Investigation of the Thiva 2020–2021 Earthquake Sequence Using Seismological Data and Space Techniques

Abstract

We investigate an earthquake sequence involving an Mw = 4.6 mainshock on 2 December 2020, followed by a seismic swarm in July–October 2021 near Thiva, Central Greece, to identify the activated structures and understand its triggering mechanisms. For this purpose, we employ double-difference relocation to construct a high-resolution earthquake catalogue and examine in detail the distribution of hypocenters and the spatiotemporal evolution of the sequence. Furthermore, we apply instrumental and imaging geodesy to map the local deformation and identify long-term trends or anomalies that could have contributed to stress loading. The 2021 seismic swarm was hosted on a system of conjugate normal faults, including the eastward extension of the Yliki fault, with the main activated structures trending WNW–ESE and dipping south. No pre- or coseismic deformation could be associated with the 2021 swarm, while Coulomb stress transfer due to the Mw = 4.6 mainshock of December 2020 was found to be insufficient to trigger its nucleation. However, the evolution of the swarm is related to stress triggering by its major events and facilitated by pore-fluid pressure diffusion. The re-evaluated seismic history of the area reveals its potential to generate destructive Mw = 6.0 earthquakes; therefore, the continued monitoring of its microseismicity is considered important.

Complexity of Recent Earthquake Swarms in Greece in Terms of Non-Extensive Statistical Physics

“…Thiva is a city in Boetoia (Central Greece), located at the transition zone between the Corinth Gulf in the south and Evia in the east. Two major rift structures, oriented WNW—ESE and NW-SE, respectively, dominate the tectonics of the area [ 76 ]. The land between the two gulfs is an area of lower strain controlled by normal faulting.…”

“…On 10 July 2021, seismic activity started at the western part of the swarm and the next seismic events present a general tendency for spatiotemporal migration towards ESE. The evolution of the swarm is related to stress triggered by its major events and facilitated by pore-fluid pressure diffusion [ 76 ].…”

A Logic-Tree Approach for Probabilistic Seismic Hazard Assessment in the Administrative Region of Attica (Greece)

“…Just north of Attica, another area that is known to host significant earthquakes is Thiva. The recent seismicity in this region includes a M w = 4.6 event on 2 December 2020 [20] and the major events (3.4 ≤ M ≤ 4.2) of a long-lasting swarm that began in July 2021 [21]. These events were also felt in Athens; however, due to their small magnitudes, they did not cause any damage.…”

Cluster Analysis of Seismicity in the Eastern Gulf of Corinth Based on a Waveform Template Matching Catalog

“…Very often, the larger events occur mid-sequence, rather than at the beginning [ 4 ], a characteristic that is exploited to distinguish between mainshock–aftershock sequences and swarms based on the skewness and kurtosis of their seismic moment release history [ 5 ]. Furthermore, swarms usually evolve in multiple stages, with bursts of earthquakes, usually concentrated in spatial clusters, either activating neighboring faults or re-activating the same fault patches [ 6 , 7 ]. The latter is related to the concept of repeating earthquakes, or multiplets, i.e., events with similar earthquake source parameters, producing nearly identical waveform recordings [ 8 ].…”

“…Mainshock–aftershock sequences, especially those involving large-magnitude events, usually trigger public interest, which, in turn, motivates seismological institutes to quickly establish temporary local networks in the affected area for closer monitoring of seismicity. The issue with earthquake swarms is that it is often not clear when a seismic excitation in a specific area, in terms of an increase in the seismicity rate, is going to last long enough [ 6 ], or evolve into a more hazardous event, such as a major earthquake [ 17 , 18 ], to merit the effort of deploying a temporary local network. As a result, most earthquake swarms usually go unnoticed or under-reported.…”