  Citation: Triantafyllou, I.; Karavias, A.; Koukouvelas, I.; Papadopoulos, G.A.; Parcharidis, I. The Crete Isl. (Greece) M w 6.0 Earthquake of 27 September 2021: Expecting the Unexpected. GeoHazards 2022, 3, 106–124. https://doi.org/10.3390/ geohazards3010006 Academic Editor: Paolo Boncio Received: 31 December 2021 Accepted: 21 February 2022 Published: 25 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). GeoHazards Article The Crete Isl. (Greece) M w 6.0 Earthquake of 27 September 2021: Expecting the Unexpected Ioanna Triantafyllou 1, *, Andreas Karavias 2 , Ioannis Koukouvelas 3 , Gerassimos A. Papadopoulos 4 and Issaak Parcharidis 2 1 Department of Dynamic Tectonic Applied Geology, Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, 15784 Athens, Greece 2 Department of Geography, Harokopio University, 17671 Athens, Greece; gp219309@hua.gr (A.K.); parchar@hua.gr (I.P.) 3 Department of Geology, University of Patras, 26504 Patras, Greece; iannis@upatras.gr 4 International Society for the Prevention & Mitigation of Natural Hazards, 10681 Athens, Greece; gerassimospapadopoulos2@gmail.com * Correspondence: ioannatriantafyllou@yahoo.gr Abstract: The 27 September 2021 damaging mainshock (M w 6.0) is the first known strong earthquake that ruptured the Arkalochori area, Crete Isl., Greece, during the entire historical period, making it an unexpected event in the long-term sense. The area is characterized by the presence of the normal active Kastelli Fault (KF) striking NNE-SSW and dipping towards ~WNW. The KF, of surface exposure only ~6 km, at its southern tip is truncated by the nearly perpendicular active Nipiditos fault. The main shock was preceded by foreshock activity lasting for ~3.9 months, thus the mainshock turned out to be an expected event in the short-term sense. Maximum ground subsidence of ~20 cm was estimated from InSAR images, but this also incorporates deformation that may have been caused by the largest aftershock (M w 5.1) of 28 September 2021. The fault model produced from the inversion of InSAR observations indicated strike 216 ◦ , dip towards ~NW at angle 53 ◦ , rake −95 ◦ , and is consistent with fault-plane solutions obtained from routine moment tensor analysis. The geodetic seismic moment calculated from the Okada’s formalism is 1.14 × 10 18 N·m (M w 6.0), while a maximum slip of 1.03 m was found at depths from 3.5 km to 5 km. The entire aftershock epicenters cloud strikes in a ~SW-NE direction but is distributed in two clusters, the southern and the northern ones. The foreshock cloud, the main slip patch, the deformation area, and the strongest aftershocks all fall within the southern cluster. The foreshocks concentration at the deepest edge of the main slip patch was a foreshadow of the mainshock nucleation area. The northern cluster, which is very likely due to the gradual expansion of aftershocks, is situated in the KF hanging wall block. To interpret the main seismic slip in the southern cluster area we propose the existence of a buried KF segment at the SSW-wards prolongation of the emerged at the surface segment. Assuming a rectangular seismic fault stress drop Δσ~7 bars was found. However, for a circular fault area, which in this case is more realistic, we get Δσ = 55 bars. This is a relatively large value for Greek earthquakes but is explainable by increased fault rigidity as a result of the long repeat time of strong earthquakes in KF. Keywords: 27 September 2021 earthquake; Crete; Greece; foreshocks; aftershocks; InSAR data inversion; source model; buried normal fault 1. Introduction On 27 September 2021 (06:17:21.3 UTC) a strong earthquake associated with normal faulting ruptured the central part of the island of Crete, Greece, at ~20 km to the south of Heraklion, the capital city of the island (Figure 1). Moment magnitude, M w , of 5.9 or 6.0 has been determined by various seismological centers (see data compilation in https://www.emsc-csem.org/Earthquake/index_tensors.php; last access 17 December 2021) but local magnitude M L 5.8 was initially calculated by the National Observatory of GeoHazards 2022, 3, 106–124. https://doi.org/10.3390/geohazards3010006 https://www.mdpi.com/journal/geohazards