Spatiotemporal evolution of the 2020 Perachora peninsula earthquake sequence (East Corinth Rift, Greece) and its association with pore-fluid pressure diffusion

In 2020, a pronounced earthquake sequence occurred at the Perachora peninsula, at the eastern edge of the active continental Corinth Rift (Greece). The sequence evolved as a swarm over the course of four months, with the largest magnitude event (M_w=3.7) occurring approximately 2 months after its initiation. The sequence was widely felt by the local population, rising public concern regarding its evolution and a possibly impending stronger and damaging event. Herein, we use seismic waveform data from the Hellenic Unified Seismic Network (HUSN) to decipher the spatiotemporal evolution of the sequence and to investigate the possible triggering mechanisms. We use a custom velocity model for the area and apply the double-difference algorithm to relocate earthquake hypocenters at the East Corinth Rift for the period January 2020 – June 2021. Although the area lacks a local dense network, the herein analysis is able to reduce the relative location uncertainties and to enhance the spatial resolution of the catalogue, providing clues on the activated structures at depth. The spatiotemporal evolution of the sequence presented distinct characteristics of earthquake migration. The Perachora earthquake swarm initiated at shallow depths at the easternmost side of the activated area and progressively migrated towards greater depths to the northwest and then west. The observed seismicity migration pattern is consistent with an expanding parabolic front of hydraulic diffusivity D=2.8 m²/s and an average velocity of 0.22 km/day, indicating pore-fluid pressure diffusion as the primary triggering mechanism. This result is further supported by the relatively high diffusion exponent of the sequence (α=0.89±0.06), which is consistent with anomalous fluid transport phenomena in heterogeneous and fractured media. Overall, the analysis and results demonstrate that the sequence was triggered by fluid overpressures. The source of fluids is likely the down-going flux of meteoric water, possibly combined with fluids of hydrothermal affinity due to the area’s proximity to the Sousaki geothermal system. The activated structures are linked with the Pisia Fault Zone, a major tectonic feature in the area that was activated during the 1981 Alkyonides earthquakes; a series of three M_w > 6 events within a period of few days, which caused severe damage and fatalities in the broader area, including Athens.          

Acknowledgements

The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Number: 00256).