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RESEARCH LETTER Characteristics on coupling along the Solomon 10.1002/2016GL070188 megathrust based on GPS observations Key Points: from 2011 to 2014 • We deployed the first continuous GPS network at the Western Solomon Yu-Ting Kuo1, Chin-Shang Ku1,2, Yue-Gau Chen1, Yu Wang3, Yu-Nung Nina Lin4, Ray Y. Chuang5, Islands since 2011 2 6 2 1,2 • GPS record reveals significantly Ya-Ju Hsu , Frederick W. Taylor , Bor-Shouh Huang , and Hsin Tung

different interseismic coupling ratios 1 2 between two adjacent segments on Department of Geosciences, National Taiwan University, Taipei, Taiwan, Institute of Earth Sciences, Academia Sinica, the Solomon megathrust Taipei, Taiwan, 3Earth Observatory of Singapore, Nanyang Technological University, Singapore, 4CGG, Singapore, • We identify a semipermanent asperity 5Department of Geography, National Taiwan University, Taipei, Taiwan, 6Institute for Geophysics, University of Texas at and a potential barrier to rupture, Austin, Austin, Texas, USA each corresponding to the subduction of geological features Abstract The Solomon megathrust along the western Solomon arc generated two megathrust

Supporting Information: in the past decade (Mw 8.1 in 2007 and Mw 7.1 in 2010). To investigate the interseismic deformation and • Supporting Information S1 inferred coupling on the megathrust, we deployed the first continuous GPS network in the Western Solomon Islands. Our 2011–2014 GPS data and the back slip inversion model show coupling ratio as high as 73% along Correspondence to: the southeastern 2007 rupture segment but only 10% on average along the segment of 2010 event. Based on Y.-G. Chen, [email protected] the spatial distribution of coseismic slip, clusters, derived coupling pattern, and paleogeodetic records, we discovered the former as a semipermanent asperity and the latter as a potential megathrust

barrier. We propose that a characteristic of magnitude not less than Mw 8 will recur in an interval Citation: Kuo, Y.-T., C.-S. Ku, Y.-G. Chen, Y. Wang, of 100 or more years by either single or doublet earthquake. Y.-N. N. Lin, R. Y. Chuang, Y.-J. Hsu, F. W. Taylor, B.-S. Huang, and H. Tung 1. Introduction (2016), Characteristics on fault coupling along the Solomon megathrust based Megathrusts developed along convergent plate boundaries are usually characterized by heterogeneous on GPS observations from 2011 to 2014, Geophys. Res. Lett., 43, doi:10.1002/ segmentations due to variations in fault geometry and frictional properties [e.g., Song and Simons, 2003; 2016GL070188. Wells et al., 2003; Loveless et al., 2010]. The subduction of particularly high geological features, like ridges or seamounts, is commonly associated with segmentation boundaries [e.g., Ichinose et al., 2007; Perfettini Received 27 JUN 2016 et al., 2010; Chlieh et al., 2011]. Among all regions with seamount subduction the Solomon megathrust is Accepted 12 AUG 2016 Accepted article online 15 AUG 2016 one of the least studied due to its geopolitical marginality and relative seismic quiescence as compared to other similar systems like Sumatra and Chile. Two earthquakes, Mw 8.1 in 2007 and Mw 7.1 in 2010, struck the islands and illuminated the need to study the seismic physics along the megathrust [Taylor et al., 2008; Chen et al., 2009; Miyagi et al., 2009; Newman et al., 2011]. It remains uncertain whether the Solomon megathrust behaves predominately aseismic or not [Cooper and

