The Source Scaling of Earthquake Swarm-genic Slow Slip Events

Abstract:

Slow slip events (SSEs) are slow ruptures that do not produce seismic waves although they often induce seismic strain release, as low- and very-low frequency earthquakes, non-volcanic tremor (i.e. tremor-genic SSEs) and ordinary earthquake swarms (swarm-genic SSEs). At subduction zones, increasing evidence suggests that aseismic slip and non-volcanic tremor correspond to the evolution of slow fracturing. However, the source duration-moment scaling of SSEs is linear on global compilation but on regional scale seems to follows a cubic scaling similar to ordinary earthquakes. Conversely, the partition of aseismic and seismic strain release during SSEs has never been addressed on global data. To date, investigations on the source scaling has been based on tremor-genic SSEs while no studies addressed the source scaling of swarm-genic SSEs.

Here, we present the first compilation of swarm-genic slow slip events in subduction, extensional, transform and volcanic environments. We find a power-law scaling of aseismic to seismic moment release during episodes of slow slip. The earthquake productivity, i.e., the ratio of seismic to aseismic moment released, is on average higher for shallow SSEs than deeper ones and scales inversely with rupture velocity. The source scaling indicates an interplay between the evolution of aseismic slip and the associated seismic response of the host medium. Additional data of tremor-genic SSEs are compatible with the scaling of swarm-genic SSEs suggesting that both events are governed by similar fracturing mechanism. Fluid pore pressure, temperature and density of asperities are the main controls on the scaling for events where seismic and aseismic slip is co-planar. In addition, static stress transfer imparted by SSEs is an additional factor in triggering off-fault swarms of ordinary earthquakes. Our data suggest that during the release of aseismic slip the proportion of seismic strain release remains always smaller than the aseismic part. However, transient changes in stress and fault rheology imparted by swarm-genic SSEs can lead to delayed triggering of major and devastating earthquakes. Our finding indicates a control of the aseismic slip on the amount of seismic moment release that should be incorporate in theoretical models of SSEs rupture propagation and seismic hazard assessments.