The Geological Structure and Tectonic Complexity of Northern Thessaly That Hosted the March 2021 Seismic Crisis

Abstract

Knowing the rich presence of active faults in northern Thessaly and the lack of any significant seismic activity since at least the mid-1940s, the 2021 seismic sequence did not surprise us. What did surprise us was the fact that (i) despite the great knowledge of the neotectonic faults in the area, the causative faults were unknown, or almost unknown; (ii) the direction of the 2021 faulting was different than the expected, and given that the focal mechanisms showed almost pure normal dip-slip motion, the extensional main axis was also different than the one we thought we knew for this area; and (iii) besides the co-seismic ruptures that occurred within the Domeniko-Amouri basin and along the Titarissios River valley, there is evidence of rupturing in the alpine basement of Zarkos mountains. After thoroughly reviewing both the alpine and neotectonic structural setting and all the available literature concerning the seismotectonic data and interpretations of the 2021 sequence, including investigations of our own, we end up in a complex tectonic setting with older alpine structures now operating as inherited faults, and we also suggest the possible occurrence of a roughly N-dipping, low-angle, detachment-type fault. This fault runs below Mt Zarkos, reaching at least the Elassona Basin, with splay faults bifurcating upwards from the main fault zone. Following this complexity, rupture of the first mainshock must have chosen a split route reaching the surface through the gneiss rocks of Zarkos and almost (?) reaching the basinal sediments of the local tectonic depressions. This seismic sequence is a perfect case study to shed some light on the tectonic and rupture processes in the context of both geodynamics and seismic hazard assessment.

The Upper Crustal Deformation Field of Greece Inferred From GPS Data and Its Correlation With Earthquake Occurrence

“…However, this NE-SW direction is compatible with the NW-SE strike of the coseismic ground ruptures that were revealed by postearthquake field observations and InSAR data (Chatzipetros et al, 2021;Kontoes et al, 2022;Koukouvelas et al, 2021;Mouslopoulou et al, 2022;Sboras et al, 2022) and is in agreement with the NE-SW extensional stress field obtained from the focal mechanisms of the 2021 Tyrnavos seismic sequence (Kassaras et al, 2022) and with geomorphic indicators which suggest repeated past ruptures on the 2021 causative faults (Mouslopoulou et al, 2022). In this context, it turns out that the orientation of the faults that ruptured during the 2021 sequence was not incompatible with the present-day crustal strain orientation as it was initially believed (Lazos et al, 2021;Sboras et al, 2022) and it appears that the occurrence of this earthquake sequence is well-justified by the active strain field.…”

An Updated Earthquake Catalogue in Crete Derived by the Development of Local 1D Velocity Models and Hypocentre Relocation

“…In area 3 (Matala), most of the events that were relocated using hypoDD belong to the earthquake sequence that occurred near Matala between 17 May 2023 and 31 May 2023. More than 130 events were located using the local velocity model obtained using the data of this study, achieving a better concentration near some of the most significant local fault sources [13,52] as a first result. The calculation of differential travel times resulted in an average number of five links per event pair and average offset between linked events of 2.07 km.…”

“…Ref. [52] shows a more detailed image of the upper crust using data from seismic surveys conducted in the area. Their P-wave velocity model is composed of four (4) layers in the uppermost 2 km and reaches 5.2-5.3 km/s below 4 km of depth.…”

“…The velocity model of the crust varies between 4.9 and 6.8 km/s for approximately 25 km, while the respective values for the neighbouring area of Rethymno are between 5.6 and 6.8 km/s for crustal thickness of less than 31 km (Figure 6b,c). The velocity model of Matala-Central Ptolemy Trench was tested in the recent earthquake sequence near Matala (17 May 2023-31 May 2023), where more than 130 events were located with a better concentration near some of the most significant local fault sources [13,52] and yielded improved error uncertainties compared to the initial model of [39].…”

Discontinuous Surface Ruptures and Slip Distributions in the Epicentral Region of the 2021 Mw7.4 Maduo Earthquake, China

“…Fault geometrical complexities, such as bends, branches, and stepovers, are widespread in strike-slip fault systems and separate the fault into segments of varying lengths [1][2][3]. These geometrical complexities can play a critical role in the initiation, propagation, and termination of earthquake ruptures [4][5][6][7][8][9][10] depending on the accumulated initial stress before an earthquake as well as the dynamic stresses during the seismic rupture [11][12][13]. Additionally, these geometrical complexities can impact the coseismic surface rupture pattern, slip distribution, and off-fault deformation, which is of significant importance for understanding fault mechanics, rupture dynamics, and seismic hazard analysis [14][15][16][17].…”