remote sensing Article Deformation and Source Parameters of the 2015 Mw 6.5 Earthquake in Pishan, Western China, from Sentinel-1A and ALOS-2 Data Yangmao Wen 1,2,*, Caijun Xu 1,2, Yang Liu 1,2 and Guoyan Jiang 2 1 School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China; [email protected] (C.X.); [email protected] (Y.L.) 2 Collaborative Innovation Center of Geospatial Technology, Wuhan University, Wuhan 430079, China; [email protected] * Correspondence: [email protected]; Tel.: +86-27-6877-1610 Academic Editors: Cheinway Hwang, C.K. Shum, Stéphane Calmant, Norman Kerle and Prasad S. Thenkabail Received: 19 December 2015; Accepted: 4 February 2016; Published: 8 February 2016 Abstract: In this study, Interferometric Synthetic Aperture Radar (InSAR) was used to determine the seismogenic fault and slip distribution of the 3 July 2015 Pishan earthquake in the Tarim Basin, western China. We obtained a coseismic deformation map from the ascending and descending Sentinel-1A satellite Terrain Observation with Progressive Scans (TOPS) mode and the ascending Advanced Land Observation Satellite-2 (ALOS-2) satellite Fine mode InSAR data. The maximum ground uplift and subsidence were approximately 13.6 cm and 3.2 cm, respectively. Our InSAR observations associated with focal mechanics indicate that the source fault dips to southwest (SW). Further nonlinear inversions show that the dip angle of the seimogenic fault is approximate 24˝, with a strike of 114˝, which is similar with the strike of the southeastern Pishan fault. However, this fault segment responsible for the Pishan event has not been mapped before. Our finite fault model reveals that the peak slip of 0.89 m occurred at a depth of 11.6 km, with substantial slip at a depth of 9–14 km and a near-uniform slip of 0.2 m at a depth of 0–7 km. The estimated moment magnitude was approximately Mw 6.5, consistent with seismological results. Keywords: radar interferometry; Tibetan Plateau; Pishan earthquake; rupture model; thrust faulting; seismic hazard 1. Introduction On 3 July 2015 (09:07:46 GMT+8), a Mw 6.5 earthquake struck Pishan in Xinjiang, western China (Figure1), causing at least four deaths, 48 injuries and hundreds of building collapses. The epicenter was located in the boundary between the southwestern Tarim Basin and the northwestern Tibetan Plateau. The collision between them, due to the northward push from the Indian plate and the subduction of the Tarim Basin, led to the development of the Pamir Plateau, the Karakoram and the west Kunlun orogens, which are associated with many well-known active tectonic features such as the Altyn Tagh fault system [1], where the 2008 Mw 7.1 Yutian earthquake occurred (Figure1). However, there are no active Holocene faults within the approximately 50 km region surrounding the 2015 Pishan earthquake, according to the map of active tectonics in China [2]. Earthquakes with well-determined source processes would help us better understand the tectonic behaviors of the Tarim and surrounding regions. However, none of the historical earthquakes in the region of Figure1 have received adequate geodetic coverage. Fortunately, complete Synthetic Aperture Radar Interferometry (InSAR) coverage of the 2015 Pishan event provides a unique opportunity to determine the location, geometry, and focal mechanism of the seismogenic fault, which may play an important role in elucidating the postcollisional convergence of India and Eurasia. Remote Sens. 2016, 8, 134; doi:10.3390/rs8020134 www.mdpi.com/journal/remotesensing Remote Sens. 2016, 8, 134 2 of 14 Remote Sens. 2016, 8, 134 The focalfocal mechanism mechanism solutions solutions from from the Unitedthe United States States Geological Geological Survey Survey (USGS), (USGS), Global Centroid Global CentroidMoment TensorMoment (GCMT) Tensor and (GCMT) China Earthquakeand China Networks Earthquake Center Networks (CENC) Center Centroid (CENC) Moment Centroid Tensor (CMT)Moment indicate Tensor that (CMT) the 2015indicate Pishan that earthquake the 2015 Pishan was primarilyearthquake thrust was faultingprimarily with thrust a minor faulting left-lateral with a minorstrike-slip left-lateral component. strike-slip However, component. there are remarkableHowever, differencesthere are inremarkable focal mechanisms, differences locations in focal and mechanisms,seismic moments locations among and these seismi resultsc moments because among of the differentthese results seismic because inversion of the methods different and seismic data inversionsources used methods [3]. Near-field and data sources geodetic used measurements [3]. Near-field with geodetic good coveragemeasurements can provide with good independent coverage canconstraints provide on independent the source parameters constraints