Aftershock sequence of 2018 Mw6.4 Hualien earthquake in eastern Taiwan from dense seismic array data set (Invited)

The February, 2018 Mw 6.4 earthquake in eastern Taiwan caused extensive damage in Hualien city. While damaging earthquakes are common in this region, there are relatively few permanent seismic stations deployed. Two days after the main shock, we deployed 70 temporary seismic stations around Hualien City for 12 days (February 8thto 19th) and station spacing was 1-5 km.During this time, 2,192 aftershocks were recorded from which 580 focal mechanisms were determined. The aftershock sequence extended about 25 km southwestward from the epicenter of the main shock, into the Longitudinal Valley and to depths of between 5-15 km. Earthquake hypocenters indicate that the aftershocks took place along a near vertical to steeply west-dipping plane in the north that becomes more diffuse in the south. Focal mechanisms are predominantly extensional different from the left-lateral strike-slip with thrust component faulting of the main shock. Very few events occurred in the uppermost crust, at depths of less that 5 km and their focal mechanisms are left-lateral strike-slip faulting. From the main shock to the aftershocks, the stress changed rapidly from a NNW-oriented compressional axis (P-axis) to the same direction for the extensional axis (T-axis). This change in the stress field from the main shock to the aftershock sequence may relate to a rapid stress relaxation rate in this complex tectonic environment. This rapid stress relaxation may account for the short recurrence time of large earthquakes in the Hualien area.