Rupture directivity of the 2019 ML 6.3 Xiulin (Taiwan) earthquake estimated by near-field seismograms

We deconvolved regional seismograms to derive the azimuth-dependent source time functions for the 2019 Xiulin (Taiwan) earthquake. Subsequently, using rupture directivity and multiple-event analyses, we investigated the earthquake’s rupture features. The rupture directivity analysis revealed a rupture length of 11.5 km, a source duration of 7.37 s, and a rupture velocity (Vr) of 1.56 km/s, approximately 0.4 times the crustal S-wave velocity. The multiple-event analysis indicated two sub-ruptures during the earthquake. Notably, the average rupture and the sub-rupture shared the same product of Δσ_SVr³ (Δσ_S: static stress drop), adhering to a specific source-scaling relationship. In summary, the 2019 Xiulin earthquake exhibited a relatively low Vr and a relatively high Δσ_S. Additionally, we observed similarities between the 2018 Hualien and 2019 Xiulin earthquakes by comparing their fault parameters. The rupture directivity analysis revealed that both earthquakes occurred on the same westward-dipping plane, suggesting that the 2019 Xiulin earthquake represented the residual stress release from the 2018 Hualien earthquake. Compared with the Hualien earthquake, the Xiulin earthquake dissipated more energy per unit area to rupture the fault.