Propagation history of the Osaka-wan blind thrust, Japan, from trishear modeling

Journal of Structural Geology 58 (2014) 79e94

Contents lists available at ScienceDirect

Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg

Propagation history of the Osaka-wan blind thrust, Japan, from trishear modeling Pamela R. Grothe a, *, Nestor Cardozo b, Karl Mueller a, Tatsuya Ishiyama c a

Department of Geological Sciences, University of Colorado, Boulder, CO, USA Department of Petroleum Engineering, University of Stavanger, Stavanger, Norway c Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyoku, 111-0032 Tokyo, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 May 2013 Received in revised form 23 October 2013 Accepted 29 October 2013 Available online 15 November 2013

Mapping the nucleation and 3D fault tip growth of the active Osaka-wan blind thrust provides an opportunity to asses how reactivated thrusts build slip from preexisting faults and the threat they pose as sources of large earthquakes. Analysis of folded growth strata, based on 2D trishear inverse modeling allows a range of best-fit models of the evolution of slip and propagation of the fault to be defined. The depth of the fault tip at 1200 ka varies between w1.5e4.5 km, suggesting the fault grew upward from high in the crust, and that it is reactivated. From its onset at w1500 ka, the fault grew rapidly along strike in w300 ky, and upwards with a P/S ratio of 2.5e3.0, but variable fault slip in space and time. Shallower depths of the fault tip at initiation and thinner basin fill correlates with slower propagation with time, contradicting models that argue for sediments as inhibitors of fault growth. Results also suggest the displacement profile of the currently active thrust is offset from its predecessor, assuming shallower depths to the original fault correlate with greater displacement in its prior history. These results suggest reactivated faults may accrue slip differently than newly developed ones, based on the history of upward fault propagation. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Osaka Bay Osaka-wan fault High-resolution seismic profiles Fault propagation folding Growth strata Trishear

1. Introduction Active blind thrust faults pose significant seismic hazards, especially where they form in rapidly subsiding sedimentary basins and are buried continuously throughout their period of activity (Suppe, 1979; King and Brewer, 1983; Stein and King, 1984; King et al., 1988; Stein and Yeats, 1989; Bullard and Lettis, 1993; Taboada et al., 1993; Shaw et al., 2002; Ishiyama et al., 2004). On 17 January 1995 the devastating Mw 6.8 Kobe earthquake struck the Osaka Bay region of southwestern Japan (Fig. 1). In response to this deadly earthquake, the Japanese government, academic and private organizations conducted seismic reflection surveys to assess seismic hazards throughout the region. Seismic reflection surveys in Osaka Bay revealed a previously unknown active blind thrust fault, named the Osaka-wan fault that did not rupture in the earthquake (Fig. 1, Kitada et al., 2001). Analysis of these data can be

* Corresponding author. Present address: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332, USA. Tel.: þ1 540 847 8617. E-mail addresses: [email protected] (P.R. Grothe), nestor.cardozo@uis. no (N. Cardozo), [email protected] (K. Mueller), [email protected] (T. Ishiyama). 0191-8141/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2013.10.014

used to define the geometry of the blind thrust and the pattern in space and time by which it has accrued slip through repeated large magnitude earthquakes. Acquisition of the high-resolution seismic reflection data, which image growth strata in extraordinary detail offers the opportunity to define how a large compressive fault-related fold grew with time. Growth of the fold was then used as a proxy for assessing how the underlying blind thrust propagated upward with increasing displacement. The resulting record is also available for comparison with other studies of fault growth and scaling, in particular for reactivated thrusts. Perhaps the greatest opportunity lies in the precise age control for deformed strata, which allows the propagation history of the thrust to be defined in previously unavailable detail. This type of history enables otherwise inaccessible blind thrusts to be evaluated in terms of their geometry and slip rate, forming a basis for assessing seismic hazards along them, including coseismic surface deformation and strong ground motion. The folded strata above the Osaka-wan fault formed as the fault propagated upwards and along strike. This fault-propagation fold is characterized by a forelimb that becomes increasingly steeper with depth (Fig. 1b), similar to the geometry of folds produced by the trishear kinematic model (Erslev, 1991; Allmendinger, 1998). In this paper, 2D trishear inverse modeling (Allmendinger, 1998; Cardozo

80

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

et al., 2011) of growth strata in five seismic profiles across the southern half of the Osaka-wan fault (black lines, Fig. 1) is used to infer the depth of nucleation of the fault and its history of fault slip and fault propagation up-dip and along-strike. 2. Geologic setting Osaka Bay sits in an actively deforming region in southwest Japan, located above the westward subducting Philippine Sea Plate slab (Fig. 1). Oblique subduction of the Philippine Sea Plate drives oblique shortening in the region, which is partitioned into an array of sub-parallel strike-slip and thrust faults (Fig. 1). The Osaka-wan fault is a 40 km long, high angle (w70 dip) thrust that trends NEe SW and splits into several strands along its northern half (Fig. 1, Yokokura, 1999). Seismic reflection profiles reveal that blind thrusts in the Osaka basin accommodate about 2.5 km of vertical offset (Sato et al., 1998). More importantly, the data reveal syntectonic growth strata deposited across the Osaka-wan fault while it was active (Fig. 1). These growth strata include finely bedded, shallowmarine and fluvio-deltaic sediments that covered the Osaka-wan fault continuously for at least the last 1200 ky are remarkably well-dated by volcanic tephra, magnetic reversals and palynology in 900 and 1600 m deep well cores (Biswas et al., 1999). Correlation of reflectors on the seismic profiles with the boreholes (Biswas et al., 1999) suggests that every reflector in the basin where nearly all reflectors extend completely across it can be tied to a specific sea level highstand. Given the large number of highstands during the Late Quaternary, these correlations offer an opportunity to track the fault growth in unprecedented detail. Most importantly, this age control bridges the gap between coseismic and paleoseismic measurements of displacements, and long-term average rates of displacement at timescales of millions of years (Mouslopoulou et al., 2009). Osaka Bay is a Late Quaternary shallow-marine basin formed by flexure in the footwall of the Kariya fault, which is exposed along the eastern edge of Awaji Island (Fig. 1). The Kariya fault puts Cretaceous granitic rocks in its hanging wall in contact with late Quaternary sediments of the Osaka Basin in its footwall. Dextral shear across the Kariya uplift is accommodated further west by the Nojima fault, which is exposed along the western edge of Awaji Island (Fig. 1). The Nojima fault extends north and west of Kobe as the Arima-Takatsuki Tectonic Line (ATTL), a set of oblique structures exposed along the base of Rokko Mountains, which form the northern margin of the Osaka Basin. Other large thrusts in this region include structures further to the east such as the Nara and Ikoma faults (Fig. 1). These faults along with the Osaka-wan and Kariya faults collectively absorb shortening north of the right lateral Median Tectonic Line (MTL), which itself accommodates shear above the subduction interface with the Philippine Sea Plate. GPS geodesy suggests that shortening in this area as defined by principal strain axes is effectively perpendicular to the trace of the Osaka-wan fault (Sagiya, 2004). Evidence for lateral displacement on the Osaka wan fault is not apparent on any of the seismic profiles except for minor offset of a few reflectors on line GS8-ME. We thus interpret it to accommodate largely dip slip although piercing points are not available to formally test this hypothesis. While the thickness of the seismogenic crust in this part of Japan lies at 13 km depth, with a relatively high crustal geothermal gradient, this part of Japan does not contain a well-developed set of arc-related volcanic centers due to a shallowly subducting slab. The Osaka Basin has subsided rapidly from the late PlioPleistocene (Huzita and Kasama, 1983) and is infilled with over 3 km of sediments (Itoh et al., 2000, 2001). Subsidence was previously proposed to be driven by either faulting on the Osaka-wan fault (Yokokura et al., 1998), which likely should only create

