003; Ranero et al., 2006; Völker et al., 2013). The relation between the volume of trench sediments and the relief of the oceanic plate strongly affects the development of the subduction channel promoting seismic segmentation (Contreras-Reyes and Carrizo, 2011; Kopp, 2013). The thicker sediment thickness along the southern region of the Chilean margin is expressed by a low mean value of Tzz (Alvarez et al., 2014). For this earthquake, we superimposed the slip distribution of Moreno et  al. (2009) with the topography-corrected Tzz from GOCE, noting that slip lobes roughly coincide with slip segmentation (Fig. 2A). For the maximum degree of the model (N = 300), Tzz presents a clear across strike segmentation, with maximum relative to the north (37.5°S) and south of the rupture (47.5°S), which can also be observed for N = 250 (white arrows in Fig. 2A and B). This narrowing of the signal could be related to seismic barriers to rupture propagation (Alvarez et al., 2012). The epicenter is flanked by relative Tzz highs (see Fig.  2A and B) to the N and NW (in the updip direction), whereas to the SW Tzz diminishes following the direction of rupture propagation. For N = 200, a high gradient contrast is observed close to the coastal line, which could be related to the downdip limit of the seismogenic zone (Fig. 2C). Different authors (Mendoza et al., 1994; Pritchard et al., 2007; Delouis et al., 2010; Loveless et al., 2010; Alvarez et al., 2014; Bassett and Watts, 2015) proposed that the high-gravity anomaly along the coastal line marks the downdip limit of the seismogenic zone.

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Topography-corrected Vertical Gravity Gradient (Tzz) obtained from GOCE (Bruinsma et al., 2013). Superimposed slip distribution for the Valdivia May 22, 1960, Mw = 9.5 earthquake (Moreno et al., 2009). White arrows indicate a narrowing of the signal that could be indicating barriers to seismic propagation. Red contours in (B) depict relatively higher Tzz anomalies related to anomalous masses that controlled rupture propagation to the south (yellow arrow). Red contour in (C) indicates a high gradient in the signal related to the upand downdip limits of the seismogenic zone. See Fig. 1 for a location in a regional perspective.

3 ­ Results and discussion

FIG. 2

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Chapter 1  Observing rupture areas from satellite gravity data

3.2 ­The Maule segment Across the forearc zone between the Juan Fernandez Ridge (JFR) and the Mocha FZ, a negative gradient signal dominates the marine forearc and is divided into a series of lobes unveiling mass heterogeneities along the seismogenic zone. In particular, the historical rupture areas in this region (1985, 1928, 1906, and 1835 earthquakes) roughly coincide with Tzz patches lower than −10 Eötvös (Fig. 3A). The Maule Mw = 8.8, 2010 earthquake ruptured bilaterally through two or three major slip patches (Lay et al., 2010; Lorito et al., 2011; Vigny et al., 2011; Moreno et al., 2012; among others) coinciding approximately the northern patch with the most likely 1928 rupture zone. In Fig. 3B (N = 250/Z = 25 km), higher densities along the inner forearc (terrestrial forearc) coincide with regions of high Vp (Vp 7.6–8.0 km/s, Vp/Vs ratio of ∼1.81 and Poisson’s ratio of 0.28) lying beneath the coast at 25 km depth, as reported by Hicks et al. (2014) from a seismological tomography. These authors noted this relationship between high positive Bouguer gravity anomaly and high seismic velocities, and associated gravity signal with dense ultramafic material where coseismic slip was reduced. In a previous work, Alvarez et al. (2014) proposed that the positive Tzz values observed in the forearc reveal the location of a seismic barrier defining the eastern edge of the rupture propagation zone for 1906 and 1985 events. The southern patch of the Maule earthquake propagated inland in a region of low Tzz, whereas the northern patch, the one of higher slip, occurred close to a minimum Tzz over the nonmarine forearc. This relationship between Tzz, seismic velocities and slip behavior suggests that gravity-derived signal is a good proxy for delimiting across and along strike segmentation in this portion of the Chilean margin. The high gradient close to the coastal line (Fig.  3C) follows the eastern termination of ruptures indicating the downdip limit of the seismogenic zone. At degree N = 200, Tzz lobes match slip maxima of the Maule earthquake, and to historic ruptures. Tzz lobes at this degree of the harmonic expansion are probably indicating the location of main asperities (i.e., locked areas where most slip occur during earthquake) along the plate interface.