Taylor, 1987]. The 2007 Mw 8.1 earthquake revealed the coupled nature at least along a certain segment on the megathrust [Miyagi et al., 2009]. Three years later, the 2010 Mw 7.1 earthquake near the of 2007 earthquake and produced a damaging tsunami [Newman et al., 2011]. The spatial distribution between the aseismic segments and the coseismic slip patches, together with the prominent bathymetric features on the seafloor and the implied roughness along the plate interface, provides us insight into the seismic physic and cycles [e.g., Bilek et al., 2003; Taylor et al., 2005; Wang and Bilek, 2011; Feng et al., 2012; Wang and Bilek, 2014; Morell, 2016]. To monitor the hanging wall deformation associated with the postearthquake slip on the Solomon megathrust, we installed 10 continuous Global Positioning System (GPS) stations on the Western Solomon Islands (Figure 1). Among them the site on Simbo Island is located on the downgoing plate only ~7 km from the trench. The proximity of these islets to the trench front, and the high plate convergence rate between the Australian Plate (AUP) and the Pacific Plate (PAP), offers a unique opportunity to monitor the crustal deformation even at the shallowest portion of the megathrust. In this paper, we present the GPS time series and secular velocities from 2011 to 2014 in the IGS08 reference frame with respect to the PAP (Figures 2 and 3 and Table 1 and Figure S1 and Table S1 in the supporting infor- mation). The secular velocities are further modeled by using the back slip approach in an elastic half space to evaluate the current coupling behavior on the Solomon megathrust. We compare our interseismic coupling

©2016. American Geophysical Union. pattern with the slip distribution and of the 2007 and 2010 earthquakes and discuss the relation- All Rights Reserved. ship between the current coupling pattern and the megathrust rupture patches.

KUO ET AL. PLATE COUPLING, WESTERN SOLOMON ISLANDS 1 Geophysical Research Letters 10.1002/2016GL070188

Figure 1. (a) Map displaying the bathymetry and plate tectonic features around the Solomon Islands (red box represents region shown in Figure 1b). (b) Plate tectonic setting for the Western Solomon Islands and the inferred rupture zones of the 2007 and 2010 earthquakes. The 2007 earthquake ruptured from the west of Rendova Island (Rov) and propagated northwestward to the triple junction between the New Britain Trench and Woodlark Ridge. The 2010 rupture is limited to the area close to the San Cristobal Trench between Rondova Island (Rov) and Tetepare Island (Tet). Black arrows indicate the motion of the Australian Plate (AUP) and the Woodlark Plate (WLP) with respect to the Pacific Plate (PAP) in ITRF2008.

2. Tectonic Setting The Solomon Islands are located in an active plate boundary, where the AUP and associated microplates (i.e., Woodlark Plate (WLP) and Solomon Sea Plate (SSP)) are underthrusting beneath the PAP (Figure 1a) [Demets et al., 1990; 1994; Beavan et al., 2002; Miura et al., 2004; Phinney et al., 2004; Taira et al., 2004; Taylor et al., 2005, 2008; Argus et al., 2011; Newman et al., 2011]. Along the southeastern Solomon Islands, the AUP is subducting

KUO ET AL. PLATE COUPLING, WESTERN SOLOMON ISLANDS 2 Geophysical Research Letters 10.1002/2016GL070188

Figure 2. (a) East, (b) north, and (c) up component of the cGPS time series in the IGS08 reference frame. The annual rates with twice the root-mean-square error above the trendline (in black). See Figure 1b for station locations.

obliquely beneath the Solomon Arc, having generated the San Cristobal Trench (Figure 1b). West of the Simbo Ridge, on the other hand, the WLP subducts and forms the New Britain Trench [Taylor and Exon, 1987; Crook and Taylor, 1994; Taylor et al., 1995; Mann et al., 1998; Taylor et al., 2005]. Altogether, these two trenches manifest the 1000 km long Solomon megathrust system, which has produced major earthquakes

in the past decades, including the last two of Mw 8.1 and Mw 7.1 earthquakes in 2007 and 2010, respectively. Unlike other well-known megathrusts (e.g., the Sunda megathrust), the western Solomon megathrust is char- acterized by rapid subduction of young Woodlark Basin oceanic crust. The AUP currently moves northwest- ward at 97 mm/yr with respect to the PAP in the reference frame of ITRF2008 [Demets et al., 1994]. Less than 5 Myr old oceanic crust is characterized by rugged seafloor topography, including two volcanic seamounts, the Kana Keoki and the Coleman, located in front of the trench south of the Rendova Island (Figure 1b) [Mann et al., 1998; Taylor et al., 2005]. The northern extension of the seamounts has probably been subducted beneath the fore arc, resulting in rapid uplift (5–7 mm/yr) near the trench [Mann et al., 1998; Taylor et al., 2005]. The intermittent eruption of the volcano Kavachi (Figure 1a) just northeast of the trench may be attrib- uted to the subduction of volcanic seamount (Figure 1b) [Mann et al., 1998; Taylor et al., 2005]. Previous stu- dies have suggested that the development and even the formation of the Solomon Islands, in particular the trench-front islets, are largely controlled by the subduction of various young bathymetric features (Figure 1b) [Mann et al., 1998; Taylor et al., 2005, 2008].