accommodation space east of the fault, or by differential subsidence of fault-bounded blocks due to its location on the margin of a proposed rhomboidal depression (Itoh et al., 2000, 2001), but with an inference of tectonic pull-apart on strike slip faults. As stated previously, the large Kariya fault, which has a minimum of 1.5 km offset (Fig. 1b), likely drives most of the subsidence in Osaka Bay in response to flexure. The Osaka Basin is filled with sediments of the Osaka Group e a continuous sequence of Plio-Pleistocene strata comprised of fluvial and lacustrine deposits (e.g. silt, sand and gravel) in the lower part, and sequences of alternating marine clay layers and freshwater sediments in the upper section (Ikebe et al., 1970; Itihara, 1993). The basin, which was situated near sea level for at least the last 1200 ky, has a stratigraphic architecture strongly influenced by marine and non-marine transitions, as recorded in the upper section of the Osaka Group of alternating marine clay and non-marine silt and sand. The age of the marine clay layers corresponds well with the global eustatic record of sea level highstands (Takatsugi and Hyodo, 1995) and hence provides high-resolution absolute elevation datums (Itoh et al., 2001). Fifteen marine clay layers were initially identified in the Osaka Group and were designated sequentially as the oldest Ma-1 through the youngest Ma13 (Itihara, 1961; Yoshikawa et al., 1997; the Ma designation is a local system used by Japanese researchers to reference the stratigraphic sequence in the region). Three additional marine clay layers were later identified from cores in the central part of the basin between Ma12 and Ma10 and were labeled Ma11.1, Ma11.2 and Ma11.3 (Furutani, 1989), totaling 17 marine layers. In addition, volcanic tephra are recognized in the Osaka Group, providing unprecedented age control on the marine clay layers (Biswas et al., 1999). In summary, the correlation of the lithostratigraphy in the boreholes is one-to-one for each reflector in the seismic reflection profiles. 2.1. Seismic and well data High-resolution seismic reflection surveys were collected in Osaka Bay in the wake of the 1995 Kobe earthquake (Fig. 1). The data were collected by multiple agencies, including the Geological Survey of Japan (GSJ), University of Tokyo (TK), Hyogo Prefecture (HG), city of Kobe (KB), Hydrographic Department (HD), the Power Reactor and Nuclear Fuel Development Corporation (NP), and Geo-Research Institute (OD). The marine seismic data were collected and processed using standard techniques (Yokokura et al., 1999). A 1700-m borehole (GS-K1, Figs. 1 and 2) at Higashinada, Kobe City, the longest core in the Osaka Basin, provided the link between the seismic reflection data and lithostratigraphy. The borehole penetrated granite basement below 1546 m, which is overlain with unconsolidated sediments of the Osaka Group (Fig. 2). Biswas et al. (1999) further refined the depositional history of the Osaka Group for the past 3200 ky by identifying two short geomagnetic reversal events at 690 ka and 1600 ka in addition to the previously mentioned age controls from volcanic tephra. Kinugasa and Mizuno (1996) first correlated strata in the cores with seismic reflectors, tying seismic line GS-NP (Fig. 1), which crosses the borehole, to the marine clay layers in the core. In addition to the tephrachronology and magnetostratigraphy, the marine clay layers from Ma-1 to Ma12 were further pinpointed in the core by diatom analyses, electrical resistivity, and sulfur content (Fig. 2, GSJ, 1996; Biswas et al., 1999). The latest marine clay layer, Ma13, was sampled in the upper 23 m of the borehole, which was not cored (Fig. 2, Biswas et al., 1999). The core contained forty-five volcanic ash layers, ten of which were dated directly using tephrachronology (GSJ, 1996; Biswas et al., 1999).

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

81

Fig. 1. a. Geological map of Osaka Bay with seismic reflection lines. Black lines are the seismic profiles used in this study. Thick gray lines are seismic profiles used to correlate age calls from the boreholes to the studied profiles. Thin gray lines are other seismic profiles. Letters on insert are as follows. N and ATTL. right lateral Nojima fault and Arima-Takatsuki Tectonic Line. K, OB, U, I and N: Kariya, Osaka Wan, Uemachi, Ikoma and Nara thrusts. MTL: Median Tectonic Line. b. Interpreted seismic profile TK-1 from Sato et al. (1998). Vertical scale is in two-way travel time.

Lithostratigraphy and faunal assemblages defined in the well cores of the Osaka Basin (Fig. 1) were also used to independently identify transitions from fluvio-deltaic to shallow-marine conditions. They were again consistent with transgressive sequence stratigraphic boundaries that correlate with specific sea level

highstands. Ma-1 to Ma4 and Ma9 to Ma13 were defined from detailed magnetostratigraphy and tephrochronology. The ages of Ma5 to Ma8 were determined by correlating stratigraphy with the oxygen isotope stages based on the climate record from pollen analysis (Itoh et al., 2000, 2001).

82

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

Lithostratigraphy borehole GS-K1 0

200

400

Depth (m)

600

Ma 12 Ma 11 Ma 10 Ma 9 Ma 8 Ma 7 Ma 6 Ma 5 Ma 4 Ma 3

127 ka 242 ka 334 ka 427 ka 528 ka 578 ka 621 ka 712 ka 787 ka 865 ka

Ma 2

960 ka

Ma 1

1070 ka

Ma 0 Ma-1

1120 ka 1200 ka

Seismic profiles GS-8ME

OD-B

HD-3

HD-4

HG-1-1M

800

1000 Marine Non-marine

1200

Granite

1400

1600

Fig. 2. Lithostratigraphy of the 1700 m GS-K1 core. Ages of marine clay layers and their interpretation in seismic profiles GS-8ME, OD-B, HD-3, HD-4 and HG-1-1M are shown. A dot means that the layer was interpreted in the seismic profile (modified after Biswas et al., 1999).

3. Methodology 3.1. Correlation of seismic reflectors The Geological Survey of Japan and the National Institute of Advanced Industrial Science and Technology provided the depthconverted seismic profiles throughout the Osaka Bay. Reflectors were interpreted and mapped on the depth-converted profiles ODA, HG-1-1M, GS-8ME, OD-B, HD-3, HD-4, HD-7, GS-11, and GS-12 (Fig. 1). This study focuses on the southern half of the fault, avoiding the northern part where the Osaka-wan fault splits into several smaller branches and where greater uncertainty exists due to the smaller displacements on each fault strand. Undeformed, nearly horizontal parallel reflectors were first mapped on seismic line OD-A, which is a strike-line located 3e 10 km from the base of the forelimb of the Osaka-wan fault-related fold (i.e. its footwall, Fig. 1). Line OD-A intersects the five seismic lines used in trishear modeling, lines GS-8ME, OD-B, HD-3, and HD4 (Fig. 3), and line HG-1-1M (Fig. 5a). Line HD-3 shows the best continuity of reflectors across the forelimb (Fig. 3). This line was used to trace the reflectors across the fold to tie into a second strike-line, GS-11, located on the hanging wall of the fault (Fig. 1). Correlations across the fold were more robust closer to its southern

termination, where displacement is smaller (e.g. on HD-3). The reflectors from profile GS-11 were then mapped and loop tied back across the fold using profiles HG-1-1M, GS-8ME, OD-B, and HD-3. GS-11 was also correlated through two additional strike lines, HD-7 and GS-12, to complete the final loop tie into HD-4 (Fig. 1). With the reflectors mapped on both sides of the fault, the reflectors across the forelimb were then correlated. In total, 14 marine clay layers were correlated throughout the seismic profiles, with the exception of a few horizons that died out at the edge of Osaka Bay on profile HD-4 (Fig. 1). For trishear modeling, only those reflectors that can clearly be traced across the forelimb were used. Fig. 4 shows the interpreted reflectors on profiles GS-8ME, OD-B, HD-3, and HD-4; and Fig. 5b those on profile HG-1-1M. Published data from Inoue et al. (2003) were used to assign age calls to the reflectors. Inoue et al. (2003) correlated the reflectors of 14 marine clay layers from several boreholes, the deep GS-K1 borehole (Kinugasa and Mizuno, 1996) and shallower boreholes near the Kansai International Airport (Itoh et al., 2001), with PeS velocity logging and VSP, or vertical seismic profile records. Ages were assigned based on Inoue et al.’s (2003) age-correlated reflectors at the intersection of seismic lines OD-A and OD-B (Fig. 1). Numbers on the interpreted beds in Figs. 4 and 5b correspond to the identification numbers of the marine clay layers (Ma) in

