remote sensing Article Co-Seismic and Postseismic Fault Models of the 2018 Mw 6.4 Hualien Earthquake Occurred in the Junction of Collision and Subduction Boundaries Offshore Eastern Taiwan Ying-Hui Yang 1, Jyr-Ching Hu 2,*, Hsin Tung 2 ID , Min-Chien Tsai 3, Qiang Chen 4, Qian Xu 4, Yi-Jun Zhang 4, Jing-Jing Zhao 4, Guo-Xiang Liu 4, Jun-Nan Xiong 1 ID , Ji-Yan Wang 1, Bing Yu 1, Chun-Ying Chiu 2 and Zhe Su 5 1 School of Civil Engineering and Architecture, Southwest Petroleum University, Chengdu 610500, China; [email protected] (Y.-H.Y.); [email protected] (J.-N.X.); [email protected] (J.-Y.W.); [email protected] (B.Y.) 2 Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan; [email protected] (H.T.); [email protected] (C.-Y.C.) 3 Seismological Center, Central Weather Bureau, Taipei 10048, Taiwan; [email protected] 4 Department of Remote Sensing and Geoinformation Engineering, Southwest Jiaotong University, Chengdu 610031, China; [email protected] (Q.C.); [email protected] (Q.X.); [email protected] (Y.-J.Z.); [email protected] (J.-J.Z.); [email protected] (G.-X.L.) 5 Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100036, China; [email protected] * Correspondence: [email protected]; Tel.: +886-092-227-3830 Received: 7 August 2018; Accepted: 28 August 2018; Published: 29 August 2018 Abstract: The ascending and descending InSAR deformations derived from ALOS-2 and Sentinel-1 satellite SAR images and GPS displacements are used to estimate the fault model of the 2018 Mw 6.4 Hualien earthquake. The sinistral strike-slip fault dipping to the west with a high dip angle of 89.4◦ and a rake angle of 201.7◦ is considered as the seismogenic fault of this event. This seismogenic fault also triggered the ruptures of the Milun fault, which dips to the east with a dip angle of ~72◦, and an unknown west-dipping fault with a dip angle of 85.2◦. Two predicted faulting models indicate that the InSAR deformation fields include more postseismic slip than those of the GPS data. The north segment of the Milun fault and west-dipping fault have been triggered by the rupture of the seismogenic fault, but the postseismic slip occurred only in the south segment of the Milun fault. The InSAR-derived co-seismic and postseismic faulting model suggests that the significant slip concentrates at depths of 2.4–15.0 km of the main fault, 0.0–14.0 km of the Milun fault. Only minor slip is detected on the west-dipping fault. The maximum fault slip of ca. 2.1 m is located at the depth of ca. 2.4 km under the Meilun Tableland. The Coulomb failure stress (CFS) change calculated by the co-seismic and postseismic faulting model shows that there is a significant CFS increase in the east of the south segment of the Milun fault, but few of the aftershocks occur in this area, which indicates a high risk of future seismic hazard. Keywords: Hualien earthquake; InSAR; co-seismic and post-seismic faulting model; triggered fault; Coulomb failure stress change 1. Introduction On 6 February 2018, a Mw 6.4 earthquake struck East Taiwan, and the epicenter (24.134◦N and 121.658◦E, USGS solution) is located at ca. 18.2 km NNE of Hualien City (Figure1). This strong Remote Sens. 2018, 10, 1372; doi:10.3390/rs10091372 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, x FOR PEER REVIEW 2 of 16 . Remote1 Introduction Sens. 2018, 10, 1372 2 of 15 On 6 February 2018, a Mw 6.4 earthquake struck East Taiwan, and the epicenter (24.134°N and 121.658°E, USGS solution) is located at ca. 18.2 km NNE of Hualien City (Figure 1). This strong earthquake caused the collapse of some buildings in Hualien City, and resulted in 17 deaths and 285 earthquake caused the collapse of some buildings in Hualien City, and resulted in 17 deaths and 285 injures as of 12 February 2018. This earthquake occurred in the northern extension of the Coastal injures as of 12 February 2018. This earthquake occurred in the northern extension of the Coastal Range which is the junction of collision and subduction boundaries between the Eurasian plate and Range which is the junction of collision and subduction boundaries between the Eurasian plate and the Philippine Sea plate. Previous studies suggested that a tear fault boundary could be the major the Philippine Sea plate. Previous studies suggested that a tear fault boundary could be the major seismogenic structure between the Taiwan orogen and Ryukyu subduction zone [1–3]. The GPS seismogenic structure between the Taiwan orogen and Ryukyu subduction zone [1–3]. The GPS measurements in this area show that the Philippine Sea plate has a movement along NW direction measurements in this area show that the Philippine Sea plate