Vertical triggering of layered faults during the 2024 MW 7.4 Hualien, Taiwan earthquake

Earthquakes have the potential to dynamically trigger other seismic events and cause a cascading effect. Most documented triggered events were laterally triggered, while vertically triggered cases are rarely reported. One of the reasons could be because data collected on the ground surface has a lower sensitivity to slip at greater depth. In this study, we report evidence of triggering between multiple layers of faults during the 2024 MW 7.4 Hualien earthquake in Taiwan. Using seismic, GPS, and InSAR measurements, we simultaneously invert for fault geometry and slip distribution. We find two possible triggering scenarios: 1) Slip initiated from a deep fault at 24 km depth, and its shear waves dynamically activated a shallow fault that is ~15 km above the hypocenter; 2) Slip initiated at the same location but ruptured along a steep and east-dipping structure and finally to the same shallow fault. The shallow fault and the east-dipping fault are likely the Takangkou-High Thrust and the deep portion of the Longitudinal Valley Fault, respectively. The deep fault coincides with one of the multiple west-dipping structures in the lower part of the Philippine Sea Plate (PSP), right below the fluid-rich upper crust. With a high convergent rate of PSP, these west-dipping structures may host past and future earthquakes with comparable size as the 2024 Hualien earthquake.