3.3 ­The central Chile segment The JFR forms a topographic barrier segmenting a trench that is partly filled with sediments (2.0– 2.5 km thick) to the south of the JFR from a trench that becomes a narrower depression with steep walls starved of sediments (Schweller et al., 1981; von Huene et al., 1997; Laursen et al., 2002; Völker et al., 2013). Sediment thickness along the trench increases again at the Peruvian Andes latitudes, where rainfall and dominant winds become predominant again from the Pacific Ocean. The absence of a thick sedimentary infill immediately to the north of JFR inception point influences the gravity response presenting higher Tzz mean values northwards (Alvarez et al., 2018) along the Chilean margin. Historic ruptures comprises one (1918, 1943, 2015, 1859, and 1946), two (1918, 1983), or more low Tzz lobes (1796, 1922). The JFR, the Nazca FZ, the Copiapó, and Taltal ridges are related to relative maxima in Tzz interposed to the low Tzz lobes, according to the hypothesis that high oceanic features promote seismic segmentation as proposed by many authors (e.g., Sparkes et al., 2010; Contreras-Reyes et al., 2010). Recent works related subduction of oceanic topography (e.g., seamounts, aseismic ridges, plateaus) to regions with a higher rate of small earthquakes (Sparkes et al., 2010; Wang and Bilek, 2011, 2014) and by a lower degree of coupling (Metois et al., 2016), thus preventing for the nucleation of great megathrust earthquakes. This could be the case of the 1909 earthquake, which is not related to a low Tzz lobe being on the contrary its rupture located over a relatively higher Tzz segment at the extrapolation of the subducting Copiapó ridge. When truncating the degree of the harmonic expansion (Fig. 4B and C), low Tzz lobes become diffuse, masking the relationship between gravity and ruptures observed to the south.

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Topography-corrected Vertical Gravity Gradient (Tzz) obtained from GOCE (Bruinsma et al., 2013). Superimposed rupture areas for 1906-Ms = 8.4, 1985-Mw = 8.0, 1928-Ms = 8.0 earthquakes, and 1835 seismic gap. Slip distribution for the Maule 2010, Mw = 8.8 is from Moreno et al. (2009). Across strike narrowing of the Tzz signal related to rupture terminations are interpreted as seismic barriers. Thick solid black contour in (B) is related to regions of high Vp (Hicks et al., 2014) that acted as along-strike seismic barriers (B). Red contours in (C) depict up- and downdip limits of the seismogenic zone. The high gradient of the gravimetric signal over the coastline is probably also related to the transition from the continental slope to the shelf (shelf break) as pointed out by Contreras-Reyes et al. (2017). Black solid contours of Tzz (ellipses) indicate main asperities related to historic ruptures and to the Maule 2010 earthquake. See Fig. 1 for a location in a regional perspective.

3 ­ Results and discussion

FIG. 3

10 Chapter 1  Observing rupture areas from satellite gravity data

FIG. 4 Topography-corrected Vertical Gravity Gradient (Tzz) obtained from GOCE (Bruinsma et al., 2013) up to N = 300 (A), N = 250 (B), and N = 200 (C). Superimposed rupture areas of historical earthquakes along central Chile. Slip distribution for the Illapel 2015, Mw = 8.3 is from Tilmann et al. (2015). Tzz lobes roughly coincide with ruptures connecting one or more main asperities along the megathrust. The 1909 rupture over the region where the Copiapo ridge subducts coincides with a region of low degree of coupling. See Fig. 1 for a location in a regional perspective.