has a movement along NW direction at at a rate of ca. 8.2 cm/year towards the Eurasian plate [4]. The high rate of the collision between a rate of ca. 8.2 cm/year towards the Eurasian plate [4]. The high rate of the collision between the two the two plates causes a very rapid uplift and develops a series of active faults in the east Coastal plates causes a very rapid uplift and develops a series of active faults in the east Coastal Range of Range of Taiwan. It also results in many destructive earthquakes, including the 1951 Hualien-Taitung Taiwan. It also results in many destructive earthquakes, including the 1951 Hualien-Taitung earthquake sequences, the 1999 Mw 7.6 Chi-Chi earthquake, and the 2016 Mw 6.5 Meinong earthquake earthquake sequences, the 1999 Mw 7.6 Chi-Chi earthquake, and the 2016 Mw 6.5 Meinong occurred in Taiwan in the past 100 years [5–8]. earthquake occurred in Taiwan in the past 100 years [5–8]. FigureFigure 1.1. ((aa)) TectonicTectonic settingsetting ofof TaiwanTaiwan characterizedcharacterized bybyopposing opposing subductionsubduction systemssystems inin thethe northnorth andand southsouth [[9].9]. ((bb)) MainMain faultfault systems (the black lines) of Taiwan orogeny.orogeny. SevenSeven GPSGPS sitessites (red(red arrows)arrows) locatedlocated atat thethe EurasiaEurasia andand PhilippinePhilippine SeaSea platesplates areare gotgot fromfrom thethe previousprevious studystudy [[10],10], whichwhich showshow aa NWNW directeddirected blockblock motionmotion with with an an average average velocity velocity of of ca. ca. 8.2 8.2 cm/year cm/year [[10].10]. TheThe yellowyellow starstar indicatesindicates thethe epicenterepicenter of of the the 2018 2018 Hualien Hualien earthquake, earthquake, and and the th beache beach balls balls are theare focal the focal mechanisms mechanisms of the of main the shocksmain shocks and major and aftershocksmajor aftershocks of the 2018 of the Hualien 2018 Hua eventslien solved events by solved USGS by and USGS BATS and based BATS on based the New on Automaticthe New Automatic Full-Waveform Full-Waveform Regional Moment Regional Tensor Moment Inversion Tensor Algorithm Inversion [11 ].Algorithm (c) SAR coverages [11]. (c) SAR (the redcoverages and blue (the lines) red and and blue aftershocks lines) and (red aftershocks circles) are (red mapped circles) on are the mapped digital on Earth the modeldigital Earth with amodel high resolutionwith a high of resolution 30 m. of 30 m. Remote Sens. 2018, 10, 1372 3 of 15 Previous studies suggest frequent strong seismic activities in the east Coastal Range of Taiwan [12,13], especially along the Longitudinal Valley fault [14]. However, only one historical big event (the 1951 ML 7.3 Hualien-Taitung earthquake) has been reported that occurred around the Hualien area and resulted in the rupture of the Milun fault [15,16]. The 2018 Mw 6.4 Hualien earthquake is the earthquake with the largest magnitude occurring in the Hualien area in the past ca. 70 years, and it triggered the Milun fault again. The focal mechanism derived from teleseismic data by USGS shows that the co-seismic fault of the 2018 Hualien event has a strike angle of 209◦ and dips to the west with a high dip angle of 73◦. BATS gives a nodal plane of strike angle of 111.5◦, dipping to the west with a dip angle of 74.2◦ and a slip of 157.3◦. It suggests that the Milun fault (dipping to east) is not the seismogenic fault of the Hualien event and, thus, it should be triggered by the motion of the seismogenic fault. Therefore, the 2018 Hualien earthquake should be a multi-fault rupture like the 2016 Mw 7.8 Kaikoura¯ earthquake that occurred in New Zealand [17]. A complex multi-fault motion, including the seismogenic fault and Milun fault rupture, should be responsible for the 2018 Hualien earthquake. However, the multi-fault geometry parameters, slip distribution, and the Coulomb failure stress (CFS) transmission from the seismogemic fault to the triggered fault are not well known so far. Here, we firstly map the seismic surface deformation fields using the ALOS-2 and Sentinel-1 synthetic aperture radar (SAR) images and the GPS observations. The ascending and descending InSAR data are used to infer the fault geometry model of the 2018 Hualien event. Then, The GPS and InSAR are respectively used to infer the co-seismic and the co-seismic and postseismic slip models of this event. We calculate the CFS change of both the triggered Milun and west-dipping faults due to the motion of the seismogenic fault. Finally, we investigate the CFS change with varying receiver parameters at the average depth of the aftershocks. 2. Materials and Methods 2.1.