Seismic swarms as intermittent quasi-static ruptures driven by pore pressure variations due to the water reservoir impoundment

Seismic swarms are characterized as time-space clustered sequences of earthquakes without a dominating strong event. The occurrence of earthquake swarms has been observed in various environments and found out to be closely related to underground migration of fluids. Mechanical interactions between fluid-conduit properties of fault and rupture determine the spatio-temporal complexity of rupturing process and earthquake occurrence. Often, seismic swarms are triggered by Water Reservoir Impoundment (WRI). Although many studies have investigated the relationship between the pore pressure changes due to WRI and observed seismicity, hydromechanical models that explain the observed processes are limited. Here, we investigate the role of hydromechanical interactions in triggering earthquake swarms in WRI conditions, using data from the Song Tranh 2 Reservoir in Vietnam as an example. We found that seismic swarms evolve due to stress redistribution and subsequent induced fluid flow. Swarm sequences form interconnected rupture increments and their aggregated behaviour is similar to a single large earthquake that ruptures the fault segment at once. The evolution of friction with the mean slip retrieved from the swarm earthquakes exhibits both, slip weakening and strengthening stages, resembling linear slip-weakening friction. The effective slip over which the weakening is completed, is modulated by the pore pressure diffusion. These results provide insights into the physical mechanisms of seismic swarms and have implications for seismic hazard.