Rupture Directivity of the 2019 ML 6.3 Xiulin (Taiwan) Earthquake Estimated by Near-Field Seismograms: Implications for Source Scaling During Faulting

This study deconvolved regional seismograms to derive the azimuth-dependent source time functions for the 2019 Xiulin earthquake in Hualien, Taiwan. Then, rupture directivity analysis was used to estimate the fault parameters, and the results revealed a rupture length of 11.5 km, a source duration of 7.37 s, and a rupture velocity ( V r ) of 1.56 km/s, approximately 0.4 times the value of the crustal S-wave velocity. Furthermore, the multiple-event analysis indicated two subruptures during the earthquake. Notably, the average rupture and the subrupture shared the same product of Δσ s V r 3 ( Δσ s static stress drop) and thus obeyed a specific source-scaling relationship. In short, the 2019 Xiulin earthquake had a relatively low V r and a relatively high Δσ s . We noted similarities between the 2018 Hualien and 2019 Xiulin earthquakes when comparing the fault parameters; rupture directivity analysis revealed that the two events occurred on a west-dipping plane with a similar strike. Therefore, the 2019 Xiulin earthquake likely constituted the remaining energy release of the 2018 Hualien earthquake.