doi: 10.3319/TAO.2019.04.03.01 * Corresponding author E-mail: jchu@ntu.edu.tw Triggered slip on multifaults after the 2018 M w 6.4 Hualien earthquake by continuous GPS and InSAR measurements Hsin Tung 1 , Horng-Yue Chen 2 , Ya-Ju Hsu 2 , Jyr-Ching Hu 1, * , Yo-Ho Chang 3 , and Yu-Ting Kuo 2 1 Department of Geosciences, National Taiwan University, Taipei City, Taiwan 2 Institute of Earth Sciences, Academia Sinica, Taipei City, Taiwan 3 Department of Natural Resources and Environmental Studies, National Dong Hwa University, Hualien County, Taiwan ABSTRACT On 6 February 2018 at 23:50 local time, a M w 6.4 earthquake struck eastern Taiwan. We characterize the instantaneous surface ground motion and the perma- nent displacement induced by this event from continuous GPS data and SAR images within a short time after the mainshock. We use high-rate GPS positioning tech- niques to obtain epoch-by-epoch positions peak ground displacement to assess po- tential seismic damage. The maximum coseismic GPS horizontal displacement of about 450 mm trending to the northeast is observed at the station HUAL located on the hanging wall of the Milun fault. The PEPU located on the footwall of the Milun fault shows a coseismic horizontal displacement of 280 mm trending to the southwest and a coseismic uplift of about 70 mm. Moreover, ascending and descending tracks of ALOS-2 and Sentinel-1 SAR images are processed to estimate coseismic surface deformation along the line-of-sight (LOS) toward the satellite. Then, wide coverage from east-west and uplift components of surface deformation is fulfilled by com- bining the LOS displacement from ascending and descending interferograms. The main deformation area revealed by both GPS results and D-InSAR interferograms is concentrated around the Milun and Lingding faults. Significant uplift on the footwall of the northern Lingding Fault implies that the Milun fault and an unknown west- dipping fault close to the Lingding fault were triggered. Both the two nodal planes of the M w 6.4 Hualien event could be different with the kinematic behavior of the Milun fault and Lingding fault. Thus we suggest that slip on multiple faults was triggered during the 0206 event. Article history: Received 13 August 2018 Revised 15 March 2019 Accepted 3 April 2019 Keywords: High-rate GPS, InSAR, Multi-fault slip triggering, Peak Ground Dis- placement Citation: Tung, H., H.-Y. Chen, Y.-J. Hsu, J.-C. Hu, Y.-H. Chang, and Y.-T. Kuo, 2019: Triggered slip on multifaults after the 2018 M w 6.4 Hualien earth- quake by continuous GPS and InSAR measurements. Terr. Atmos. Ocean. Sci., 30, 285-300, doi: 10.3319/ TAO.2019.04.03.01 1. INTRODUCTION At 21:56:41 local time on 4 February 2018, a M w 6.0 (0204 event) occurred in the Hsinchen Ridge offshore the city of Hualien (Fig. 1). According to the focal mechanism of this event, the possible seismogenic fault is a nearly E-W- striking and gently north-dipping thrust fault with a minor dextral strike-slip component. This event could be interpret- ed as a regular subduction zone earthquake that occurred at the complex junction of convergence boundary of the Philippine Sea plate and Eurasian plate. Two days later, a M w 6.4 earthquake (0206 event) occurred near the epicenter of 0204 event with a different focal mechanism. The epi- center of this second event was located in the offshore area ~16.5 km northeast of city of Hualien at a depth of 6 km resulting in 17 deaths with 285 injured. Based on the Cen- tral Weather Bureau (CWB) intensity scale, the peak ground acceleration (PGA) larger than 400 gals were measured in Hualien. This extremely large ground shaking caused severe building damage, including four that had partially collapsed. Thousands of aftershocks were recorded by Geophysical Database Management System (https://gdms.cwb.gov.tw/ index.php) of CWB within 14 days (Kuo-Chen et al. 2019). Aftershocks were propagated southward from the hypocen- ter of the mainshock to the Longitudinal Valley (Chang et al. 2019; Chen et al. 2019). According to field investigations, the major surface rupture was identified along the Milun fault (Huang et al. 2019; Lin et al. 2019; Wu et al. 2019), an active fault that Terr. Atmos. Ocean. Sci., Vol. 30, No. 3, 285-300, June 2019