[S-CG60] Shallow Fault Zone Structure and Seismic Hazard Assessment

[SCG60-09] Source characteristics of the 2022 Guanshan and Chihshang earthquakes in southeast Taiwan determined by the strong motion waveforms

*Yin-Tung Yen¹, Yi-Ying Wen^2,3, Yen-Yu Lin^4,5 (1.Sinotech Engineering Consultants, INC., 2. Department of Earth and Environmental Sciences, National Chung Cheng University, 3.Environment and Disaster Monitoring Center, National Chung Cheng University, 4.Department of Earth Sciences, National Central University, 5.Earthquake-Disaster & Risk Evaluation and Management Center, National Central University)

Keywords:Source-Scanning Algorithm, Empirical Green's Function method, storng motion, waveform modeling, Longitudinal Valley

In September 2022, two devastating earthquakes struck southeast Taiwan. The first earthquake, the Guanshan earthquake (M_L 6.4), occurred on September 17th, 2022, at 13:41 UT and was caused by a near-vertical fault with a northeast-southwest strike direction. The second earthquake, the Chihshang earthquake (M_L 6.8), which took place on September 18th, 2022, at 06:44 UT, had a similar fault type. Both earthquakes were located in the Longitudinal Valley (LV), a complex fault system in the plate suture between the Eurasian and Philippine plates. The earthquakes generated large pulse-like velocity and acceleration, causing damage to bridges and buildings. Geologists observed obvious strike-slip surface rupture along the valley. Using the Source-Scanning Algorithm (SSA) technique, we determine that both centroid locations for the Guanshan and Chihshang earthquakes were in the south and north of the hypocenters with a shallower depth, respectively. We then implement the Empirical Green's Function method (EGFM) to determine the source properties of the earthquakes by considering broad-band waveforms (0.15 to 10 Hz). The waveform modeling of strong motion using the EGFM suggests that the Guanshan earthquake can be explained by a single Strong Motion Generation Area (SMGA) of 2.8 x 4.2 km², but the Chishang earthquake needs two. One SMGA with an area of 0.8 x 0.4 km² was located near the hypocenter, while the main SMGA with an area of 4.0 x 4.2 km² was located 5 km north-northeast of the epicenter near its centroid location. The stress drops of SMGA for both events are higher than those of moderate-large events worldwide. The likelihood of a major earthquake occurring along the LV region remains high due to the ongoing deficit in the stored moment. It is crucial to conduct seismic hazard assessments and implement mitigation measures in this high-seismic-potential but largely unknown region.