Source properties of the 2019 M6.3 Hualien, Taiwan, earthquake, determined by the local strong motion networks

The 2019 M 6.3 Hualien earthquake struck the northern Longitudinal Valley (LV) and generated not only large strong motions (intensity of 7, as defined by the Central Weather Bureau, Taiwan) locally but also widespread strong shaking in metropolises in northern Taiwan. In this study, we analyse strong motion records from local seismic networks to understand the source properties of the 2019 event. We first obtain the centroid location of the 2019 event using the source-scanning algorithm (SSA) technique by applying the unfiltered records. The determined centroid location is 121.55°E, 24.10°N, with a depth of 22.5 km. This location is 5.5 km north-northwest of and 3.8 km deeper than the Central Weather Bureau hypocentre, suggesting that the 2019 event occurred on the high-angle west-dipping plane of the focal solution. The centroid time delay is 3.35 s. Then, we obtain strong motion generation areas (SMGAs) of the 2019 event using the empirical Green's function method by considering the broad-band waveforms (0.4-10 Hz). Unlike other moderate-sized earthquakes in Taiwan, which have one SMGA, we determine that there were two SMGAs in the 2019 event. SMGA1 initiated at the CWB hypocentre with a size of 4.00 km2, and SMGA2 initiated at the centroid location determined by the SSA approach with a size of 3.63 km2. Such small areas cause high stress drops of 13.7 and 27.4 MPa for SMGA1 and SMGA2, respectively. We infer that the localized high stress drop of SMGAs is one of the important factors responsible for high peak-ground accelerations (PGAs) in Taiwan in addition to a strong directivity effect coupled with the radiation pattern reported by the previous study. Furthermore, previous moderate-sized earthquakes on an active structure called the Xiulin segment revealed similar source properties with a high stress drop and generated large PGA locally as well as in the metropolises of northern Taiwan. Considering the stored moment deficit, the probability of a future large earthquake in the northern LV region remains high. It is essential to consider seismic hazard assessment and mitigation for this not-well-known but high-seismic-potential region.