3 ­ Results and discussion

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The Illapel 2015 Mw = 8.3 earthquake nucleated immediately to the north of the subducting JFR (where a relative high Tzz is observed) and rupture propagated to the N-NW and updip of the epicenter toward a low-gravity gradient up to the Challenger FZ. The slip model (Tilmann et al., 2015) depicts a higher slip inland in a region where the high Tzz over the terrestrial forearc is interrupted (Fig. 4B), being the rupture flanked north and south by higher Tzz values (as shown in Alvarez et al., 2017a). Similar to the Maule, segment across-strike (inception of JFR and Challenger FZ) and along-strike (seismic barriers) segmentations are related to the density structure of the forearc zone.

3.4 ­Northern Chile-southern Peru The Mw 8.4 Arequipa earthquake in 2001 reactivated the northern portion of the 1868 rupture, leaving the southern segment unbroken (Fig.  5), with rupture propagating unilaterally to the southeast over 300 km (Bilek and Ruff, 2002; Giovanni et al., 2002; Audin et al., 2008). In this region, climatic conditions allowed accumulation of higher sediment thicknesses along the trench than in northern Chile to the south, revealed by lower values of Tzz (less than -5Eötvös). Here low Tzz signal correlates well with high seismic slip over the marine forearc (as shown by Alvarez et al., 2015). This gravity low could be related to the gravimetric expression of a forearc basin over the continental shelf, developed because of the Nazca FZ subduction (Wells et al., 2003; Bassett and Watts, 2015). A narrowing of the gradient signal or maximum relative is observed at both lateral endings of the slip distribution. At degree N = 200, positive Tzz (+5 Eötvös contour) to the SE, NW, and W of the hypocenter could be indicating different material properties impeding rupture propagation in these directions (Fig. 5C). Relative Tzz minima are probably indicating a heterogeneity that acted as a path to rupture propagation to the south and further amplification close to the Tzz minima lobe. On April 1, 2014 the Iquique Mw = 8.2 earthquake ruptured the plate boundary interface between the Nazca and South America plates (Ruiz et  al., 2014; Schurr et  al., 2014) over the region recognized as the Iquique seismic gap, where the largest recorded historical earthquake occurred in 1877 with magnitude Mw ~ 8.5–8.8 (Lomnitz, 2004), and estimated rupture zone from Arica to Antofagasta (see Fig. 5). This earthquake was preceded by an intense foreshock activity developed in the previous months to the main event, which accelerated toward the final foreshock sequence (Ruiz et al., 2014) and by a decrease in the b value 3 years prior to earthquake occurrence (Schurr et al., 2014). Geersen et al. (2015) imaged multiple large seamounts along the plate interface under the marine forearc in the intermediate coupled central part (19° to 20.5°S) of the northern Chile seismic gap (Metois et al., 2012). Slip patch for this earthquake shows a certain correlation to minimum Tzz (Fig. 5A). At lower degrees (N = 200), Tzz low in the region of maximum slip is substituted by a positive Tzz signal. This high variability in the signal could be produced by the subducted northern part of the Iquique ridge beneath the marine forearc. Subducted relief not only may act as barriers to seismic propagation but also as asperities linked to seismic rupture (Husen et al., 2002; Bilek et al., 2003), but generating networks of small-scale fractures and faults causing unfavorable conditions for seismic rupture propagation (Cloos, 1992; Mochizuki et al., 2008; Wang and Bilek, 2011; Kopp, 2013). Lay (2015) highlighted that larger slip for this earthquake was unusually concentrated. In this scenario, the positive gradient signal where the foreshock sequence and maximum slips took place could be related to subducted seamounts and basal erosion associated with the subduction of the Iquique ridge. The Mw = 8.1 Antofagasta earthquake on 1995 ruptured the subduction interface over a length of 180 km from the southern part of the Mejillones peninsula to the south (Fig. 5). Several earthquakes in

12 Chapter 1  Observing rupture areas from satellite gravity data

FIG. 5 Topography-corrected Vertical Gravity Gradient (Tzz) obtained from GOCE (Bruinsma et al., 2013) up to N = 300 (A), N = 250 (B), and N = 200 (C). Superimposed slip distributions for the 2001 Mw = 8.4 Arequipa, 2007 Mw = 7.7 Tocopilla, and 1995 Mw = 8.1 Antofagasta earthquakes from Chlieh et al. (2004, 2011). Slip for the April 1, 2014 Mw = 8.2 Pisagua and April 3, 2014 Mw = 7.7 Iquique are from Schurr et al. (2014). References: Red stars are the epicenters for the different earthquakes shown in this figure. Blue dashed line: 1877 Mw = 8.6 reduced zone from Metois et al. (2013). See Fig. 1 for map location in a regional perspective.

