Variations in earthquake source properties across the Taiwan collision zone

In 2024, the Mw 7.4 eastern Taiwan earthquake highlights Taiwan’s status as one of the most seismically active areas, driven by the complex interaction between the European Plate (EP) and Philippine Sea Plate (PSP). This dense seismicity provides a unique opportunity to investigate the earthquake rupture properties. The corner frequency, a key parameter for understanding kinematic rupture properties, is challenging to measure due to the site effect and the trade-off between corner frequency and attenuation parameter t*. Jian and Kuo (2024) proposed the Cluster Event Method 2 (CEM2), which mitigates these challenges by incorporating joint datasets of spectra and spectral ratios. This study applied CEM2 to the eastern Taiwan earthquakes recorded by local broadband stations. Site-effect patterns were first retrieved during the initial inversion stage for each station, enabling corrected corner frequency estimates in the subsequent inversion. Using the relationship of stress drop, corner frequency and seismic velocity at the source area (Madariaga, 1976), we calculate the stress drop (SD) for individual events. To prevent overestimation caused by extreme values, we calculated the average SD in log-scale. The overall average SD for earthquakes shallower than 40 km is 50 MPa. Our results reveal significant spatial variations in SD across the Taiwan collision zone. In the southern collision zone, SD in the PSP are approximately twice as high as those in the EP. To the north, where the PSP subducts beneath the EP, SD increases with longitude and depth. However, west of the Longitudinal Valley Fault (LvF), the vertical variation of SD is reversed: shallow SD (<10 km) is about twice as high as deep SD (>10 km). Considering self-similarity model of faults with constant stress drop of earthquakes, further exploration of the parameters controlling tectonic-related stress drop variability are necessary. Overall, earthquakes within the PSP exhibit higher stress drops than those in the EP. These findings provide valuable insights for different earthquake rupture properties in the tectonically complex region.