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

83

Fig. 3. Seismic profiles a. GS-8ME, b. OD-B, c. HD-3, and d. HD-4. Profiles are in depth and have no vertical exaggeration.

Fig. 4. Interpretation of seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and d. HD-4. In all profiles, the rectangle shows the area where the current fault tip was searched in the trishear inversions. Numbers on the interpreted beds correspond to the identification numbers of the marine clay layers (Ma) in borehole GS-K1 (Fig. 2).

84

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

Fig. 5. a. Seismic profile HG-1-1M and b. Interpretation. The profile is in depth and has no vertical exaggeration. In b, the rectangle shows the area where the current fault tip was searched in the trishear inversions. Numbers on the interpreted beds correspond to the identification numbers of the marine clay layers (Ma) in borehole GS-K1 (Fig. 2).

borehole GS-K1 (Fig. 2). Overall, the interpreted profiles show a decrease of fault displacement, fault uplift and fault related folding from profile HG-1-1M (north) to profile HD-4 (south; Figs. 1 and 3e 5). With the exception of profile HG-1-1M where the fault breaks through the forelimb (Fig. 5), the marine clay layers Ma-1 to Ma12 are continuous across the forelimb profiles (Figs. 3 and 4).

used for the dip of the Osaka-wan fault. Although uncertainties affect this estimation, the results of trishear inverse modeling (Section 4) are similar for a fault dip angle between 65 and 75 . Trishear inversions with this fault-dip angle range give essentially the same results as with a fixed 72 fault dip angle. These values are similar to the dip of the nearby Kariya thrust (Sato et al., 1998).

3.2. Estimation of fault dip

3.3. Trishear inverse modeling

Trishear inverse modeling is more robust with independent data for fault geometry such as might be available from boreholes that directly penetrate a fault, or direct fault plane reflections (e.g. Allmendinger and Shaw, 2000; Cardozo and Aanonsen, 2009). The seismic data do not reveal a clear fault-dip angle based on tightly constrained stratigraphic cutoffs. However, indirect evidence is available that constrains the dip of the Osaka-wan fault. Time section TK-1 (Fig. 1; Sato et al., 1998) in the Osaka Bay area shows a sharp transition from the unfolded footwall to the forelimb of the Osaka-wan fault-propagation fold where shear is concentrated along the synclinal axial surface (Fig. 6a). Points marking the sharp synclinal hinge on different stratigraphic levels (white depth corrected dots in Fig. 6a) are also aligned onto a planar surface. This synclinal axial surface was thus used as a proxy for the dip of the Osaka-wan fault. Points along the synclinal axial surface were converted to depth using stacking velocity functions from Yokokura et al. (1998, Fig. 6b). The depth-converted points suggest a dip of the synclinal axial surface of about 72 (Fig. 6b). This angle was then

Trishear is a kinematic model of fault-propagation folding where the decrease in displacement along the fault is accommodated by heterogeneous shear in a triangular zone radiating from the fault tip (Erslev, 1991; Allmendinger, 1998). Trishear explains features observed in the Osaka-wan fault-propagation fold such as (i) changes in stratigraphic thickness and dip of fold forelimbs, (ii) footwall synclines, (iii) rounded and angular anticlinal and synclinal hinges, and (iv) steepening of fold limbs (i.e. larger strain) toward the fault tip (Figs. 3e5). The applications of trishear to faultpropagation folds include predicting finite strain and fracturing (Allmendinger et al., 2004; Cardozo et al., 2005), incorporating growth strata geometries (Hardy and Ford, 1997; Gawthorpe and Hardy, 2002), and applying fault-propagation fold theory to seismic hazards (Allmendinger and Shaw, 2000; Champion et al., 2001; Lin et al., 2007). The key advantage of trishear in these studies is the ability to invert fold geometry to determine the geometry, displacement and propagation of underlying, seismogenic blind thrusts similar to the Osaka-wan fault.

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

85

Fig. 6. a. Seismic profile TK-1 (Sato et al., 1998) in the Osaka Bay area. White circles indicate the synclinal axial surface. b. Time-to-depth conversion of synclinal axial surface. Black dots are data from Yokokura et al. (1998), and white circles are the points on the synclinal axial surface of a. Dip of axial surface is a proxy for the dip of the Osaka-wan fault.

In this paper, 2D asymmetric trishear modeling (Zehnder and Allmendinger, 2000; their equations 9bec) was used to infer from the geometry of the folded growth strata (Figs. 4 and 5b), the evolution of slip and the propagation of the Osaka-wan fault. As stated in Section 3.2, a fault dip angle of 72 was used to invert for the fault tip location (x and y), fault-propagation to fault-slip ratio (P/S), apical angle of the triangular zone or trishear angle, fault slip, and ratio of hanging wall versus total trishear angle (l) that best restores the growth strata to their pre-folding orientation. The parameter l is a measure of the asymmetry of the trishear zone. When the trishear zone is symmetric to the fault, l ¼ 0.5, giving the standard, linear in the x direction trishear velocity field (Zehnder and Allmendinger, 2000; their equation 6). A l > 0.5 indicates that the trishear zone is located preferentially toward the hanging wall, whereas when l is < 0.5, the trishear zone is located preferentially toward the footwall. In the limit where l ¼ 1.0, the hanging

wall slides rigidly pass the footwall without folding (Zehnder and Allmendinger, 2000). The fitness of the model, which is a particular combination of the above parameters, to the data is measured by an objective function (fobj) that by convention is small when the fit is good. Trishear inverse modeling is thus a minimization problem because it searches for the model that minimizes fobj. To solve this problem, a global optimization, simulated annealing routine was used (Sen and Stoffa, 2013). The specific algorithm is from the MATLAB Global Optimization ToolboxÔ and is called simulannealbnd (The MathWorks, Inc., 2013). Besides reducing significantly the time of the inversion in comparison to brute force grid-search methods (Cardozo and Aanonsen, 2009), simulated annealing has the advantage of being able to sample the parameter space for local minima. This capability means that rather than providing a single solution or best-fit model, the algorithm provides a range of models