3 ­ Results and discussion

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the Antofagasta region (Ruiz and Madariaga, 2018) preceded the 1995 earthquake. The rupture process resulted in a smooth and slightly heterogeneous slip distribution that has been extensively studied using seismological and geodetic techniques (Chlieh et al., 2004). Although different observations indicate that this earthquake ruptured the deeper part of the plate interface (e.g., Delouis et al., 1997; Ihmlé and Madariaga, 1996), evidence was found that rupture reached near the trench (Ruiz and Madariaga, 2018). The 2007, Mw = 7.7 Tocopilla earthquake ruptured only the deeper portion of the seismogenic zone (Peyrat et al., 2010). Although the rupture of this earthquake occurred in a highly locked area of the megathrust, rupture was limited to a small fraction in the downdip end of the locked fault zone (Chlieh et  al., 2011). The last authors reported that 1-m slip contour appears to have ruptured only a small portion of the southern downdip end of the locked fault zone and of the 1877 event. For the Tocopilla 2007 earthquake, Contreras-Reyes et al. (2012) proposed the influence of the along dip geometry of the Nazca plate and Schurr et al. (2014) proposed different friction properties along dip to explain the position of this event near the bottom of the plate interface (Ruiz and Madariaga, 2018). Different to other earthquakes previously analyzed along the Chilean margin, where hypocenters nucleate close to a Tzz high and rupture propagates to a relatively low Tzz, slip models for the ruptures for the 1995 and 2007 earthquakes indicate that rupture occurred mostly in the downdip portion of the megathrust. Besides both earthquakes nucleated close to a Tzz low and propagated over Tzz highs. This region of the Chilean margin, between 21°S and 25°S, presents a high positive Tzz signal along the marine forearc, different to the rest of the margin. Along this segment, there is a clear correlation between the gravity high along the coastal line and the downdip limit of the seismogenic zone, as observed along the rest of the margin. From 23 to 26°S, no giant megathrust earthquakes are known, reason by which this segment is considered an atypical segment in which only moderately large events have occurred (Ruiz and Madariaga, 2018). Probably the high Tzz is evidencing different seismogenic conditions for nucleation of a great megathrust earthquake, or at least with a great rupture area developing along the outermost forearc. The high Tzz signal seems to be dominated by the lack of sediments along the trench in a region dominated by basal tectonic erosion of the forearc crust. Different works (Sobiesiak et al., 2007; Llenos and Mc Guire, 2007; Tassara, 2010) have focused on these Central Andean forearc asperities and their link to gravity highs (mafic bodies) reflected in a high vertical stress anomaly (VSA) that accounts for the component of normal stress due to the weight of the overlying crustal column (Tassara, 2010). Whereas this anomaly is a relevant parameter for northern Chile, the Southern Andes forearc is felsic-dominated (low-density) producing neutral-to-negative VSA.

3.5 ­Peru On August 15, 2007, a Mw = 8.0 earthquake stroke about 20 km offshore of Pisco (Peru) producing a tsunami (Pritchard and Fielding, 2008; Wei et al., 2008; Fritz et al., 2008). The rupture was associated with a slip up to 8 m (Perfettini et al., 2010) and propagated southward below the Paracas Peninsula and offshore before arresting on the northern edge of the Nazca ridge (Sladen et al., 2010), which is subducting obliquely beneath the South American plate (Fig. 6) at a convergence rate of about 6 cm/ year (Kendrick et al., 2003). To the south of the Pisco 2007 rupture, the 1942 Mw 8.0 and the 1996 Mw 7.7 Nazca earthquakes occurred (Salichon et al., 2003; Pritchard et al., 2007). The last ruptured only the deeper portion of the seismogenic zone (“Domain C” proposed by Lay et al., 2012) and had similar characteristics to the 2007 Mw 7.7 Tocopilla event (Chlieh et al., 2011). The 1942 and 1996 ruptures