3. The 2007 and 2010 Earthquakes

A Mw 8.1 earthquake struck Western Solomon Islands on 1 April 2007. Analyses based on teleseismic data suggest that the rupture started from the northwestern San Cristobal Trench near Rendova Island, ramped over the trench-trench-transform triple junction to the west of the Simbo Ridge, propagated toward the southeastern New Britain Trench, and terminated at another triple junction where the Woodlark Rise is being subducted Figure 3. Map displaying the vertical (red vector) and horizontal (blue (Figure 1) [Taylor et al., 2008; Furlong vector) cGPS rates in mm/yr from 2011 to 2014. Vectors in ITRF2008 et al., 2009]. Miyagi et al. [2009] found stable Pacific reference frame [Altamimi et al., 2012]. large slip on the megathrust near the

KUO ET AL. PLATE COUPLING, WESTERN SOLOMON ISLANDS 3 Geophysical Research Letters 10.1002/2016GL070188

Table 1. The Vectors in ITRF2008 Stable Pacific Reference Frame [Altamimi et al., 2012] and the Result From Model in Three Componentsa ITRF2008-PAP The Result From the Optimal Model

Site North (mm/yr) East (mm/yr) Up (mm/yr) North (mm/yr) East (mm/yr) Up (mm/yr)

SEGE À3.4 ± 0.5 8.6 ± 0.9 À8.2 ± 1.3 À1.0 6.2 À0.1 MUDA 0.0 ± 1.7 25.0 ± 1.7 0.2 ± 5.0 3.6 19.5 À1.2 HOPO À4.9 ± 2.3 À4.0 ± 2.5 2.3 ± 6.4 3.2 9.9 À1.6 RIGI 10.5 ± 1.4 18.9 ± 1.4 À6.4 ± 3.6 5.1 24.6 À3.2 NUSU À3.0 ± 1.4 44.4 ± 1.4 À11.6 ± 4.7 11.4 36.0 À12.9 LALE 29.2 ± 0.6 72.5 ± 0.6 À9.3 ± 1.1 21.6 56.0 À21.9 MARA 9.3 ± 1.0 23.9 ± 1.5 À13.8 ± 3.8 10.6 28.6 À9.9 BULI 23.8 ± 0.8 31.2 ± 0.8 À23.2 ± 1.4 17.3 38.7 À17.2 TEPA 14.2 ± 1.2 0.4 ± 1.5 25.1 ± 3.3 2.6 3.9 À0.2 SIBO 43.75 ± 0.6 78.78 ± 06 À6.9 ± 0.9 aThe parameters for absolute plate rotation pole of PAP in ITRF2008 are À0.411 ± 0.007 (wx (mas/a)), 1.036 ± 0.007 (wy (mas/a)), À2.166 ± 0.009 (wz (mas/a)), 0.677 ± 0.002 (w (∘/Ma)), 0.42 (WRMS of east), and 0.44 (WRMS of north).

extension of the WLP-AUP-PAP triple junction (Figure 1b), with maximum slip of ~10 m around Ranongga Island, close to the centroid reported by the global centroid moment tensor (CMT) (Figure 1b) [Miyagi et al., 2009]. Miyagi et al. [2009] further suggested that this slip pattern results from the strong coupling associated with the rugged bathymetry along the plate boundary.

Another Mw 7.1 earthquake struck the Western Solomon Islands along the San Cristobal Trench in 2010, generating severe landslide and tsunami hazards on nearby islands (Figure 1b) [Newman et al., 2011]. A postearthquake survey revealed coseismic ground subsidence even on the trench-front islands, suggesting that the 2010 rupture must have been limited to the area between the islands and the trench. The Global CMT solution and the joint inversion results based on tsunami runup and coastal uplift data also confirm the low-angle thrusting nature with a narrow rupture width (<20 km) [Newman et al., 2011].

