Seismic and Geodetic Imaging (DInSAR) Investigation of the March 2021 Strong Earthquake Sequence in Thessaly, Central Greece

Abstract

Three strong earthquakes ruptured the northwest Thessaly area, Central Greece, on the 3, 4 and 12 March 2021. Since the area did not rupture by strong earthquakes in the instrumental period of seismicity, it is of great interest to understand the seismotectonics and source properties of these earthquakes. We combined relocated hypocenters, inversions of teleseismic P-waveforms and of InSAR data, and moment tensor solutions to produce three fault models. The first shock (Mw = 6.3) occurred in a fault segment of strike 314° and dip NE41°. It caused surface subsidence −40 cm and seismic slip 1.2–1.5 m at depth ~10 km. The second earthquake (Mw = 6.2) occurred to the NW on an antithetic subparallel fault segment (strike 123°, dip SW44°). Seismic slip of 1.2 m occurred at depth of ~7 km, while surface subsidence −10 cm was determined. Possibly the same fault was ruptured further to the NW on 12 March (Mw = 5.7, strike 112°, dip SSW42°) that caused ground subsidence −5 cm and seismic slip of 1.0 m at depth ~10 km. We concluded that three blind, unknown and unmapped so far normal fault segments were activated, the entire system of which forms a graben-like structure in the area of northwest Thessaly.

Complex Coseismic and Postseismic Faulting During the 2021 Northern Thessaly (Greece) Earthquake Sequence Illuminated by InSAR Observations

“…The total seismic moments released by the main fault and the secondary fault are ∼3.1 × 10 18 Nm, corresponding to M W 6.3. Compared to a relatively large residual fringe revealed by a single fault model (Papadopoulos et al, 2021;Figure S11d), our two-fault model shows a noticeably better fit to the observed data with the root-mean-square (rms) of 8.5 mm (Figures 2b-2d).…”

“…The focal mechanism solutions from the National Observatory of Athens (NOA), German Research Centre for Geosciences (GFZ; https://geofon.gfz-potsdam.de/) and U.S. Geological Survey (USGS; https://www.usgs.gov/; Tables S1–S3 in Supporting Information ) indicate that the Thessaly seismic sequence composed of the aforementioned three M w > 5.5 earthquakes ruptured on a set of nearly NW‐SE‐trending gently dipping normal faults, consistent with the overall spatial distribution of aftershocks (Figure 1c). Although preliminary geodetic analysis (Ganas et al., 2021; Papadopoulos et al., 2021; Tolomei et al., 2021) highlights the activation of three buried normal faults during the Thessaly seismic sequence, De Novellis et al. (2021) proposes four unknown seismogenic faults accounting for this seismic sequence, which consist of two NE‐dipping faults responsible for Tyrnavos earthquake, one NE‐dipping fault responsible for Elassona earthquake and one SW‐dipping fault responsible for Verdikoussa earthquake.…”

“…Although preliminary geodetic analysis (Ganas et al, 2021;Papadopoulos et al, 2021;Tolomei et al, 2021) highlights the activation of three buried normal faults during the Thessaly seismic sequence, De Novellis et al (2021) proposes four unknown seismogenic faults accounting for this seismic sequence, which consist of two NE-dipping faults responsible for Tyrnavos earthquake, one NE-dipping fault responsible for Elassona earthquake and one SW-dipping fault responsible for Verdikoussa earthquake. However, the agreement between the predicted data by previously published single fault model and two-fault model for the Tyrnavos event and the observed data is rather poor (Papadopoulos et al, 2021;Figure S1) and the dipping orientation of the seismogenic faults for the Elassona and Verdikoussa events have not been agreed upon in these early studies. Besides, previous studies only focus on the coseismic deformation as well as the related source models but ignore the postseismic deformation along with the relevant mechanism.…”

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

“…The broad Thiva area is characterized by normal faulting (Figure 1). It is worth noting that recent strong and destructive earthquakes in Greece have occurred on normal faults, such as the Lesvos 2017 [5,17,18], Kos 2017 [5,19,20], Samos 2020 [21][22][23][24], Thessaly 2021 [25][26][27][28][29][30], and Arkalochori (Crete) 2021 [31,32] earthquakes. Fault segmentation is common near Thiva, with the presence of a multitude of smaller normal faults all around the area.…”

Structural Damage Prediction of a Reinforced Concrete Frame Under Single and Multiple Seismic Events Using Machine Learning Algorithms

“…Thus, the seismic performance of structural systems subjected to successive ground motions receives increasing attention the last years. The recent disaster held on March 2021 in Tyrnavos-Elassona region, Thessaly of Greece due to a pair of compatible magnitude (Mw=6.3, Mw=6.1) [1] shallow earthquakes with more than 1800 damaged or non-serviceable buildings, emerge the necessity of predicting the damage potential caused by mainshock-aftershock sequences in order to assess the seismic risk. It should be noted that the final, accumulated, damage includes the initial damage caused by major earthquake and the incremental damage caused by the following seismic sequence.…”