86

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

at the scale of the figure). In the northern profile with the greatest fault slip, profile GS-8ME (Fig. 7a), the best-fit models are remarkably consistent, showing aligned initial and final fault tips, larger fault slip in older beds with the exception of bed Ma1, more rapid early propagation, constant high trishear angle (w80 ) through time, and asymmetry of the trishear zone toward the footwall (l ¼ 0.3e0.4). This consistency is despite the best-fit models being obtained from trishear inversions on different beds. In the next profile to the south with a lesser fault slip, profile OD-B (Fig. 7b), some consistency does exists among the best-fit models for the different beds, although their predicted initial and final fault tips are not as aligned as in GS-8ME. Interestingly in profile OD-B, the best-fit models suggest a trishear angle of w40e60 with a trishear zone symmetric to asymmetric toward the hanging wall (l ¼ 0.5e0.6). The next two profiles to the south, profiles HD-3 and HD-4 (Fig. 7ced), have even smaller fault slip than profile OD-B and show less consistency between the best-fit models. The best-fit models for profile HD-3 (Fig. 7c) suggest a symmetric trishear zone (trishear angle w65e85 ) in lower beds, and a much broader trishear zone asymmetric toward the footwall in upper beds. The southernmost profile HD-4 with the least fault slip (Fig. 7d) is the example where the best-fit models for the different beds are less consistent. Here the best-fit models suggest a broad trishear zone (trishear angle w80 ) that is asymmetric toward the footwall, however propagation does not show a consistent pattern.

that can fit the structure. This range is useful in trishear inverse modeling where the parameter space often has local minima and large uncertainties in the input data occur (Cardozo et al., 2011). 4. Results The analysis was first run in profiles GS-8ME, OD-B, HD-3 and HD-4 where the marine clay layers Ma-1 to Ma12 are continuous across the forelimb (Fig. 4). As a first step, trishear models were searched to best fit the beds in the profiles on a bed-by-bed basis, i.e. running separate trishear inversions for each bed. The parameter space for the group of all possible combinations that might fit a bed was defined by a range of P/S of 1.0e4.0, trishear angle of 30e 90 , fault slip of 0e2 km, and l of 0.3e0.7. In addition, the fault tip was searched within the rectangular areas shown in Fig. 4. The location and extent of these rectangular areas were defined by the offset of the top basement reflector across the fault and the discontinuity of the forelimb. An initial estimate at the center of the parameter space was used in all inversions. Fig. 7 shows the best-fit models (i.e. those with the lowest fobj) for the growth strata in each of the four profiles. Numbers on the predicted initial location of the fault tip refer to the inverted bed. In general, the agreement between the beds (gray lines in Fig. 7) and the trishear models (black lines in Fig. 7) is very good (i.e. the gray beds in Fig. 7 are almost uniformly overlain by the black model lines

0

a

b

GS-8ME

OD-B

1

2 Depth (km)

12

6

3

12

9 3

11

11 10

10 6 9

1 0

4

-1

3 0 1 -1

5

6

c

d

HD-3

HD-4

Depth (km)

0

1

2

-1 0 3

3 0

-1 12 6 11 1 9

12 6 9 1

1

2

11 10

3 x (km)

4

5

6

0

1

2

3

4

5

6

x (km)

Fig. 7. Best-fit models from inversions of beds in seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and c. HD-4. Gray lines are beds and black lines are best-fit models. Black and red circles are initial and current locations of the fault tip, respectively. Numbers on initial locations of the fault tip correspond to the inverted bed (bed numbers are as in Fig. 4). Dashed lines indicate trishear zones. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

A better picture of the significance of the trishear inversions can be obtained by plotting the distribution of their iterations (i.e. the tested trishear models). Figs. 8 and 9 show the distribution of trishear models with fobj < 1.0, an upper limit for trishear models that closely fit bed geometries. In both figures, beds are plotted on the horizontal with goodness of fit of their iterations shown by horizontal ticks that are wider for better fits. Figs. 8 and 9 show a larger spread of models for upper beds (e.g. Ma11 and Ma12), suggesting a wide range of trishear combinations that can fit the younger, less folded growth strata. Trishear inversions of older growth strata, particularly in the northern profiles with greater fault slip, are key to defining some of the trends. Possible trishear models for growth strata Ma-1 to Ma9 in profile GS-8ME clearly indicate a P/S of 3.0e4.0, and a large trishear angle of w85e90 (Fig. 8a). Inversions for the same beds in profile OD-B suggest either a high P/S of w3.0e4.0 or a lower P/S of w1.5e2.0, and a narrower trishear angle between 40 and 60 (Fig. 8b). Possible models for the older growth strata in profiles HD-3 and HD-4 suggest a high P/S of w3.0e4.0 or a lower P/S of w1.0e2.0 (with models for the oldest bed Ma-1 in HD-4 indicating a P/S w 2.5), and a wide trishear angle of 60e90 (Fig. 8c and d). Fault slip in all four profiles is well constrained with some exceptions in profile HD-3 (Fig. 8, bottom). This consistency results from having well-defined growth strata across and outside the fault-propagation fold (Fig. 4). Fault slip decreases from north (GS-8ME) to south (HD-4). Fault slip-rate varies significantly between profiles (Fig. 8, bottom row) with lower slopes of the line connecting the model points corresponding to lower rates.

87

Since fault slip through time is well constrained in the trishear inversions, the vertical position of the fault tip through time (fault slip*P/S*sine of fault dip angle) is also well constrained (Fig. 9, top). The inversions suggest that between beds Ma-1 and Ma1, the fault tip was at a depth of 4.5e6.5 km in profile GS-8ME (with most models around 5 km, Fig. 9a, top), 2.5e4.5 km in profile OD-B (Fig. 9b, top), 2.0e3.0 km in profile HD-3 (Fig. 9c, top), and 1.0e 2.0 km in profile HD-4 (Fig. 9d, top). The asymmetry of the trishear zone also varies along strike with possible models suggesting clear asymmetry of the trishear zone toward the footwall in profile GS8ME (Fig. 9a, bottom), a trishear zone slightly asymmetric toward the hanging wall to asymmetric toward the footwall in profiles HD3 and HD-4 (Fig. 9c and d, bottom), and a trishear zone asymmetric toward the hanging wall in profile OD-B (Fig. 9b, bottom). In general, profiles with a possible location of the fault tip aligned with the synclinal axial surface (profile GS-8ME and to a minor extent profiles HD-3 and HD-4, Fig. 4) require larger trishear deformation in the footwall. The northernmost profile, profile HG-1-1M (Fig. 5), has the greatest fault slip, but less continuity of growth strata across the forelimb. In fact, in this profile, older growth strata Ma-1 to Ma3 may have been offset by the fault. Trishear inversions of beds on this profile can further constrain the results. Fig. 10 shows the results of trishear inversions for profile HG-1-1M, using the same parameter limits as before and searching for the current fault tip within the rectangle of Fig. 5b. In this case, the best-fit models for the different beds are quite consistent, particularly for beds Ma0 to Ma10 (Fig. 10a) where fault slip is less in younger beds, and a

Fig. 8. Distribution of P/S (top), trishear angle (middle) and fault slip (bottom) for models that fit the beds well in seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and d. HD-4. For each bed, there are ticks of three different colors and lengths proportional to the goodness of fit. These ticks correspond to models whose difference in fobj with the best-fit model is: Below 0.2 (black long ticks), from 0.2 to 0.4 (blue intermediate ticks), and above 0.4 (red short ticks) the range in fobj of the possible models. Bed numbers are as in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

