The 27 September 2021 Earthquake in Central Crete (Greece)—Detailed Analysis of the Earthquake Sequence and Indications for Contemporary Arc-Parallel Extension to the Hellenic Arc

Abstract

The Arkalochori village in central Crete was hit by a large earthquake (Mw = 6.0) on 27 September 2021, causing casualties, injuries, and severe damage to the infrastructure. Due to the absence of apparent surface rupture and the initial focal mechanism solution of the seismic event, we initiated complementary, multi-disciplinary research by combining seismological and remote sensing data processing, followed by extensive field validation. Detailed geological mapping, fault surface measuring accompanied with tectonic analysis, fault photorealistic model creation by unmanned aerial system data processing, post-seismic surface deformation analysis by DInSAR image interpretation coupled with accurately relocated epicenters recorded by locally established seismographs have been carried out. The combination of the results obtained from these techniques led to the determination of the contemporary tectonic stress regime that caused the earthquake in central Crete, which was found compatible with extensional processes parallel to the Hellenic arc.

Satellite-Observed Thermal Anomalies and Deformation Patterns Associated to the 2021, Central Crete Seismic Sequence

“…The InSAR results show that the area surrounding Arkalochori moved roughly downwards and are in a good agreement with previous works [21,22,53]. Note that dipslip earthquakes have a very asymmetrical displacement pattern: i.e., the subsidence will be larger than the uplift for normal-faulting earthquakes.…”

“…After the decomposition, the ground deformation in the vertical (up-down) direction showed subsidence up to 20 cm, while no uplift displacement was detected (Figure 7a). The horizontal (east-west) displacement map indicated a maximum displacement of about 8 cm towards the east of the footwall block and about 6 cm towards the west of the hanging wall (Figure 7b) in agreement with that observed in [21,22] and modeled in [53], probably attributed to the strike-slip component of the earthquake focal mechanism as suggested in [21]. The InSAR results show that the area surrounding Arkalochori moved roughly downwards and are in a good agreement with previous works [21,22,53].…”

“…On 27 September 2021, a rare and strong onshore earthquake of magnitude Mw6.0 occurred in Central Crete, near the village of Arkalochori at 06:17:21 UTC [21]. The epicenter of the seismic event was located at latitude 35.15 N and longitude 25.27 E, while the focal depth was about 10 km and associated with the Kastelli Fault Zone [22] whose mean recurrence interval has been estimated at approximately 800 years [23]. Since the beginning of June, almost 4 months earlier, a significant number of foreshocks had beenrecorded in the broader area, while a rich aftershock sequence was observed in the months after the main shock occurrence [21,22,24,25].…”

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

“…It is also worth noting that the evolution of a seismic swarm is not always similar to the one of the present study. Recently, on 27 September 2021, while the Thiva swarm was still active, a destructive earthquake (M w = 6.0) occurred in Arkalochori in central Crete [31,32]. In Arkalochori, the seismic activity was initiated about 4 months before the mainshock, in early June 2021, as an earthquake swarm.…”

“…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.…”