Reactivation of basement faults by deep fluids during the 2017 Changdao earthquake swarm, Eastern China

Ancient basement faults within plate interiors may be reactivated by external forces, generating intraplate seismicity. However, the driving mechanisms remain unclear. The 2017 Changdao earthquake swarm in the Bohai Bay Basin, eastern China, provides an opportunity to investigate such processes. A high-resolution earthquake catalog, constructed using matched filter detection and waveform-based relocation, reveals that the swarm occurred at 7-13 km depth along an X-shaped fault network, with migration following sqrt(t) diffusion (D = 0.08 – 1.2 m² s-1). High b-values and external forcing rate, and substantial isotropic components (13-38%) in the moment tensors indicate fluid involvement. Combined with low VP/VS ratios and proximity to CO2-rich hydrocarbon fields, we propose a fault-valve model where highly compressible fluids episodically breach fault intersections, triggering transient crack opening and swarm migration. This study reveals how deep fluids reactivate ancient basement faults in intraplate regions, and provides new insights for seismic hazard assessment in hydrocarbon-bearing basins. A distinctive migration pattern consistent with fluid pressure diffusion is identified within a high-resolution earthquake catalogue of the 2017 Changdao swarm and suggests a fault-valve mechanism driven by overpressure CO2-rich fluids was responsible.