4. GPS Network and Processing Ten GPS stations were set up between 2009 and 2012. Nine were deployed on the Solomon Arc and one on the Simbo Ridge where is actually situated on the subducting AUP (SIBO; Figure 1b). We analyzed time series data between 2011 and 2014, using the GIPSY-OASIS software (GOA II) [Blewitt, 1989; Zumberge et al., 1997]. This package includes effect corrections of the International GPS Service (IGS) final orbit and clock informa- tion, the antenna calibration, the first order ionospheric bias, the residual wet and dry tropospheric delays, ocean loading, etc. The point positions are computed in a global navigation satellite system IGS08, a free-network reference frame that undertakes specific transformation parameters to obtain positions (Figures 2 and S1 and Table S1) [Dow et al., 2009]. Figure 2 shows the time series results from our GPS network in the IGS08 reference frame. Logistical difficulties on remote islands and unstable power supply limited the actual recording period and continuity of data. Thanks to the rapid convergence rate of ~97 mm/yr, nonethe- less, most time series records still demonstrate significant linear trends in all three components, with larger seasonal variations as well as observational uncertainties in the vertical component. The secular velocities were originally computed in the IGS08 reference frame (Figure S2) and then trans- formed into the PAP reference frame (Figure 3) by using the ITRF2008 plate rotation model from Altamimi et al. [2012]. A lateral gradient of decreasing velocities is found in the Solomon Arc from northwest to south- east (Figure 3). Toward the northwestern end, the trench-front station LALE shows similar motion as the station SIBO in the AUP, suggesting the overriding plate being coupled and dragged by the motion of the underthrusting plate. The secular velocities gradually decrease away from the trench, indicating decreased coupling at depth. To the southeastern end, smaller rates and incoherent motion in the secular velocities suggest that these stations are related more to the stable PAP and not coupled to the underthrusting AUP. The differences in secular rates suggest both along-strike and downdip variation of interseismic coupling on the Solomon megathrust. We use these secular velocities to constrain a back slip model and quantitatively assess the spatial variation of coupling on the subduction interface.

KUO ET AL. PLATE COUPLING, WESTERN SOLOMON ISLANDS 4 Geophysical Research Letters 10.1002/2016GL070188

Figure 4. Map displaying the back slip inversion model and the comparison of the GPS velocities for modeled and observed. (a) Values and vectors of back slip for each patch. (b) Back slip model overlain by coseismic slip contours and aftershocks. The 2007 and 2010 are shown in red stars. Aftershocks of 2007 and 2010 earthquakes from U.S. Geological Survey (USGS) catalog (M > 2.5, within 1 year of the main shock) are shown in blue and cyan circles, respectively. The fault polygons and slip contours are shown in black and purple for the 2007 earthquake [Furlong et al., 2009] and the 2010 earthquake [Newman et al., 2011], respectively. (c) Observed and modeled vectors in horizontal component. (d) Observed and modeled vectors in vertical component. 5. Back Slip Model A simplified fault geometry is defined for the megathrust used in the back slip inversion model [Savage, 1983; Hsu et al., 2012] due to lack of constraints from either background seismicity or exploration seismic profiles. We use the strike of the San Cristobal Trench (N55°W) and assigned an uniform dip angle of 20° by averaging the shallow part of the Slab 1.0 model (0–20 km at depth) [Hayes et al., 2012]. This geometry is identical to that

of the 2007 Mw 8.1 Solomon earthquake from Miyagi et al. [2009]. The Green's function is computed based on dislocations in a homogeneous elastic half space [Okada, 1985] with Poisson's ratio of 0.25. To constrain the model roughness, we incorporated a Laplacian smoothing operator [Harris and Segall, 1987] in our inversion model, with the weight determined by a cross-validation method [Matthews and Segall, 1993]. We also constrained the slip direction being right-lateral and downdip, consistent with the relative motion of GPS stations as well as the motion of the AUP with respect to the stable PAP (Figure 3). After taking all above constraints into account, we finally chose a fault model consisting of 24 35 × 35 km patches, derived by the trade-off between the weighted root-mean-square (WRMS) of data misfit and the model roughness (Figure S3). Figure 4 shows our optimal model. This model does not fit the observation perfectly, yet it fulfills the cross- validation test (Figures 4c, 4d, and S3 and Table 1) and therefore should represent the best prediction within