88

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

Fig. 9. Distribution of fault tip depth (yti, top), and ratio of hanging wall versus total trishear angle (l, bottom) for models that fit the beds well in seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and d. HD-4. For each bed, there are ticks of three different colors and lengths proportional to the goodness of fit. These ticks correspond to models whose difference in fobj with the best-fit model is: Below 0.2 (black long ticks), from 0.2 to 0.4 (blue intermediate ticks), and above 0.4 (red short ticks) the range in fobj of the possible models. Bed numbers are as in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. a. Best-fit models, and distribution of b. P/S, c. Trishear angle, d. Fault slip, and e. Fault tip depth (yti), for beds in seismic profiles HG-1-1M. In a, gray lines are beds and black lines are best-fit models. In b to e, for each bed, there are ticks of three different colors and lengths proportional to the goodness of fit. These ticks correspond to models whose difference in fobj with the best-fit model is: Below 0.2 (black long ticks), from 0.2 to 0.4 (blue intermediate ticks), and above 0.4 (red short ticks) the range in fobj of the possible models. Bed numbers are as in Fig. 5b. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

trishear angle is 60e90 , asymmetric toward the hanging wall, which is a result of constraining the fault tip to the backlimb of the structure (Fig. 5b). The distribution of models with fobj < 1.0 suggests a P/S of 3.0e4.0 in bed Ma0, and either 3.0e3.5 or 2.0e2.5 in bed Ma4. The trishear angle shows large spread and has values of 50e90 (Fig. 10c). With the exception of bed Ma0, which cannot be traced across the forelimb and as shown by the abrupt widening of the range of values (Fig. 5b), fault slip is well constrained (Fig. 10d). Within the uncertainties of the data, the results suggest fault slip of 0.6e0.8 km for bed Ma4 (Fig. 10d). The depth of the fault tip suggested by the inversions is between 3.0 and 7.0 km at 1120 ka (bed Ma0), and 2.5e4.0 km at 787 ka (bed Ma4, Fig. 10e). The results are not conclusive, but they suggest high fault-propagation rates between beds Ma0 and Ma4, which are greater than those in profile GS-8ME for the same period, but that then decrease during the period between beds Ma4 to Ma12. Possible models suggest a trishear zone asymmetric toward the hanging wall. l (not shown in Fig. 10) in beds Ma0 and Ma4 ranges between 0.5 and 0.6. In younger beds Ma7 to Ma11, l shows a larger spread and ranges between 0.5 and 0.7. The inversions suggest some variation in trishear parameters occurs with time, although models with constant values of these parameters can be made to generally fit the growth strata. This hypothesis was tested in the four profiles of Fig. 4 by choosing

a

possible trishear models (i.e. specific combinations of fault tip, P/S, trishear angle and l) for the oldest bed Ma-1 (this time with fobj < 0.5 to reduce the number of models), and inverting for the fault-slip increments required for each model to fit the younger beds. Figs. 11 and 12 show the results of these inversions as faultslip history (Fig. 11), or vertical fault-propagation history (Fig. 12). Gray lines in both figures are models defined as possible because they result in a decrease of fault slip with time less than or equal to 10%. Black lines are the ten models with the lowest total fobj (the sum of fobj of the inversions of all beds). In profile GS-8ME, the inversions suggest two fault-slip scenarios where current fault slip is 0.9 or 1.1 km or where the lowest fobj models favoring a current fault slip of 0.9 km and lower fault-slip rates (Fig. 11a). The P/S of the models is between 3.0 and 4.0, giving a depth of the fault tip of 5, 5.5, or 6.5 km at 1200 ka, with the lowest fobj models supporting a P/S of 3.2 and a depth of the fault tip of 5 km (Fig. 12a). In profile ODB, the inversions are more widely spread with the lowest fobj models supporting a current fault slip between 0.45 and 0.55 km (Fig. 11b), a P/S w1.5e2.0 or w3.0e3.5, and depth of the fault tip of 2.5 or 3.5 km at 1200 ka (Fig. 12b). In profile HD-3, the trishear models also show a large spread, but the lowest fobj models narrow the solution and indicate a current fault slip of 0.25 km (Fig. 11c), a P/S of 2.9, and depth of the fault tip of 2.5 km at 1200 ka (Fig. 12c). The southernmost profile HD-4 with the lowest fault slip and least

b

GS-8ME Beds

-1 0 1

3

6

89

9 10 11 12

OD-B Beds

-1 0 1

3

6

9 10 11 12 N = 243

600

400

N = 171

Fault slip (km)

1.2

0.8

0.4

0 1200

1000

800

600

400

200

0

1200

1000

800

Age (ka)

c

HD-3

-1 0 1

d

Beds 3

6

200

0

Age (ka)

9 10 11 12

HD-4

-1 0 1

Beds 3

6

9 10 11 12 N = 551

N = 183

Fault slip (km)

1.2

0.8

0.4

0 1200

1000

800

600 Age (ka)

400

200

0

1200

1000

800

600

400

200

0

Age (ka)

Fig. 11. Fault slip versus time (or bed number) in seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and d. HD-4. The graph was constructed assuming that the possible models for the oldest bed (Ma-1 in Fig. 2) were applicable to the younger beds. Gray lines are possible models. Black lines are the ten models with lowest total fobj. N is the number of models. Bed numbers are as in Fig. 4.

90

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

a

b

GS-8ME

OD-B

Beds -1 0 1 1

3

Beds

6

9 10 11 12 N = 171

600

400

-1 0 1

3

6

9 10 11 12 N = 243

600

400

yti (km)

3

5

7 1200

1000

800

200

0

1200

1000

800

Age (ka)

c HD-3 -1 0 1 1

d

Beds 3

6

200

0

Age (ka)

9 10 11 12

HD-4 Beds

-1 0 1

3

6

9 10 11 12

N = 183

N = 551

yti (km)

3

5

7 1200

1000

800

600

400

200

Age (ka)

0

1200

1000

800

600

400

200

0

Age (ka)

Fig. 12. Fault tip depth yti versus time (or bed number) in seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and d. HD-4. The graph was constructed assuming that the possible models for the oldest bed (Ma-1 in Fig. 2) were applicable to the younger beds. Gray lines are possible models. Black lines are the ten models with lowest total fobj. N is the number of models. Bed numbers are as in Fig. 4.

folding is the solution where the inversions show the largest spread. The lowest fobj models indicate a present fault slip between 0.13 and 0.23 km (Fig. 11d), a P/S between 1.0 and 3.5, and depth of the fault tip of 1.2e1.7 km at 1200 ka (Fig. 12d). Notice that in all four profiles, fault slip increases between beds Ma-1 and Ma0 in all possible models (Fig. 11). This outcome suggests that the fault was active from profiles GS-8ME to HD-4 at 1200 ka. A significant feature of Figs. 11 and 12 is the large variation in fault slip and propagation rates among profiles over distances along strike of 5e10 km. Significant variations of fault slip and propagation rates also occur in single profiles with low rate periods (e.g. from beds Ma0 to Ma1 in profile GS-8ME, and Ma11 to Ma12 in profiles GS-8ME, OD-B and HD-3), and high rate periods (e.g. from beds Ma10 to Ma11 in profiles GS-8ME, OD-B and HD-3, Figs. 11 and 12). These slower or more rapid periods of fault slip and propagation are not synchronous along strike. For example, the quiet period between beds Ma0 and Ma1 in profile GS-8ME is characterized by greater fault-slip rates in the other profiles to the south, although the low fault slip of bed Ma-1 in profile GS-8ME may be associated to uncertainties in the interpretation (Fig. 4). Likewise the rapid period between beds Ma1 and Ma3 in profile GS-8ME is characterized by low fault-slip rates as compared to in the other profiles to the south (Figs. 11 and 12).

