Dual Seismic Migration Velocities in Seismic Swarms

Fluid-induced earthquake sequences generally appear as expanding swarms activating a particular fault. The recent analysis of a swarm in the Corinth rift has revealed a dual migration pattern, with a global slow expansion (m day(-1)) and episodes of rapid migration (km day(-1)). Such swarms are generally interpreted as fluid diffusion, which ignores the possibility of static, dynamic, or aseismic triggering and the existence of rapid migration. Here, we propose a new model for such swarms, where earthquakes consist in the failure of asperities on a creeping fault infiltrated by fluid. For that, we couple rate-and-state friction, nonlinear diffusivity, and elasticity along a 1D interface. This model reproduces the dual migration speeds observed in real swarms. We show that migration speeds increase linearly with the mean pressurization and are not dependent on the hydraulic diffusivity, as traditionally suggested. Plain Language Summary The common interpretation of earthquake swarms assumes a fluid diffusion from a source at depth, which destabilizes critically stressed faults. In this model, seismicity is localized at the fluid front. However, earthquake swarms expand faster than fluid is likely to diffuse. Recent observations in the Corinth rift (Greece) also report that swarms consist in the succession of bursts of rapidly migrating events, which is not explained by the diffusion model. To account for these features, one needs to consider slow slip on faults (undetected at the surface) in addition to fluid diffusion. Here, we propose a physics-based model coupling fluid diffusion, slow slip, and earthquake triggering on a 1D fault. Our model reproduces the dual migration (slow expansion and rapid bursts) pattern of some seismic swarms. We also show that migration speeds are controlled by the increase of the mean pore pressure within the fault and not by hydraulic diffusivity.

Automated summary

Earthquake swarms are commonly interpreted as being caused by fluid diffusion from a deep source, destabilizing critically stressed faults. However, these swarms expand faster than fluid diffusion is likely to occur. Recent observations in the Corinth rift in Greece also reveal that swarms consist of bursts of rapidly migrating events, which cannot be explained by the diffusion model. To account for these features, it is necessary to consider slow slip on faults in addition to fluid diffusion.