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Topography-corrected Vertical Gravity Gradient (Tzz) obtained from GOCE (Bruinsma et al., 2013) up to N = 300 (A), N = 250 (B), and N = 200 (C). Superimposed rupture areas for the 1996 M = 7.5; 1970 M = 7.6/1966 M = 7.5; 1940 M = 8.0; 1942 M = 8.1 and slip models for the 1996 M = 7.7 Nazca, 2007 M = 8.0 Pisco, and 1974 M8.1 earthquakes (Langer and Spence, 1995; Swenson and Beck, 1999; Sladen et al., 2010; Pritchard et al., 2007; Chlieh et al., 2011). References: Red stars are the epicenters for the different earthquakes shown in this figure. See Fig. 1 for a location in a regional perspective.

Chapter 1  Observing rupture areas from satellite gravity data

FIG. 6

3 ­ Results and discussion

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seem to have overlapped and stopped on the southern side of the Nazca ridge (Salichon et al., 2003). A reassessment of the 1942 earthquake (Okal and Newman, 2001) suggests that both events probably ruptured inland of the coast (Sladen et al., 2010). Apparently, no historical event ruptured through the segment corresponding to the subduction point of the Nazca ridge, suggesting that this area could be a permanent barrier to earthquake rupture propagation (Dorbath et al., 1990; Perfettini et al., 2010; Chlieh et al., 2011). Other relatively minor events (Mw > 7.5) occurred in the subduction segment located between the Mendana FZ to the north and the Nazca ridge to the south such as the 1966 (Mw 8.0), the 1974 Mw 8.0 Lima earthquake (Okal, 1992), 1970 M7.6, 1996 M7.5, 1940 M8.0, 1966 M7.5 (Dorbath et al., 1990; Pritchard et al., 2007). North of the Nazca ridge well-developed offshore forearc basins exist whereas to the south none basins were formed (Clift et al., 2003; Krabbenhoft et al., 2004). This is reflected by a lower mean value (more negative) of the Tzz signal (Fig. 6A) as observed along the southern Chilean margin. The 2007 Pisco rupture presents a good anticorrelation to Tzz signal, with negative Tzz over higher slip areas as explained by Alvarez et al. (2015). In this region, the distance between the trench and the coastline increases from 100 km (south of Pisco) to 200 km to the north, coinciding with a very distinct salient of the coastline (Sladen et al., 2010). This feature is generally associated with the downdip extent of the seismogenic zone (Ruff and Tichelaar, 1996). Regarding the positive Tzz signal along the coastline (Fig. 6A and B), slip models for the 2007 Pisco and 1996 Nazca earthquakes present higher displacements inland over relatively lower Tzz, as observed along the south-central Chilean margin. Particularly the last earthquake propagated downdip in a region of relatively lower Tzz signal along the coast as observed for other events that propagated in the lower portion of the megathrust. The 1974 rupture presents two slip patches coinciding with relatively lower Tzz lobes along the marine forearc, but events located to the north of it to the Mendana FZ present no correlation to the Tzz signal. This is probably due to the high spatial resolution of GOCE model and relatively small rupture areas. In a recent work, Alvarez et al. (2015) found that if event magnitude increases (and consequently rupture area) the correlation between low Tzz lobes (10−4 mGal/m) and high slip (m) increases for Mw > 8.0 events attributing this to the high spatial resolution of GOCE only models (160 km).