KUO ET AL. PLATE COUPLING, WESTERN SOLOMON ISLANDS 5 Geophysical Research Letters 10.1002/2016GL070188

the data uncertainties and model variance as asserted by the roughness constraint. According to this model, there is large oblique back slip on the Solomon megathrust at the northwestern section of the fault, ranging from 31 to 71 mm/yr (Figure 4b). The azimuth of these back slip vectors generally matches the AUP-PAP plate-motion vector (Figure 4a). The back slip velocities gradually diminish to less than 10 mm/yr at the south- eastern part of the model, revealing dramatic lateral variation along the megathrust. However, that the misfit on magnitude and azimuth in between observed and modeled may imply some localized geological effects as the detail of the fault geometry or the effect of the fluid other than the simplified fault model used in this study. For site HOPO the significant misfit on moving direction may be caused by the contribution source other than interseismic.

6. Discussion 6.1. Asperity and Barrier on the Megathrust Our back slip model shows significantly different rates between the segments ruptured in 2007 and 2010, respectively. For the southeastern part of the 2007 segment, covered by our back slip model, the maximum back slip is 70.8 mm/yr (Figure 4b), equivalent to 73% coupling given the plate convergent rate of 97 mm/yr [Demets et al., 1994]. The strongly coupled patches are located from the trench all the way down to ~24 km depth, in between the two larger coseismic slip patches of the 2007 earthquake (Figure 4b) [Taylor et al., 2008; Chen et al., 2009; Furlong et al., 2009; Miyagi et al., 2009]. The facts that this region also has relatively few after- shocks, and only several years after the main shock the GPS-observed result shows high coupling ratios, prob- ably suggest that the stress on the plate interface was not fully released during the 2007 event. This indicates that there is potential for a strong earthquake. Coral records on the Rannonga Island also reveal coseismic uplifts in the past 4000 years that are larger than the 2007 readings [Thirumalai et al., 2015]. Jointly taking into account the current coupling pattern and paleogeodetic records, we thus define the semipermanent nature of this asperity. For the 2010 segment, our GPS data show nearly uncoupled motion between the two plates, with an average back slip of ~5 mm/yr, equivalent to <10% coupling. These patches are also covered by dense aftershocks of both the 2007 and 2010 earthquakes (Figure 4b). This is consistent with the observations of other earthquakes (e.g., Nias earthquake [Hsu et al., 2006] and Maule earthquake [Lin et al., 2013]), in which afterslip takes place on nearby patches after the main shock. The change in the slope recorded by the stations TEPA and HOPO between mid-2012 and early 2013 through the start time and total duration of our data do not allow a robust fit to postseismic decay. However, we are currently observing a transition from the postseismic to an interseis- mic period, when the elastic surface deformation is nearly zeroed, leading to apparent low coupling ratios. It is possible that this segment may be still situated in a period of gradual recovery of a higher coupling ratio. The downdip termination of the coseismic slip patches, together with the aftershocks distribution edges, helps decipher the eastern and the deeper part of the 2010 segment as a seismic barrier, where large rupture initiates/terminates and afterslip tends to occur. Analogous examples have been reported in the Mejillones Peninsula in northern Chile [Pritchard and Simons, 2006; Victor et al., 2011] and the Arauco Peninsula in southern-central Chile [Lin et al., 2013]. In summary, we identify a semipermanent asperity (i.e., southeastern 2007 rupture) and a potential barrier (i.e., 2010 rupture) along the Solomon megathrust. The asperity coincides with subduction of the Simbo Ridge and the associated transform fault, whereas the boundary between the asperity and the barrier strongly correlates to the seamount subduction (Figure 4). This observation seems self-contradictory, just like what is being observed along several other subduction zones [Kelleher, 1972; Kanamori and Mcnally, 1982; Kodaira et al., 2003, 2004; Chlieh et al., 2011]. To really understand how the geological features controls the asperity/barrier effect and even frictional properties on the megathrust, more data are needed to acquire in the future, including detailed megathrust geometry, composition of the accretionary prism, geochemistry and heat flow associated with the oceanic crust features, and the most essential paleoseismology records that can be utilized to reasonably fit evidence into the historical and statistical framework.