Fig. 13 shows the evolution of the Osaka-wan fault as interpreted from the lowest total fobj (best-fit) models of Figs. 11 and 12. The fit between the data (gray lines in Fig. 13) and the trishear models (black lines in Fig. 13) is effectively one to one, which further validates the hypothesis that the growth strata can be simulated with constant trishear parameters except for fault-slip through time. The best-fit models suggest a fault that at 1200 ka propagated from a depth of 5.0 km in profile GS-8ME, 3.5 km in profile OD-B, 2.5 km in profile HD-3, and 1.6 km in profile HD-4 (Fig. 13, first column). The P/S in the profiles is around 3.0, with the exception of profile HD-4 where the P/S is 2.54. The trishear zone is either broad (profile GS-8ME) or narrow (profile OD-B), and asymmetric toward the footwall (profiles GS-8ME and HD-4) or slightly asymmetric toward the hanging wall (profiles OD-B and HD-3). The current fault slip is 0.89 km in profile GS-8ME, 0.5 km in profile OD-B, 0.26 km in profile HD-3, and 0.18 km in profile HD-4 (Fig. 13, fourth column). At 865 ka (bed Ma3), 25, 28, and 19% of the total fault slip had accumulated in profiles GS-8ME, OD-B, and HD3, respectively (Fig. 13, second column). At 427 ka (bed Ma9), 57, 58, 54, and 41% of the total fault slip had accumulated in profiles GS8ME, OD-B, HD-3, and HD-4, respectively (Fig. 13, third column). This analysis indicates steadier fault-slip rates at longer timescales of w500 ky, a behavior observed in other faults from other

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

0

a

GS-8ME

1200 ka

865 ka

91

427 ka

Present

Depth (km)

1 2 3

P/S = 3.2

4

= 0.3 Slip = 0

Slip = 0.22 km

Slip = 0.51 km

Slip = 0.89 km

Slip = 0.14 km

Slip = 0.29 km

Slip = 0.5 km

Slip = 0.05 km

Slip = 0.14 km

Slip = 0.26 km

Slip = 0.076 km

Slip = 0.185 km

5 0

b

OD-B

Depth (km)

1 2 3

P/S = 3.19

4

= 0.57 Slip = 0

5 0

c

HD-3

Depth (km)

1 2 3

P/S = 2.92

4

= 0.54 Slip = 0

5 0

d

HD-4

Depth (km)

1 2 3

P/S = 2.54

4

= 0.43 Slip = 0

5

Fig. 13. Incremental restoration of seismic profiles a. GS-8ME, b. OD-B, c. HD-3 and d. HD-4, using the lowest total fobj models of Figs. 11 and 12. Gray lines are the restored beds (data) and black lines are the trishear models. Plots have no vertical exaggeration.

extensional and contractional settings (Mouslopoulou et al., 2009). The best-fit models of Fig. 13 are by no means a unique solution to the Osaka-wan fault, but are a good representation of the structure and its evolution within the uncertainties of the data and the assumptions of the trishear inversions. 5. Discussion Fig. 14 summarizes the results of the trishear inversions in terms of the depth of the fault tip along strike and through time. The overall pattern of fault growth portrayed by the inversions is that of a fault that propagated from profile HG-1-1M (north) to profile HD4 (south) early in its history at about 1200 ka. Extrapolation of sedimentation rates to the base of the growth sequence in the footwall of the Osaka-wan fault indicates that the onset of sedimentary growth and faulting was w1500 ka. The Osaka-wan fault thus propagated rapidly along strike to its final length (in w300 ky), and then subsequently accumulated slip in the last 1200 ky. The shallow depth of the fault tip early in its history (Fig. 14) suggests

that the fault was reactivated rather than a new fault originating from the base of the seismogenic crust (e.g. Allmendinger and Shaw, 2000, their Fig. 1c). The evidence of rapid early lateral propagation of the Osaka-wan thrust, which is consistent with models for fault reactivation based on fault-scaling relationships (Walsh et al., 2002), further supports this hypothesis. The depth of the fault tip early in its history may have been influenced by the depth of the basement rock that decreases from north to south (Figs. 4 and 5, and Yokokura et al., 1998; their Fig. 10). At 1200 ka the fault had propagated from a depth of 2.3 km below the basementsediment interface in profile GS-8ME, to 1, 0.3, and 0.1 km below that interface in profiles OD-B, HD-3, and HD-4, respectively (Fig. 14). Fault-slip and fault-propagation rates show some variation among profiles and within single profiles (Fig. 14). At 1120 ka (bed Ma0), the fault had accumulated more slip and upward faultpropagation in profile HG-1-1M, than in profile GS-8ME to the south (Fig. 14). In profile HG-1-1M, fault-propagation rates are greater for beds Ma0 to Ma4 (1070e787 ka) than from beds Ma7 to

92

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

HD-3

1

OD-B

GS-8ME

HD-4

HG-1-1M

0

Pres

ent

Ma11 Ma9 Ma7

2

dash

Ma6

to ed =

p ba

e sem

nt

Depth (km)

Ma4 Ma-1 = (1200 ka)

Ma3

3

Ma0 = (1120 ka) Ma3 = (865 ka) Ma4 = (787 ka)

4

Ma0 Ma6 = (621 ka) Ma9 = (427 ka) Ma11 = 242 ka)

Ma-1

5

0

20

10

30

Distance along strike (km) Fig. 14. Fault tip depth yti along strike and through time, suggested by seven of the lowest total fobj models of Figs. 11 and 12 (black lines in these figures), and fobj < 0.25 models in profile HG-1-1M (Fig. 10). Symbols and continuous lines are defined by the best-fit (lowest fobj) trishear models (Figs. 10a and 13). The ranges of models define the error bars. Thick dashed line is the top of the basement in the footwall of the Osaka-wan fault. Data points slightly offset in x-direction on plot to increase clarity. Possible fault tip depths for Ma3 and Ma6 in profile HG-1-1M are based on the inversions of beds Ma4 and Ma7.

Ma12 (578e127 ka, Fig. 14). Large early fault-propagation rates in profiles HG-1-1M are not present in the other profiles to the south. In fact, from beds Ma-1 to Ma1 (1200e1070 ka), fault-propagation rates are similar and low from profile GS-8ME to profile HD-4 (Fig. 14). Profile GS-8ME started to build more rapid fault slip and fault-propagation at w1070 ka (bed Ma1). Fault-propagation rates from Ma1 to Ma11 (1070e242 ka) are greater in profile GS-8ME than in the other profiles to the south (Fig. 14). In general, faultpropagation rates for this time interval decrease progressively to the south (Ma1 to Ma11 rates are greater in OD-B than in HD-3, and in HD-3 than in HD-4, Fig. 14). From Ma11 (242 ka) to present faultpropagation rates are greater in profile GS-8ME than in the other profiles to the south, and possibly greater than those in profile HG1-1M to the north (Fig. 14). In summary, the modeling suggest a blind thrust that lengthened rapidly (in w300 ky) after the onset of slip (1500 ka), accumulated rapid fault slip and fault-propagation at an early stage, and then slowed down in profile HG-1-1M, and stabilized at w1000 ka, building fault slip and fault-propagation at greater rates in profile GS-8ME, than in the other profiles to the south. Additional comparisons between the history of upward propagation of the Osaka-wan blind thrust and the depth of the fault tip also yield insight into the boundary conditions that might control its development. For instance, the section of the fault with the shallowest starting depths at profile HD-4 correspond to the least amount and slowest rate of upward propagation. Conversely, the greatest starting depth at profile GS-8ME corresponds with the largest and most rapid amount of upward propagation. This coupled with the thickness of sedimentary fill in the basin, which

