Aftershock sequence of the 2018 M _w 6.4 Hualien earthquake in eastern Taiwan from a dense seismic array data set

The February 2018 M _w 6.4 earthquake in eastern Taiwan caused extensive damage in Hualien City. Although damaging earthquakes are common in this region, there are relatively few permanent seismic stations deployed. Two days after the mainshock, we deployed 70 temporary seismic stations around Hualien City for 12 days (8–19 February), and station spacing was 1–5 km. During this time, 2192 aftershocks were located from which 580 focal mechanisms were determined. The aftershock sequence extended about 25 km southwestward from the epicenter of the mainshock into the Longitudinal Valley and to depths between 5 and 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 than the left-lateral strike slip with thrust-component faulting of the mainshock. Very few events occurred in the uppermost crust at depths of less than 5 km, and their focal mechanisms are left-lateral strike-slip faulting. From the mainshock to the aftershocks, the stress changed rapidly from a north-northwest-oriented compressional axis (P axis) to the same direction for the extensional axis (T axis).