6.2. Earthquake Scenario Estimation By assuming our model is able to represent the characteristic coupling pattern along the 2007 segment of the Solomon megathrust, we estimate earthquake scenarios based on the coseismic moment release [Heaton

KUO ET AL. PLATE COUPLING, WESTERN SOLOMON ISLANDS 6 Geophysical Research Letters 10.1002/2016GL070188

and Heaton, 1989] and our current back slip model. With average crustal rigidity of 30 GPa, a Mw 8 event should occur every 69 years. However, in reality the largest historic earthquake from 1900 to 2007 is merely

Mw 7.2 [Cooper and Taylor, 1987; Chen et al., 2011]. The recurrence interval of an earthquake of Mw 8.1 is estimated as 107 years (1900–2007). These estimates suggest either our high coupling ratio is an overshoot,

or that we are expecting another Mw 8.1 earthquake within this asperity patch alone or even a bigger one if the slip is to propagate northwestward or southeastward outside this patch, or a fraction of accumulated

strain may be released by aseismic slip events. The latter scenario, with Mw 8.1 as the lower bound, agrees with the conclusion from the aforementioned paleogeodetic observations. Actually, among the six major events in the coral records, some events were temporally close enough to suggest doublet earthquakes [Thirumalai et al., 2015]. We therefore propose a longer recurrence interval (>100 years) with larger coseismic

moment release (no less than Mw 8.1) in either single or doublet form as the most likely earthquake scenario along this part of the Solomon megathrust.

7. Conclusion Our GPS measurements from 2011 to 2014 reveal a spatially variant slip-coupling pattern for the 2007 and 2010 earthquake corresponding megathrust segment. After synthesizing the prominent coseismic slip patch, aftershock distribution and even paleogeodetic records, we conclude that the high coupling patch on the 2007 rupture related segment probably represents a semipermanent asperity developed on the megathrust, whereas the currently low coupling patch on the 2010 rupture related segment perhaps shows only the signal of a seismic barrier. The location of the asperity and the barrier correlate with subducting oceanic crust bathymetric features, while the exact controlling mechanism remains unknown. The inferred characteristic earthquake scenario for the study area can be hypothesized as a single or doublet earthquake with magni-

tude no less than Mw 8 in a recurrence interval of 100 or more years.

