Cascading occurrence of moderate magnitude seismicity in the active fault system of Thiva (central Greece)

Intense and continuous seismicity in the last two years (October 2020 – September 2022) that took place in a rather small area near the city of Thiva is investigated here. The activity started with an M4.6 earthquake (2 of December 2020) followed by its own aftershock sequence. The activity migrated slightly to the west, with a persistent swarm in July 2021-September 2022 (largest magnitudes M4.3, on 11 July 2021, M4.0 on 2 September 2021 and M4.3, on 10 April 2022). Aiming to constrain the geometry and kinematics of the activated fault segments along with the spatiotemporal evolution of the seismic activity, processing of the recording of the regional seismological network was accomplished that includes the determination of the focal coordinates using the HYPODD software. The above information will provide a better understanding of the seismic sequence and seismic hazard in the region so that there is better prevention and preparation against a future strong earthquake. Aiming to study in detail the properties of this seismicity manifestation, the recordings of the Hellenic Unified Seismological Network (HUSN) are used to accurately determine the seismic parameters of earthquakes with magnitudes M≥1.5. Phases (P, S phases) are gathered from the Geophysical Department of the Aristotle University of Thessaloniki and the Geodynamics Institute of National Observatory of Athens from October 2020 to September 20212. Then, the bulletins were merged, and an initial earthquake catalogue was compiled containing ~6000 events. Earthquake relocation was initially performed using HYPOINVERSE software and all the available manually picked P and S phases. An appropriate local velocity model and the V_P/V_S ratio were necessary to defined. The Wadati method was applied to the dataset and the resulting V_P/V_S ratio equals to 1.76. The one-dimensional velocity model used for this study is calculating by the VELEST software. Time corrections relative to the crustal model were calculated considering the mean residual for each station. For the relocation of the events the calculated time delays were taken into account. To improve the obtained locations, we relocate the earthquakes using the double difference inversion algorithm, hypoDD with differential times derived from phase-picked data.

Τo define the stress regime in the area, the moment tensors of earthquakes with ML ≥ 3.5 were estimated using the ISOLA and FPFIT software. The fault plane solutions from the largest earthquakes of the seismic sequence have been used for Coulomb stress changes calculation. The stress field is calculated according to the focal mechanism of the next large event, whose triggering is inspected, so it can be checked if foreshocks contributed to the occurrence of the largest earthquakes of the sequence and the possible sites for future strong earthquakes can be assessed as well.