decreases from GS-8ME to HD-4, appears to indicate that a thicker sequence of sedimentary fill does not necessarily inhibit upward propagation, in contrast to previous models based on fault mechanics, at least for a reactivated fault (Nino et al., 1998). Another interesting aspect of the upward propagation history of the thrust is the observation that the displacement profile, (i.e. the distribution of displacement to length, or D/L) appears to differ between that at the end of the prior history of slip on the thrust and its current one. For instance, if it is assumed that shallower depths to the starting fault tip correspond to greater slip in the early displacement history of the thrust, assuming a dip slip fault with a convex upward slip profile, the most recent history of slip is greater in the opposite direction (i.e. toward the north at GS-8ME). Greater offset corresponds with a higher degree of fault maturity and lower strength, implying that older faults may accrue slip regardless of an earlier displacement history (i.e. that might correlate with more rapid propagation in the current history of fault slip and fault propagation). A more likely cause may instead be the interaction between the Osaka-wan blind thrust and the nearby Kariya fault where displacement varies between the two along strike, regardless of fault maturity. With regard to the modeling, although there is some spread in the inversions, an interesting result is that it suggests that the fault propagated with a constant P/S of 2.5e3.0 through time in all the five analyzed profiles. Being that P/S is the parameter in trishear that is most related to the strength of the rock material (Hardy and Allmendinger, 2011), a constant P/S through time suggests no major competence contrasts in the growth strata sequence. Similar values of P/S have been obtained for faults propagating through relatively

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

homogenous sedimentary sequences at scales of seismic reflection profiles (P/S of 2.6 for the Puente Hills thrust in Neogene sediments of the Los Angeles Basin, Allmendinger and Shaw, 2000), outcrop scale (P/S of 2.5 in near surface strata folded by the Chelungpu Fault in west-central Taiwan, Lin et al., 2007; Cardozo et al., 2011), and analog scale models (P/S of 2.0 in clay models of extensional faultpropagation folds, Cardozo et al., 2011). In the case of the Osakawan thrust, the P/S, however, could have been greater when the fault was propagating through basement rock, a scenario that has not been accounted for in the inversions. The trishear angle and asymmetry of the trishear zone, which are parameters that are more model-dependent and artificial than the P/S, also show some spread, more than the spread of P/S, in the inversions. A larger spread of trishear angle than P/S is a feature commonly seen in trishear inverse modeling (e.g. Allmendinger et al., 2004; Cardozo and Aanonsen, 2009; Cardozo et al., 2011). In the Osaka-wan thrust, both trishear angle and asymmetry of the trishear zone, as defined by the parameter l, change along strike. In the modeling, the variation of these parameters results from constraining the current fault tip (based on seismic data, Figs. 3e5) to be below the synclinal axial surface (e.g. profile GS-8ME, Fig. 4a), or toward the backlimb of the structure (e.g. profile HG-1-1M, Fig. 5b). In general, a current fault tip below the synclinal axial surface requires a broad trishear zone asymmetric toward the footwall (l < 0.5 and higher strain toward the footwall), while a current fault tip toward the backlimb requires a narrower trishear zone slightly asymmetric toward the hanging wall (l ¼ 0.5e0.6 and higher strain toward the hanging wall). This requirement is regardless of the uncertainties in the dip angle of the fault, but assuming that the fault has a high dip angle of w70 . What controls the position of the fault with respect to the developed fault-related fold is enigmatic, but it seems to be less dependent on the competence of the sediments than in the case of the P/S. Of the model parameters searched in the inversion, the fault slip is the one that shows the least amount of spread. This relative lack of variation is because the growth strata can be traced continuously with minor uncertainty in interpretation from the footwall to the hanging wall of the fault, across the fault-related fold. These continuous traced reflectors (Figs. 4 and 5b) provide a template for the trishear model, thus constraining fault slip in the inversions (Cardozo et al., 2011). Alternatively P:S is poorly constrained in all the profiles except in line GS-8-ME and HG-1-1M. Trishear angle is moderately to well constrained with tighter results (for all beds) in GS-8-ME. The two intermediate lines ODB and HD-3 and HG-1-1M show a systematic tightening of the model for older beds. In fact, as a general rule, profiles with greater strain, or older beds in general yield results that are more tightly constrained for any given parameter. The depth of the fault tip is well constrained for all the beds in the profiles and is an indication of the robustness of the models, a primary goal of this study. Finally, trishear asymmetry is interesting in that while well constrained, values vary markedly between adjacent profiles, indicating fundamental differences in the way shear is accommodated in the trishear envelope over the short (w5 km) distances between the seismic profiles. Results presented in this paper have implications for the assessment of seismic hazards, largely as they relate to the geometry of blind thrusts and the depth at which folding is replaced by broader shear in the forelimbs of trishear fault-propagation folds. For instance, the ability to predict the location and magnitude of folding at the ground surface as defined by trishear inversions would allow seismic hazards posed by blind thrusts to be better assessed. This includes predictions of likely surface deformation in addition to strong ground motions that depend on an understanding of fault dip and the upper limit of faulting in the brittle crust (e.g. Allmendinger and Shaw, 2000). The validity of the

93

methods outlined here can also be tested in future blind thrust earthquakes where suitable inversions of fault slip from seismologic data are available, and where sufficient subsurface data exist to image fault and fold geometry. This capability may be particularly relevant for other active blind thrusts in the Osaka region where growth strata are adequately imaged by high-resolution seismic data (Fig. 1). For blind thrusts in offshore areas, this method might also allow models of tsunami generation to be assessed by application of likely patterns of seafloor deformation, a case relevant to the Osaka-wan fault. Acknowledgments We would like to acknowledge the Geologic Survey of Japan, the Geo-Research Institute, the Hydrographic Department, the University of Tokyo, the Hyogo Prefecture (HG), and Power Reactor and Nuclear Fuel Development Corporation (NP) for collection of the high-resolution marine seismic reflection surveys, for which we use in this study. John Brandenburg and an anonymous reviewer provided helpful comments that improved the manuscript considerably. References Allmendinger, R.W., 1998. Inverse and forward numerical modeling of trishear fault-propagation folds. Tectonics 17, 640e656. Allmendinger, R.W., Shaw, J.H., 2000. Estimation of fault propagation distance from fold shape: implications for earthquake hazard assessment. Geology 28, 1099e 1102. Allmendinger, R.W., Zapata, T.R., Manceda, R., Dzelalija, F., 2004. Trishear kinematic modeling of structures, with examples from the Neuquen Basin, Argentina. In: McClay, K. (Ed.), Thrust Tectonics and Hydrocarbon Systems, American Association of Petroleum Geologists, Memoir, vol. 82, pp. 356e371. Biswas, D.K., Hyodo, M., Taniguchi, Y., Kaneko, M., Katoh, S., Sato, H., Kinugasa, Y., Mizuno, K., 1999. Magnetostratigraphy of Plio-Pleistocene sediments in a 1700m core from Osaka Bay, southwestern Japan and short geomagnetic events in the middle Matuyama and early Brunhes chrons. Palaeogeogr. Palaeoclimatol. Palaeoecol. 148, 233e248. Bullard, T.F., Lettis, W.R., 1993. Quaternary fold deformation associated with blind thrust faulting, Los Angeles Basin, California. J. Geophys. Res. 98, 8349e8369. Cardozo, N., Aanonsen, S., 2009. Optimized trishear inverse modeling. J. Struct. Geol. 31, 546e560. Cardozo, N., Allmendinger, R.W., Morgan, J.K., 2005. Influence of mechanical stratigraphy and initial stress state on the formation of two fault propagation folds. J. Struct. Geol. 27, 1954e1972. Cardozo, N., Jackson, C.A.L., Whipp, P.S., 2011. Determining the uniqueness of bestfit trishear models. J. Struct. Geol. 33, 1063e1078. Champion, J., Mueller, K., Tate, A., Guccione, M., 2001. Geometry, numerical models and revised slip rate for the Reelfoot fault and trishear fault-propagation fold, New Madrid seismic zone. Eng. Geol. 62, 31e49. Erslev, E.A., 1991. Trishear fault propagation folding. Geology 19, 617e620. Furutani, M., 1989. Stratigraphical subdivision and pollen zonation of the Middle and Upper Pleistocene in the coastal area of Osaka Bay, Japan. J. Geosci. Osaka City Univ. 32, 91e121. Gawthorpe, R., Hardy, S., 2002. Extensional fault propagation folding and base-level change as controls on growth-strata geometries. Sediment. Geol. 146, 47e56. GSJ e Geological Survey of Japan, 1996. Report on the Deep-drilling and Its Appurtenant Surveys in Higashinada, Kobe. Hardy, S., Allmendinger, R.W., 2011. Trishear: a review of kinematics, mechanics, and applications. In: McClay, K., Shaw, J., Suppe, J. (Eds.), Thrust Fault-related Folding, AAPG Memoir, vol. 94, pp. 95e119. Hardy, S., Ford, M., 1997. Numerical modeling of trishear fault propagation folding. Tectonics 16, 841e854. Huzita, K., Kasama, T., 1983. Geology of the Osaka-Seihokubu District. Geological Survey of Japan. Ikebe, N., Iwatsu, J., Takenaka, J., 1970. Quaternary geology of Osaka with special reference to land subsidence. J. Geosci. Osaka City Univ. 13, 39e98. Inoue, N., Kitada, N., Itoh, Y., Takemura, K., Nakagawa, K., 2003. Integrated study of high resolution geophysical and geological information of Osaka Bay, Southwest Japan. J. Asian Earth Sci. 22, 1e11. Ishiyama, T., Mueller, K., Togo, M., Okada, A., Takemura, K., 2004. Geomorphology, kinematic history and earthquake behavior of the active Kuwana wedge-thrust anticline, central Japan. J. Geophys. Res.-Solid Earth 109, 12408. http:// dx.doi.org/10.1029/2003JB002547. Itihara, M., 1961. Some problems of the Quaternary sediments in the Osaka and Akashi areas, Japan. Jour. Inst. Polytech. Osaka City Univ. 5, 13e30. Itihara, M. (Ed.), 1993. The Osaka Group. Sogensha, p. 340.