with on the high source accuracy, parameters especially with when high no accuracy, surface break especially is observed when noto constrainsurface break the faultis observed geometry to [constrain4–7]. Such the high-precision fault geometry source [4–7]. parameters Such high-precision can enhance source our parametersunderstanding can ofenhance the tectonics our understanding in the southwestern of the tectonics Tarim Basin. in the southwestern Tarim Basin. Figure 1. Tectonic setting of the 2015 Pishan earthquake,earthquake, western China. The focal mechanism of the 2015 Pishan earthquake is from the GCMTGCMT catalog.catalog. Red circles areare thethe aftershocksaftershocks withwith MM ¥≥ 2.0 from the International Seismological Centre [8] [8] betweenbetween 3 July 2015 and 2222 NovemberNovember 2015.2015. Blue boxes outline the spatial coverage of the Sentinel-1A SAR images (ascending trac trackk T056A and descendisng track T136D) and the ALOS-2 Phased Array type L-band Synthetic Aperture Radar-2 (PALSAR-2) images (ascending(ascending tracktrack P160A).P160A). Black Black lines lines are are the the published published faults faults from from the the map map of of active active tectonics tectonics in inChina China [2]. [2]. Blue Blue vectors vectors are are the the interseismic interseismic GPS GPS velocities velocities relative relative to a Eurasianto a Eurasian reference reference frame frame with with95% confidence95% confidence [9]. [9]. According toto thethe GCMTGCMT catalog, catalog, only only two two moderate moderate earthquakes earthquakes (5 <(5 M< M <6) < were6) were recorded recorded in the in theearthquake earthquake region region in the in pastthe 40past years, 40 years, implying implyi thatng the that seismicity the seismicity is low. is There low. wereThere more were (~1012) more (~1012)aftershocks aftershocks (M ¥ 2) (M [8 ]≥ in 2) the[8] in region the region before before 22 November 22 November 2015 2015 (Figure (Figure1). Among 1). Among these these events, events, the thelargest largest aftershock aftershock is an MLis an 5.0 ML event, 5.0 whichevent, followed which followed approximately approximately two hours two later. hours These later. aftershocks These aftershocksare spatially are confined spatially to aconfined ~50 by ~40to a km ~50 nearly by ~40 NW-SE-trending km nearly NW-SE-trending block (Figure block1). The (Figure block almost1). The blockcorresponds almost tocorresponds the surface to projection the surface of projection the ruptured of the thrust ruptured fault ofthrust the 2015fault Pishanof the 2015 earthquake, Pishan earthquake,the distribution the patterndistribution of which pattern was of similar which towa thats similar of the to 2008 that Mw of 6.8the Zemmouri2008 Mw 6.8 earthquake Zemmouri in Algeriaearthquake [10] in and Algeria the 2013 [10] Mw and 6.6 the Lushan 2013 Mw earthquake 6.6 Lushan in Chinaearthquake [11]. in China [11]. In this study, the coseismic deformation caused by the 2015 Pishan earthquake is measured with the InSAR technique using both ascending and descendingdescending Sentinel-1A andand ascending ALOS-2 SAR data. The The quasi-eastward quasi-eastward and and quasi-upward quasi-upward surface surface displacements displacements are are determined based on the ascending and descending observations. The fault geometry and slip distribution are retrieved from a combination of nonlinear and linear inversions together with the Helmert variance component Remote Sens. 2016, 8, 134 3 of 14 ascending and descending observations. The fault geometry and slip distribution are retrieved from a combination of nonlinear and linear inversions together with the Helmert variance component estimation (HVCE) weighting method [12]. The stress state in the surrounding area is also investigated through Coulomb failure stress analysis. Finally, we discuss the relationship among the rupture geometry, the fault slip patterns and the potential seismic hazards in this area. 2. Geological Background The 3 July 2015 Pishan earthquake occurred in the southwest boundary of the Tarim basin, which is the largest inland basin in China. The Tarim basin is bounded by the Tianshan Mountains to the north, the western Kunlun Mountains and the Altyn Tagh Mountains to the south and southeast, and the Pamir Plateau to the west (Figure1). The basin is covered by a thick foreland sequence of Mesozoic to Cenozoic continental sedimentary rocks with a maximum thickness of 17 km [13]. In the region of this event, there is a famous fault system, the Altyn Tagh fault system, which is located in the Altyn Tagh Mountain range in northern Tibet, extends for ~1200 km along the strike, which consists of the ENE-trending Altyn Tagh fault in the middle, the WNW-trending western Kunlun fault, and