Reactivation of the basement faults by lower-crust fluid during the 2017 Changdao earthquake swarm in Eastern China

Ancient basement faults, long considered geologically stable, can be reactivated by deep fluid migration, significantly influencing earthquake activities. However, the triggering mechanisms and behaviors of fault-fluid interaction remain poorly understood. In 2017, an intensive earthquake swarm occurred in the Changdao area, eastern China, providing a rare chance to investigate fluid-driven reactivation of basement faults at intraplate regions. Using template matching detection and waveform cross-correlation-based relocation techniques, we obtained a high-resolution spatio-temporal distribution of earthquakes. The earthquakes mostly occurred at the depth range of 7 to 13 km, and depicted a complex system with a listric fault intersected by a steeply dipping fault and several sub-faults in the hanging wall, forming a X-shaped fracture network. The seismicity displayed distinct spatio-temporal migration, consistent with a fluid diffusion-driven process, and the inferred fluid diffusivities range from 0.08 to 1.2 m ² /s varying across different stages and fault segments. The seismicity mostly occurred in the top region of a vertical zone with relatively low V _P /V _S ratios, while the surrounding regions have relatively high V _P /V _S ratios, suggesting that fluid intrusion and pressure perturbations from the deep region were the primary driver for the swarm and fault reactivation. We inferred that the swarm sequence was driven by a combination of fluid pressure diffusion and cascade stress transfer from moderate-size earthquakes. In addition, the episodic fluid flows likely interacted with the earthquakes through a fault-valve mechanism. These findings provided new insights into the dynamic interactions between deep-seated fault systems and migrating fluids.