Characteristics on Fault Coupling Along the Solomon Megathrust Based On

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Characteristics on fault 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 earthquakes 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, aftershock 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 earthquake 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 hypocenter 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 information). 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 aftershocks 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 characterized by rapid subduction of young Woodlark Basin oceanic crust. The AUP currently moves northwestward 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

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