3.6 ­Ecuador-Colombia The Musine Mw = 7.8 thrust earthquake in 2016 ruptured nearly 200 km along the plate interface, in an area similar to the rupture zone of the Mw = 7.8 1942 earthquake. The 2016 earthquake occurred at a margin characterized by moderately big to giant earthquakes such as the 1906 (Mw = 8.8). A heavily sedimented trench explains in part the abnormal lengths of the rupture zones in this region because it inhibits the role of natural barriers on the propagation of rupture zones. A high amount of sediment thickness is associated with tropical climates, high erosion rates, and eastward Pacific dominant winds that provoke orographic rainfalls over the Pacific slope of the Ecuadorian Andes. This high trench infill volume is denoted by a low-gravity signal (Fig. 7A) as observed in southern Chile. In particular, the rupture zone of the 2016 Mw = 7.8 Ecuador earthquake developed through a relatively low-density zone of the forearc sliver (Alvarez et al., 2017b). When truncation degree to N = 200, Tzz minima lobes show a good fitting to historical rupture areas in this region. 1979, 1958, and 2016 earthquakes nucleated close to a region of relatively lower Tzz and propagated toward minimum Tzz lobes (Fig.  7C). The 1942 earthquake nucleated at the center

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Topography-corrected Vertical Gravity Gradient (Tzz) obtained from GOCE (Bruinsma et al., 2013) up to N = 300 (A), N = 250 (B), and N = 200 (C). Superimposed rupture areas of the main earthquakes: 1906 Mw = 8.8; 1942 Mw = 7.8; 1958 Mw = 7.7, 1979 Mw = 8.2, and 2016 Mw = 7.8 (Kanamori and McNally, 1982; Mendoza and Dewey, 1984; Swenson and Beck, 1996; Ye et al., 2016). Plate convergence rate is from Nocquet et al. (2014). See Fig. 1 for a location in a regional perspective.

Chapter 1  Observing rupture areas from satellite gravity data

FIG. 7

4 ­ Concluding remarks

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of the low Tzz minima low and propagated roughly radially. The 1906 earthquake, the one of higher magnitude in this region, occurred at the center of the relative maxima and propagated bilaterally to both minima Tzz lobes. Relatively higher Tzz signal (barriers) and low Tzz lobes (asperities) correlate to earthquake nucleation position and rupture propagation behavior, suggesting that the forearc density structure strongly affects seismogenesis.

4 ­Concluding remarks Along the southern Chilean margin, in the region where two of the most giant earthquakes registered occurred (the 1960 Mw = 9.5 Valdivia and the 2010 Mw = 8.8 Maule earthquakes), the Tzz signal presents its lower mean values reaching less than −20 Eötvös. For these earthquakes, Tzz minima lobes present a good spatial correlation to the maximum registered displacements and also to historic rupture areas (1835, 1928, 1906, and 1985). Similarly along the Ecuador-Colombia margin, Tzz minima lobes are also coincident with the maximum slip values of the 2016 Mw 7.5 Musine earthquake and also to historical ruptures (1942, 1906, 1979). Main asperities seem to be located over low Tzz lobes for N = 200 as shown by refined slip distributions for recent great earthquakes (2001 Mw 8.4 Arequipa, 2007 Mw 8.0 Pisco, 2010 Mw 8.8 Maule 2010, 2015 Illapel Mw 8.3 and 2016 Musine 7.8). The relationship between minimum Tzz (<0 Eötvös) lobes and highly coupled regions acting as seismic asperities was observed in previous works (Alvarez et al., 2014, 2015, 2017a,b, 2018) and associated with subducted sediments and forearc basins. On the other hand, relative maximum Tzz signal over the marine forearc in general coincides with lateral rupture bounds as explained by Alvarez et al., 2014. Tzz relative maxima in these regions are mainly related to different types of subducting oceanic plate roughnesses (seamounts, aseismic ridges, etc.), associated with higher rates of low degree seismicity, lower interseismic coupling, and thus controlling a high degree of seismic segmentation along the margin. The Mw 8.4 Pisagua earthquake on 2014 took place in a region where a Tzz minimum lobe over the marine forearc does not continue at depth (for N = 200), being replaced by a positive gradient signal. This particular event was preceded by an intense foreshock sequence, which has been associated with subducted seamonts (related to the northern border of the Iquique ridge) under the region of the main rupture. These differences on rupture processes are shared by a different gradient signal behavior along the margin. The 1909 earthquake at the Copiapó latitudes could have presented a similar behavior. The central Chilean segment between the Taltal ridge and the JFR presents a higher mean value of Tzz in a region of the margin that has been characterized mainly by subduction erosion. Here historical ruptures seem to comprise different numbers of asperities if they are mapped by the Tzz signal (one: 1966, 1859, 1946, two: 1918, 1983, three or four: 1922, 1796). Slip model of the Mw = 8.32015 Illapel earthquake indicates propagation to a region of Tzz minima, whereas historical ruptures of the 1943 and 1918 earthquakes coincide with the minimum Tzz lobe between the JFR and the Challenger FZ. The highly positive Tzz signal along the coastal line marks the downdip limit of the rupture zone in many of the studied cases. Other authors reported that coseismic slip models and aftershocks sequences are located seaward of the positive gravity-derived anomalies along the Chilean coast, evidencing a direct relationship of these maxima with the downdip limit of the seismogenic zone (Mendoza et al., 1994; Delouis et al., 2010; Loveless et al., 2010; Alvarez et al., 2014; Bassett and Watts, 2015). Ruiz and Madariaga (2018) observed that rheology along dip also controls the dynamic rupture process of