Acknowledgments References This work is funded by MOST grants 100-2116-M-002-012-MY3 and Altamimi, Z., L. Métivier, and X. Collilieux (2012), ITRF2008 plate motion model, J. Geophys. Res., 117, doi:10.1029/2011JB008930. 104-2116-M-001-013 and NSF grant Argus, D. F., R. G. Gordon, and C. DeMets (2011), Geologically current motion of 56 plates relative to the no-net-rotation reference frame, EAR 1119211. We thank the Geochem. Geophys. Geosyst., 12, doi:10.1029/2011GC003751. fi Section of the Solomon Islands Beavan, J., P. Tregoning, M. Bevis, T. Kato, and C. Meertens (2002), Motion and rigidity of the Paci c Plate and implications for plate boundary – Ministry of Mines, Energy, and Rural deformation, J. Geophys. Res., 107(B10), ETG 19-1 ETG 19-15, doi:10.1029/2001JB000282. fl – Electrification and the government of Bilek, S. L., S. Y. Schwartz, and H. R. DeShon (2003), Control of sea oor roughness on earthquake rupture behavior, Geology, 31(5), 455 458. the Western Province for the support. Blewitt, G. (1989), Carrier phase ambiguity resolution for the Global Positioning System applied to geodetic baselines up to 2000 km, – We are grateful to Li-Wei Kuo, J. Geophys. Res., 94(B8), 10,187 10,203. Horng-Yue Chen, Kuan-Chuan Lin, Yu-Ju Chen, M.-C., C. Frohlich, F. W. Taylor, G. Burr, and A. Q. van Ufford (2011), Arc segmentation and seismicity in the Solomon Islands arc, SW fi – Kuo, and John Suppe for their useful Paci c, Tectonophysics, 507,47 69. comments and assistance in data Chen, T., A. V. Newman, L. Feng, and H. M. Fritz (2009), Slip distribution from the 1 April 2007 Solomon Islands earthquake: A unique image of collecting and processing. We near-trench rupture, Geophys. Res. Lett., 36, doi:10.1029/2009GL039496. appreciate the Editor, Jeroen Ritsema, Chlieh, M., H. Perfettini, H. Tavera, J. P. Avouac, D. Remy, J. M. Nocquet, F. Rolandone, F. Bondoux, G. Gabalda, and S. Bonvalot (2011), and two anonymous reviewers for their Interseismic coupling and seismic potential along the Central Andes subduction zone, J. Geophys. Res., 116, doi:10.1029/2010JB008166. constructive and detailed comments Cooper, P., and B. Taylor (1987), The spatial distribution of earthquakes, focal mechanisms and subducted lithosphere in the Solomon Islands that have greatly improved and in Marine Geology, Geophysics and Geochemistry of the Woodlark Basin-Solomon Islands, Earth Sci. Ser., vol. 7, edited by B. Taylor and N. F. strengthened the paper. The software Exon, pp. 67-88, Circum-Pac. Counc. Energy and Mineral. Resour., Houston, Tex. GIPSY-OASIS II and precise GPS orbit Crook, K. A. W., and B. Taylor (1994), Structure and Quaternary tectonic history of the Woodlark triple junction region, Solomon-Islands, Mar. – products of JPL have both made Geophys. Res., 16(1), 65 89. – significant contribution. The Demets, C., R. G. Gordon, D. F. Argus, and S. Stein (1990), Current Plate Motions, Geophys. J. Intl., 101(2), 425 478. topography and the seismic data shown Demets, C., R. G. Gordon, D. F. Argus, and S. Stein (1994), Effect of recent revisions to the geomagnetic reversal time-scale on estimates of – in this study are accessible to the Marine current plate motions, Geophys. Res. Lett., 21(20), 2191 2194, c. Geoscience Data System and the U.S. Dow, J. M., R. E. Neilan, and C. Rizos (2009), The international GNSS service in a changing landscape of Global Navigation Satellite Systems, – Geological Survey, respectively. In J. Geod., 83, 191 198. addition, if original observation data are Feng, L. J., A. V. Newman, M. Protti, V. Gonzalez, Y. Jiang, and T. H. Dixon (2012), Active deformation near the Nicoya Peninsula, northwestern interested please directly contact with Costa Rica, between 1996 and 2010: Interseismic megathrust coupling, J. Geophys. Res., 117, doi:10.1029/2012JB009230. – authors. Furlong, K. P., T. Lay, and C. J. Ammon (2009), A great earthquake rupture across a rapidly evolving three-plate boundary, Science, 324, 226 229. Harris, R. A., and P. Segall (1987), Detection of a locked zone at depth on the Parkfield, California, segment of the San-Andreas Fault, J. Geophys. Res., 92(B8), 7945–7962. Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, doi:10.1029/2011JB008524. Heaton, T. H., and R. E. Heaton (1989), Static deformations from point forces and force couples located in welded elastic Poissonian half-spaces—Implications for seismic moment tensors, Bull. Seismol. Soc. Am., 79(3), 813–841. Hsu, Y. J., M. Simons, J. P. Avouac, J. Galetzka, K. Sieh, M. Chlieh, D. Natawidjaja, L. Prawirodirdjo, and Y. Bock (2006), Frictional afterslip following the 2005 Nias-Simeulue earthquake, Sumatra, Science, 312, 1921–1926. Hsu, Y. J., S. B. Yu, T. R. A. Song, and T. Bacolcol (2012), Plate coupling along the Manila subduction zone between Taiwan and northern Luzon, J. Asian Earth Sci., 51,98–108.

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