94

P.R. Grothe et al. / Journal of Structural Geology 58 (2014) 79e94

Itoh, Y., Takemura, K., Ishiyama, T., Tanaka, Y., 2000. Basin formation at a contractional bend of a large transcurrent fault: Plio-Pleistocene subsidence of the Kobe and northern Osaka Basins, Japan. Tectonophysics 321, 327e341. Itoh, Y., Takemura, K., Kawabata, D., Tanaka, Y., Nakaseko, K., 2001. Quaternary tectonic warping and strata formation in the southern Osaka Basin inferred from reflection seismic interpretation and borehole sequences. J. Asian Earth Sci. 20, 45e58. King, G.C.P., Brewer, J., 1983. Fault related folding near the Wind river thrust, Wyoming, USA. Nature 306, 147e150. King, G.C.P., Stein, R.S., Rundle, J.B., 1988. The growth of geological structures by repeated earthquakes, 1. Conceptual framework. J. Geophys. Res. 93, 13307e 13318. Kinugasa, Y., Mizuno, K., 1996. Underground geology of Kobe region. Tech. Rep. Jpn. Ind. Technol. Assoc. 265, 77e80. Kitada, N., Takemura, K., Ito, K., Saito, R., Miyakawa, C., Mitamura, M., Nanayama, F., Iwabuchi, Y., 2001. Stratigraphic correlation between the OB-1 and OB-2 cores and its application to the activity evaluation of the Osaka-wan fault. Annu. Rep. Active Fault Paleoearthq. Researches 1, 153e166. Lin, M.L., Wang, C.P., Chen, W.S., Yang, C.N., Jeng, F.S., 2007. Inference of trishearfaulting processes from deformed pregrowth and growth strata. J. Struct. Geol. 29, 1267e1280. Mouslopoulou, V., Walsh, J.J., Nicol, A., 2009. Fault displacement rates on a range of timescales. Earth Planet. Sci. Lett. 278, 186e197. Nino, F., Philip, H., ChÈry, J., 1998. The role of bed-parallel slip in the formation of blind thrust faults. J. Struct. Geol. 20, 503e516. Sagiya, T., 2004. A decade of GEONET: 1994-2003 eThe continuous GPS observation in Japan and its impact on earthquake studies. Earth Planets Space, 29e41. Sato, H., Hirata, H., Ito, T., Tsumura, N., Ikawa, T., 1998. Seismic reflection profiling across the seismogenic fault of the 1995 Kobe earthquake, southwestern Japan. Tectonophysics 286, 19e30.

Sen, M.K., Stoffa, P.L., 2013. Global Optimization Methods in Geophysical Inversion, second ed. Cambridge University Press, p. 304. Shaw, J.H., Plesch, A., Dolan, J.F., Pratt, T.L., Fiore, P., 2002. The Puente Hills blindthrust system, Los Angeles, California. Bull. Seismol. Soc. Am. 92, 2946e2960. Stein, R.S., King, G.C.P., 1984. Seismic potential revealed by surface folding; 1983 Coalinga, California, Earthquake. Science 224, 869e872. Stein, R.S., Yeats, R.S., 1989. Hidden earthquakes. Scientif. Am. 260, 48e57. Suppe, J., 1979. Structural interpretation of the southern part of the northern Coast Ranges and Sacramento Valley, California. Geol. Soc. Am. Bull. 90, 327e330. Taboada, A., Bousquet, J.C., Philip, H., 1993. Coseismic elastic models of folds above blind thrust faults in the Betic Cordilleras (Spain) and evaluation of seismic hazard. Tectonophysics 220, 223e241. Takatsugi, K.O., Hyodo, M., 1995. A geomagnetic excursion during the late Matuyama chron, the Osaka group, southwest Japan. Earth Planet. Sci. Lett. 136, 511e524. The MathWorks, Inc, 2013. Global Optimization Toolbox, User’s Guide. Walsh, J.J., Nicol, A., Childs, C., 2002. An alternative model for the growth of folds. J. Struct. Geol. 24, 1669e1675. Yokokura, T., 1999. Seismic investigation of an active fault off Kobe; another disaster in the making? Lead. Edge 18, 1417e1421. Yokokura, T., Kano, N., Yamaguchi, K., Miyazaki, T., Ikawa, T., Ohta, Y., Kawanaka, T., Abe, S., 1998. Seismic profiling of deep geological structure in the Osaka Bay area. Bull. Geol. Surv. Jpn. 49, 571e590. Yokokura, T., Yamaguchi, K., Kano, N., Miyazaki, T., Ikawa, T., Ohta, Y., Kawanaka, T., Abe, S., 1999. Seismic profiling of deep geological structure in Kobe and Ashiya areas, Japan. Bull. Geol. Surv. Jpn. 50, 245e267. Yoshikawa, S., Tsukuda, E., Mitamura, M., Nakagawa, K., Mizuno, K., Higashiwaki, A., Kataoka, K., Takahashi, M., 1997. Litho- and tephrostratigraphy of the Yuhigaoka boring cores from the Osaka Plain, central Japan. Bull. Geol. Surv. Jpn. 48, 661e672. Zehnder, A.T., Allmendinger, R.W., 2000. Velocity field for the trishear model. J. Struct. Geol. 22, 1009e1014.