18

Chapter 1  Observing rupture areas from satellite gravity data

earthquakes and seismic wave attenuation over the Chilean margin. From this study, we observed that for N = 200 this relationship becomes notorious. Particularly for the Maule earthquake, we found a correlation between high Tzz and regions with high Vp (Hicks et al., 2014 from a seismic tomography) and vice versa. The last authors reported that high Vp and gravity anomaly highs anticorrelate to coseismic slip for the downdip portion of the rupture. In some cases when rupture propagated inland (e.g., the southern patch of Maule, the eastern side of the Illapel rupture, main aftershock Mw 7.7 of the Pisagua earthquake), maximum slip occurred also over anomalous Tzz regions (i.e., over a relative minima along the coastal line). If high Vp and high gradient signal were related to slip reduction, forecasting regions of variable slip from Earth gravity field models would be possible. Events that occurred almost entirely along the deeper portion of the plate interface onshore, e.g., the 1995 Mw = 8.1 Antofagasta, 1996 M = 7.7 Nazca, and 2007 Mw = 7.7 Tocopilla, could not be related to Tzz minima lobes following this analysis. Other alternative for these occurrences can be found in Tassara (2010) and in Bejar-Pizarro et al. (2013), who proposed that large-scale structures in the overriding plate can influence the frictional properties of the seismogenic zone at depth suggesting that the occurrence of megathrust earthquakes in northern Chile is controlled by the surface structures that built Andean topography. Following this proposal, relative gravity highs along the forearc are associated with high-density crustal bodies that impose large vertical stresses on top of the interplate seismogenic zone; and for a given value of friction and pore pressure along the subduction channel, this region acts as a seismic asperity (i.e., high shear strength, high levels of seismicity, and large coseismic slip). Different approaches have been tested to explain earthquake directivity as fault segmentation, the history of previous earthquake ruptures, preferential orientation of structures on the fault interface, or the superposition of different materials across the fault zone (McGuire et al., 2002; Rubin and Gillard, 2000; Pritchard et al., 2007). Many of the analyzed events presented this directivity behavior and in many cases rupture propagated to the minimum Tzz lobe (e.g., in Ecuador-Colombia events, Musine 2016, Pisco 2007, Arequipa 2001, Valdivia 1960, Maule 2010, Illapel 2015). Greatest and recent events in the South American active margin (e.g., 2010 Mw 8.8 Maule, 2001 Mw 8.4 Arequipa) presented a high correlation between location of Tzz minima lobes and higher slip suggesting the location of main asperities (mainly when located at domain “B”). Finally, forearc density distribution could explain the directivity effect in many cases.

­Acknowledgments The authors acknowledge the use of the GMT-mapping software of Wessel and Smith (1998). The authors would like to thank to Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and to CICITCAPROJOVI (Project nº: 80020170300015SJ) de la Secretaría de Ciencia y Técnica-Universidad Nacional de San Juan, for funding sources.

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Chapter 1  Observing rupture areas from satellite gravity data

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