Induced seismic swarm triggered by tunnel excavation and pore pressure diffusion mechanism

SUMMARY Seismic activity induced by underground engineering projects often involves complex causal mechanisms, and represents significant hazards, including ground subsidence, disruption of surface and underground water systems, ecological damage, structural damage to buildings and even casualties. Consequently, induced seismicity has become an important topic in the risk assessment and protective measures for underground engineering projects. During the construction of the Hongtu Tunnel on the Dafenghua Expressway in Guangdong, China, a series of earthquakes occurred nearby, with the biggest of magnitude ML = 3.7, alongside significant water inflows at multiple locations. This study analysed seismic network data from 2017 to 2022 around the tunnel area to investigate the potential relationship between the seismic swarm and tunnel construction and uncover the underlying mechanisms. After velocity model corrections and double-difference relocation, the earthquakes were primarily distributed at depths of 1∼4 km. Three concealed, steeply dipping NE-trending faults, each 3∼7 km in length, were identified based on the earthquake distribution. The swarm began about one month after the onset of water inflows in the tunnel and grew significantly after the peak daily inflow, culminating in the ML 3.7 main shock. A strong spatiotemporal correlation was observed between the seismic swarm and the water inflows. During the first year of the swarm, the seismicity displayed migration characteristics consistent with pore pressure diffusion, with an initial diffusion depth of approximately 2 km and a diffusion rate of 0.0039∼0.0446 m² s−1, and best fit by the classical parabolic diffusion model (α = 0.5). After 2021, the earthquakes occurred more consistently, mainly exhibiting stress-triggering characteristics. Over time, the seismicity gradually extended to greater depths, with focal mechanisms changing from normal faulting to strike-slip faulting. The local stress field shifted from extensional to shear, which reflected the sustained influence of pore pressure diffusion on fault activation. Fluid diffusion not only initially activated the faults but also promoted repeated fault slip during the seismic swarm, indicating that prolonged water inflow significantly altered fault activity patterns and the regional stress field. This study is the first to reveal the phenomenon of long-distance induced seismicity caused by tunnel water inflow and the role of pore pressure diffusion in triggering such events, which offers new insights into the safety of underground construction and the study of fluid-related geological processes.