Rupture behavior and interaction of the 2018 Hualien earthquake sequence and its tectonic implication

© 2019 Seismological Society of America. All Rights Reserved. The M w 6.4 disastrous 2018 Hualien earthquake hitting the city of Hualien in eastern Taiwan on 6 February 2018 was preceded by unusual foreshock activities, starting with the largest M w 5.9 foreshock two days earlier. To better understand the source processes and tectonic significance of this earthquake sequence, we investigated the rupture behaviors of both the largest foreshock and mainshock by the backprojection (BP) method. Results reveal that the foreshock ruptured northeastward and downward on a west-northwest-striking, north-northeast-dipping subhorizontal fault, and the mainshock propagated southwestward along a north-northeast–south-southwest-striking high-angle subvertical fault. The mainshock was not properly represented by a single point-source model, as suggested by a significant compensated linear vector dipole (CLVD) component of the moment tensor solution. A multiple point-source model clarifies the cause of a high CLVD and indicates that the mainshock was composed of one M w 6.2 and three smaller subevents. The largest one is an east-dipping sinistral strike-slip fault distinct from the other predominant thrusting subevents. The largest subevent was triggered by receiving the increased stress of at least 10 bars induced from the foreshock and initial subevent of the mainshock through the static Coulomb stress-transfer mechanism. The 2018 Hualien earthquake sequence represents a combined consequence of oblique subduction and lateral compression ongoing concurrently beneath the northern tip of the Taiwan–Luzon arc collision zone.