The 2018 Mw6.4 Hualien earthquake: Dynamic slip partitioning reveals the spatial transition from mountain building to subduction

The Hualien earthquake of 6 February 2018 occurred on the east coast of Taiwan in the collision zone between the Eurasia and Philippine Sea plates. Even though an event with a moderate magnitude of Mw6.4, it caused widespread surface rupture in and around the city of Hualien, leading to significant loss of life and property damages. In this study, we investigate the rupture process of this earthquake using teleseismic, strong motion, GPS and SAR observations. We first employ the geodetic data to search for the geometry of multiple fault planes, followed by a multi-point source inversion using teleseismic and strong motion waveforms to determine the possible change in mechanism during the rupture process. We parameterize the fault model with three curved segments to represent the complicated fault system involved in the rupture process. An offshore fault was ruptured by the Hualien earthquake showing a curved geometry with the dip angle changing from shallow (∼45°) in the north to sub-vertical (∼80°) in the south. Evidence from tomographic velocity model indicates that the offshore fault is the boundary between the subducted Coastal Range and Meilun Tableland, which is also a plate interface. Finally, a joint inversion is carried out to constrain the complete rupture process. The inversion result reveals that the rupture initiated on the offshore fault interface and propagated unilaterally towards south-southwest. The rupture then jumped on to the sub-vertical Meilun Fault and reached the ground surface. Significant dynamic slip partitioning occurred during the rupture process near the Meilun Fault, with the majority of thrust and strike-slip motions occurring on the off-shore fault and the Meilun Fault, respectively. The afterslip model is inverted with SAR timeseries spanning 6 months after the mainshock. The afterslip model shows a strong complementary pattern with co-seismic slips. The comparison between the 1951 and 2018 Hualien earthquake shows that the rupture behavior of the Meilun Fault does not follow a repeating earthquake pattern: thrust motion may occur on the two side walls of the Meilun Tableland with a “teeterboard” uplift pattern in different earthquake cycles. The change in geometry and slip on the inter-plate fault appears to be related to the transition of the Coastal Range from mountain building to subduction.