Multisegment ruptures and Vp/Vs variations during the 2020–2021 seismic crisis in western Corinth Gulf, Greece

Abstract

SUMMARY On 2020 December 23, a seismic crisis initiated in the western Corinth Gulf offshore Marathias, lasted several months, and generated thousands of small magnitude earthquakes. The Gulf of Corinth is well known for earthquake swarm occurrence and short-lived burst-like earthquake sequences, mostly triggered by crustal fluids. Here, we perform a detailed seismic analysis aiming to identifying earthquake clusters within the seismic crisis and define their spatial and temporal characteristics. Thanks to the dense seismic station coverage in the area, operated by the Hellenic Unified Seismological Network and Corinth Rift Laboratory, we relocate shallow seismicity and compile a high-resolution earthquake catalogue containing ∼1400 earthquakes spanning the first two months of the seismic crisis. We identify 19 earthquake clusters by applying spatio-temporal criteria and define the geometry (strike and dip) using principal component analysis for 11 of them. Our results are consistent with moment tensor solutions computed for the largest earthquake in each cluster. A striking feature of the seismic activity is the west-towards-east migration with a notable increase in Vp/Vs values for each cluster and a slight increase of the dip angle for the identified fault segments. Furthermore, we find that each cluster contains several burst-like, short interevent time, repeating earthquakes, which could be related to aseismic slip or fluid migration. Overall, we show that the 2020–2021 seismic crisis consists of earthquake clusters that bifurcate between swarm-like and main shock–aftershock-like sequences and ruptured both north- and south-dipping high-angle fault segments. The 2020–2021 seismic activity is located between 5 to 8 km, shallower than the low angle north-dipping (∼10°) seismic zone which hosts long-lived repeating sequences at ∼9–10 km depth. This study supports a hypothesis that the low-angle north-dipping seismicity defines the brittle-ductile transition in the western Corinth Gulf, with seismic bursts occurring at shallower depths in the crust.