Imbricated Aseismic Slip and Fluid Diffusion Drive a Seismic Swarm in the Corinth Gulf, Greece

Abstract

The primary processes driving seismic swarms are still under debate. Here, we study the temporal evolution of a seismic swarm that occurred over a 10-day period in October 2015 in the extensional rift of the Corinth Gulf (Greece) using high-resolution earthquakes relocations. The seismicity radially migrates on a normal fault at a fluid diffusion velocity (~125 m/day). However, this migration occurs intermittently, with periods of fast expansion (2 to 10 km/day) during short seismic bursts alternating with quiescent periods. Moreover, the growing phases of the swarm illuminate a high number of repeaters. The swarm migration is likely the results of a combination of multiple driving processes. Fluid upflow in the fault may induce aseismic slip episodes, separated by phases of fluid pressure build-up. The stress perturbation due to aseismic slip may activate small asperities that produce bursts of seismicity during the most intense phases of the swarm.Plain Language Summary Seismic swarms are clusters of numerous earthquakes of small magnitudes. To maintain such seismic activity, a driving mechanism is required, but it is still an open question. Here, we focus on a small, prolific earthquake swarm recorded by a dense network of seismic stations in the Corinth Gulf (Greece). We find that the overall expansion of the swarm is related to fluid diffusion. However, in detail, bursts of events with fast migration and earthquakes sharing similar waveforms suggest that most of the slip on the fault does not radiate seismic waves. We therefore suggest that the fluid pressure mainly induces aseismic deformation that, then, triggers the seismicity by perturbing stress.

Dual Seismic Migration Velocities in Seismic Swarms

“…Note that asperities are reactivated several times during the sequence, which could be seen as repeating earthquakes. Such repeating sequences were observed within seismic swarms (De Barros et al., 2020) and during induced seismicity (Cauchie et al., 2020).…”

“…In Figure 3, we also represented the estimations of global expansion speed and rapid migration speeds for the Corinth swarm analyzed in De Barros et al. (2020). These observations are compatible with a pressurization of the order of 10 Pa s −1 (or 0.864 MPa day −1 ) if one assumes a lithostatic stress of the order of 150 MPa.…”

“…Recent in situ fault slip reactivation experiments (Duboeuf et al., 2017;Guglielmi et al., 2015) and seismological observations (Bourouis & Bernard, 2007; Cauchie et al., 2020; De Barros et al., 2020; Eyre et al., 2019, 2020; Hatch et al., 2020; Lengliné et al., 2017; Lohman & McGuire, 2007; Wei et al., 2015) have revealed that significant slow (aseismic) slip has to be taken into account in the triggering of fluid‐induced seismicity. In this alternative model, the pore pressure changes trigger an aseismic slip front propagating on faults, leading to the failure of preexisting asperities (earthquakes).…”

“… Location and migration of the seismicity during the 2015 Corinth gulf swarm (De Barros et al., 2020). (a ) 3D location of the seismicity, view from South.…”

“…A recent analysis of an earthquake swarm occurring in the Corinth rift (Greece) has revealed a particular migration pattern shown in Figure 1 (De Barros et al., 2020). It consists of a slow (125 m day −1 ) expansion of the swarm, involving several episodes of rapid (∼10 km day −1 ) earthquake migrations.…”