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  
[PERSON]  
1 Section of Geography and Climatology, Department of Geology and Geoenvironment, School of Sciences, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, 15784 Athens, Greece; [EMAIL_ADDRESS] (K.A.K. ); [EMAIL_ADDRESS] (S.P. ); [EMAIL_ADDRESS] (G.-A.T.)
2 Section of Geophysics-Geothermics, Department of Geology and Geoenvironment, School of Sciences, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, 15784 Athens, Greece; [EMAIL_ADDRESS] (G.K.); [EMAIL_ADDRESS] (V.K. ); [EMAIL_ADDRESS] (J.A. ); [EMAIL_ADDRESS] (N.V.)
3 Department of Physical and Environmental Geography, Aristotle University of Thessaloniki (AUTH), 54124 Thessaloniki, Greece; [EMAIL_ADDRESS] (E.P. ); [EMAIL_ADDRESS] (M.F.) 4 Institute of Geodynamics, National Observatory of Athens, Lofos Nymfon, 11810 Athens, Greece; [EMAIL_ADDRESS] (C.P.E. ); [EMAIL_ADDRESS] (V.K)
5 Correspondence: [EMAIL_ADDRESS]  
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In particular, MB (Figure 1) acts as a supra-detachment basin [2,4] which lies on top of the hanging wall of the south Cretan extensional detachment [3]. The latter is located at the active margin of the rapidly southwestwards-moving Aegean micro-plate [5, 6, 7]. It is an extensional, south-dipping structure, similar to the north-dipping Cretan Detachment [8, 9, 10]. Figure 1: (a) Location of Crete (red rectangle) in southern Greece. Bold lines marked with dents or arrows indicate major tectonic structures in the vicinity of the Hellenic Arc; ACZ: Apulian Collision Zone, CTFZ: Cephalonia Transform Fault Zone, NAT: North Aegean Trough, NAF: North Anatolian Fault. Lines indicate the traces of seismogenic sources from the Euro-Mediterranean Seismic Hazard Model 2013 (ESHM13; [11]). The NNE-pointing vector at the lower-left indicates the direction and convergence rate of the Nubian plate [12]. (b) Seismotectonic map of Crete (red rectangle in panel (a). Black lines are active faults from the NOAfaults v3.0 database [13]. Beachballs indicate focal mechanisms of earthquakes (\(M_{\mathrm{w}}\geq 4.0\)) from the compilation of [14] and sources referenced therein (see also Data Availability). Circles (\(M\) < 5.0) and stars (\(M\geq 5.0\)) represent past seismicity of years 1000–1899 (SHEEC; [15]) and 1900–2009 [16] for events with focal depths down to 70 km. The maximum (S\({}_{1}\)) and minimum (S\({}_{3}\)) principal stress axes of the regional crustal stress field [14] are  
depicted by blue and red arrows, respectively, with arrow length proportional to the cosine of the plunge angle. The epicenter of the Arkalochori earthquake is presented by a star within MB and HB areas. MB has been considered an E-W-trending, typical graben structure [17]; however, it is characterized by the absence of classic continuous bounding faults at the north and south margins of the basin. Its northern margin is probably covered with Miocene and/or Pliocene sediments. Therefore, the exact boundaries between MB and HB can hardly be observed and, consequently, several studies, with different arguments [18; 19; 20], implicate arc-parallel extension dynamics, trending along the local Hellenic trench axis (Figure 1). Intense historical and recent seismicity reveal the continuous contemporary tectonic activity of the entire Cretan territory (e.g., [21]). Destructive earthquakes have occurred in the broader Crete region since antiquity and during the pre-instrumental era, until 1900 AD. The earthquake of 8 August 1303 [15; 22], with an estimated magnitude of 8.3 [23], is one of the largest that has ever been reported in the entire Mediterranean region. This earthquake also caused a large tsunami that was observed at Crete, Rhodes, Peloponnese, Egypt, Syria, the Adriatic Sea, and along the eastern Mediterranean coast.