The Mw = 5.6 Kanallaki Earthquake of 21 March 2020 In
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geosciences Article The Mw = 5.6 Kanallaki Earthquake of 21 March 2020 in West Epirus, Greece: Reverse Fault Model from InSAR Data and Seismotectonic Implications for Apulia-Eurasia Collision Sotiris Valkaniotis 1,* , Pierre Briole 2 , Athanassios Ganas 3 , Panagiotis Elias 4 , Vassilis Kapetanidis 5 , Varvara Tsironi 3,6 , Anna Fokaefs 3, Helena Partheniou 3,5 and Panagiotis Paschos 7 1 Koronidos Str., 42131 Trikala, Greece 2 Ecole Normale Supérieure, PSL Research University, Laboratoire de Géologie—UMR CNRS, 8538 Paris, France; [email protected] 3 National Observatory of Athens, Institute of Geodynamics, Lofos Nymfon, Thission, 11810 Athens, Greece; [email protected] (A.G.); [email protected] (V.T.); [email protected] (A.F.); [email protected] (H.P.) 4 National Observatory of Athens, Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, Vas. Pavlou & I. Metaxa, GR-15 236 Penteli, Greece; [email protected] 5 Department of Geology, National and Kapodistrian University of Athens, 15784 Athens, Greece; [email protected] 6 Department of Geology, University of Patras, 26504 Rio, Greece 7 Hellenic Survey of Geology and Mineral Exploration, Unit of Epirus, 48100 Eleonas, Preveza, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-210-3490186 Received: 20 October 2020; Accepted: 5 November 2020; Published: 11 November 2020 Abstract: We identify the source of the Mw = 5.6 earthquake that hit west-central Epirus on 21 March 2020 00:49:52 UTC. We use Sentinel-1 synthetic aperture radar interferograms tied to one permanent Global Navigation Satellite System (GNSS) station (GARD). We model the source by inverting the INSAR displacement data. The inversion model suggests a shallow source on a low-angle fault (39◦) dipping towards east with a centroid depth of 8.5 km. The seismic moment deduced from our model agrees with those of the published seismic moment tensors. This geometry is compatible with reverse-slip motion along the west-verging Margariti thrust fault that accommodates part of the convergence within the collision zone between Apulia and Eurasia. We also processed new GNSS 1 data and estimate a total convergence rate between Apulia and Eurasia of 8.9 mm yr− , of which the shortening of the crust between the Epirus coastal GNSS stations and station PAXO in the Ionian 1 Sea (across the Ionian Thrust) is equivalent to ~50% of it or 4.6 mm yr− . By back-slip modelling we found that a 60-km wide deformation zone takes up nearly most of the convergence between Apulia-Eurasia, trending N318◦E. Its central axis runs along the southwest coast of Corfu, along the northeast coast of Paxoi, heading toward the northern extremity of the Lefkada island. The island of Paxoi appears kinematically as part of the Apulian plate. Keywords: deformation; earthquake; InSAR; inversion; fault; convergence; Apulia; Epirus; Greece 1. Introduction The tectonics of Epirus is characterised by on-going compression due to the active collision between the Apulian continental block [1–4] and Eurasian (Aegean) plate. The Apulian continental lithosphere subducts beneath Epirus and was imaged at 70–80 km depth by [5]. Both seismological and Geosciences 2020, 10, 454; doi:10.3390/geosciences10110454 www.mdpi.com/journal/geosciences GeosciencesGeosciences 20202020,, 1010,, 454x FOR PEER REVIEW 22 ofof 2626 lithosphere subducts beneath Epirus and was imaged at 70–80 km depth by [5]. Both seismological geodeticand geodetic data data show show that activethat active deformation deformation is occurring is occurring by crustal by crustal shortening shortening directed directed in a NNE–SSWin a NNE– 1 toSSW NE–SW to NE–SW orientation orientation [6–11] [6–11] (Figure (Figure1). The 1). overall The overall tectonic tectonic strain rate strain was rate estimated was estimated at 40–50 at ns 40–50 yr − ofns contractionyr−1 of contraction by [11 by] for [11] west-central for west-central Epirus. Epirus. At leastAt least 50 50 km km of of horizontal horizontal shortening shortening has has beenbeen suggestedsuggested forfor EpirusEpirus byby geologicalgeological datadata [[12–15]12–15] accompaniedaccompanied byby largelarge scalescale clockwiseclockwise rotationsrotations [[12].12]. Figure 1. Map of Epirus ( A) showing active fault traces (black lines with ticks on the overriding side), Global NavigationNavigation SatelliteSatellite System System (GNSS) (GNSS) stations stations (blue (blue triangles) triangles) and and epicentres epicentres determined determined for for the the 21 March21 March 2020 2020 Mw Mw= 5.6 = 5.6 earthquake. earthquake. Panel Panel (B) ( showsB) shows the the study study area area in the in the NW NW corner corner of Greece. of Greece. Panel Panel (C) shows(C) shows the