Aftershock Sequences of 2018 Mw6.4 and 2019 Mw6.2 Hualien Earthquakes in Eastern Taiwan from Dense Geophone Arrays

In the Hualien area, two Mw6.4 and Mw6.2 earthquakes, 20 km apart, occurred in February 2018 and April 2019 respectively. The former to the northeast, located offshore to the Liwu river, triggered several earthquake clusters along the Milun fault and southward to the Longitudinal Valley, the suture of the Eurasian plate and the Philippine Sea Plate; the latter to the southwest, located in the Central Range, also triggered several seismic swarms but in the Central Range, along the Liwu river to the northeast and at Ji'an to the southeast. Except for the Milun fault, neither GPS nor InSAR observations detects significant surface deformation after the occurrence of these two main shocks, indicating that the earthquake ruptures mainly developed within the crust. Therefore, seismic observation becomes the efficient tool for revealing the seismotectonics of the two earthquake sequences.

For monitoring the aftershock sequences, two days after the main shocks, we deployed two geophone arrays, 70 Z-component (RefTek 125ATexan) in 2018 and 47 three-component (FairfieldNodal Z-Land) in 2019, around the Hualien City both for two weeks and station spacing was 1-5 km. These earthquake swarms were well recorded and analyzed through the dense seismic networks. The seismicity of the aftershock sequences were both significantly reduced within two weeks after the main shocks and to depths of between 5-15 km in 2018 and 3-25 km in 2019 respectively. The numbers of aftershock sequences manually identified via our data sets are two-fold more than that issued by the Central Weather Bureau, Taiwan. Due to the dense ray coverage of body waves and seismic networks, we further apply both body wave tomography and joint inversion tomography of body and surface waves. The improved resolution at shallow part of the crust (at depth less than 5 km) provides new constraints to get detailed (with grid spacing 500 m) and reliable Vp, Vs, and Vp/Vs velocity models at local scale for the first time. Combined with the high resolution velocity models and the spacial distribution of these more complete aftershocks, our results clearly depict several local tectonic structures that have not been observed at the northern tip of the Longitudinal Valley, not only a suture but also a transitional area from collision to subduction.