Domino-style earthquakes along blind normal faults in Northern Thessaly (Greece): kinematic evidence from field observations, seismology, SAR interferometry and GNSS

Abstract

Here we present a joint analysis of the geodetic, seismological and geological data of the March 2021 Northern Thessaly seismic sequence, that were gathered and processed as of April 30, 2021. First, we relocated seismicity data from regional and local networks and inferred the dip-direction (NE) and dip-angle (38°) of the March 3, 2021 rupture plane. Furthermore, we used ascending and descending SAR images acquired by the Sentinel-1 satellites to map the co-seismic displacement field. Our results indicate that the March 3, 2021 Mw=6.3 rupture occurred on a NE-dipping, 39° normal fault located between the villages Zarko (Trikala) and Damasi (Larissa). The event of March 4, 2021 occurred northwest of Damasi, along a fault oriented WNW-ESE and produced less deformation than the event of the previous day. The third event occurred on March 12, 2021 along a south-dipping normal fault. We computed 22 focal mechanisms of aftershocks with M≥4.0 using P-wave first motion polarities. Nearly all focal mechanisms exhibit normal kinematics or have a dominant normal dip-slip component. The use of InSAR was crucial to differentiate the ground deformation between the ruptures. The majority of deformation occurs in the vertical component, with a maximum of 0.39 m of subsidence over the Mw=6.3 rupture plane, south and west of Damasi. A total amount of 0.3 m horizontal displacement (E-W) was measured. We also used GNSS data (at 30-s sampling interval) from twelve permanent stations near the epicentres to obtain 3D seismic offsets of station positions. Only the first event produces significant displacement at the GNSS stations (as predicted by the fault models, themselves very well constrained by InSAR). We calculated several post-seismic interferograms, yet we have observed that there is almost no post-seismic deformation, except in the footwall area (Zarkos mountain). This post-seismic deformation is below the 7 mm level (quarter of a fringe) in the near field and below the 1 mm level at the GNSS sites. The cascading activation of the three events in a SE to NW direction points to a pattern of domino-style earthquakes, along neighbouring fault segments. The kinematics of the ruptures point to a counter-clockwise change in the extension direction of the upper crust (from NE-SW near Damasi to N-S towards northwest, near Verdikoussa).

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

“…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.…”

Seismicity Patterns Prior to the Thessaly (Mw6.3) Strong Earthquake on 3 March 2021 in Terms of Multiresolution Wavelets and Natural Time Analysis

“…The earthquake occurred in a region that is one of the most seismically active in Greece, mainly characterized by normal faulting along NW-SE striking faults, which belong to the Thessaly fault zone [1][2][3][4][5][6][7][8][9][10]. Based on the provided focal plane solutions [11], the mainshock was generated by the activation of an NW-SE striking normal fault (Figure 1) [12]. The mainshock was widely felt in the Thessaly basin and in the surrounding areas, from Athens in the south to the northern borders of Greece.…”

“…The Thessaly basin has a well-known history of large earthquakes, with mainshocks presenting typical magnitudes between 6.0 and 7.0 [3][4][5][6][7][8][9][10][11][12][13][14][15]. During the last century, eight major earthquakes, with magnitudes equal to or larger than 6.0, have occurred in this area.…”

The scaling of PGA with IV2p and its potential for Earthquake Early Warning in Thessaly (Central Greece)

“…However, the highest PGA, recorded at station GINA at the outskirts of the city, was ~140 cm/s 2 , well below the limit given by the national building code, i.e., ~235 cm/s 2 (ΕΑΚ, 2003). This event was followed by another strong earthquake with Mw=6.1 (Ganas et al, 2021), located ~10 km NW.…”