relocatedthe relocated epicentre epicentre of the of mainshock the mainshock (this study;(this study; white white star) and star) its and aftershocks its aftershocks (red circles; (red periodcircles; 20period March 20 March 2020–23 2020–23 May 2020).May 2020). Fault Fault traces traces modified modified after after [16 [16,17].,17]. The The black black line line betweenbetween thethe PaxoiPaxoi andand CorfuCorfu islandsislands indicatesindicates thethe tracetrace ofof thethe IonianIonian thrustthrust (ticks(ticks onon thethe overridingoverriding side).side). Geological datadata (field (field mapping mapping and and borehole borehole loggings) loggings) indicate indicate that that the the region region between between Corfu Corfu and Ioanninaand Ioannina (Figure (Figure1) is the 1) siteis the of site Neogene of Neogene thrusting thrusting and folding and folding [ 2,3,12] [2,3,12] involving involving mainly thin-skinnedmainly thin- thrustingskinned thrusting onto the Apulianonto the marginApulian along margin Triassic along evaporite Triassic devaporiteécollements décollements [13]. The rock [13]. formations The rock informations most of Epirus in most consist of Epirus of Mesozoic consist and of CenozoicMesozoic carbonate and Cenozoic and clastic carbonate strata ofand the clastic Ionian strata basin of of thethe externalIonian basin Hellenides of the belt,external which Hellenides have been belt, thrusted which westwards have been onto thrusted the Apulian westwards foreland. onto Thethe Apulian inferred traceforeland. of the The main inferred basal trace thrust of the of main Eurasia basal over thrust Apulia, of Eurasia that is over part Apulia, of the that Dinaric–Hellenic is part of the Dinaric– orogen, passesHellenic off orogen,shore Corfu–Paxoi passes offshore islands Corfu–Paxoi with a general islands NNW–SSE with a general orientation NNW–SSE (Figure orientation1A). In the region(Figure of1A). Parga–Kanallaki In the region of (Figure Parga–Kanallaki1), there are (Figure two onshore 1), there thrusts are oftwo the onshore Ionian zonethrusts running of the N-SIonian and zone are west-directedrunning N-S and (i.e., are the west-directed Margariti and (i.e., Paramithia the Margariti thrusts, and [Paramithia14]). Offshore thrusts, Parga, [14]). two Offshore west-verging Parga, reversetwo west-verging faults are reported reverse faults by [15 ]are (mapped reported on by an [1 E–W5] (mapped oriented on geological an E–W cross-sectionoriented geological in that paper,cross- seesection section in that B in paper, their see Figure section 3) to B oinff theirset Pliocene–Miocene Figure 3) to offset sediments,Pliocene–Miocene the western sediments, one representing the western theone contact representing of the overthrusted the contact Ionianof the zoneoverthrusted onto the pre-Apulian Ionian zone zone onto of the the Hellenidespre-Apulian (i.e., zone the Ionianof the Thrust,Hellenides see Figure(i.e., the1A). Ionian The IonianThrust, zone see Figure is a well-known 1A). The Ionian petroliferous zone is a area well-known of Greece petroliferous [ 3,13], yet public area dataof Greece on the [3,13], details yet of public the geological data on structurethe details (including of the geological the detailed structure pattern (including of crustal the shortening) detailed arepattern lacking. of crustal On the shortening) other hand, are openlacking. data On on the the other active hand, tectonics open ofdata Epirus on the can active be provided tectonics byof SpaceEpirus Geodesy can be provided (InSAR and by Spac GNSS,e Geodesy [10,11]), (InSAR which can and be GNSS, combined [10,11]), with which published can be data combined from crustal with seismicitypublished surveysdata from [6 ,crustal9]. seismicity surveys [6,9]. Geosciences 2020, 10, 454 3 of 26 Herein we use Sentinel-1 synthetic aperture radar interferograms, tied at the offsets measured at a nearby permanent Global Navigation Satellite System (GNSS) station (located at Gardiki, station GARD, see Figure1A), to map the co-seismic displacement field of the 21 March 2020 Mw = 5.6 earthquake that occurred near Kanallaki, southwest Epirus (Figure1C). We relocated seismicity data to constrain the hypocentre of the mainshock and its moment tensor solution (Section2). InSAR displacement data was used to invert for fault parameters (Section3). Our inversion approach has resulted in the best-fitting fault model, providing its geometry and kinematics (strike/dip-angle/rake and concluding on its dip-direction). The geodetic observations and the seismicity data allow to discriminate the seismic fault between the two nodal fault planes of the double-couple focal mechanism of the mainshock. We then discuss our findings in terms of the geological structure (Section4) and in particular the depth of the geodetic centroid