remote sensing Article Ground Deformation Modelling of the 2020 Mw6.9 Samos Earthquake (Greece) Based on InSAR and GNSS Data Vassilis Sakkas Department of Geophysics-Geothermics, National Kapodistrian University of Athens, Panepistimiopolis Zografou, 15784 Athens, Greece; [email protected]; Tel.: +30-2107274914 Abstract: Modelling of combined Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) data was performed to characterize the source of the Mw6.9 earthquake that occurred to the north of Samos Island (Aegean Sea) on 30 October 2020. Pre-seismic analysis revealed an NNE–SSW extensional regime with normal faults along an E–W direction. Co- seismic analysis showed opening of the epicentral region with horizontal and vertical displacements of ~350 mm and ~90 mm, respectively. Line-of-sight (LOS) interferometric vectors were geodetically corrected using the GNSS data and decomposed into E–W and vertical displacement components. Compiled interferometric maps reveal that relatively large ground displacements had occurred in the western part of Samos but had attenuated towards the eastern and southern parts. Alternating motions occurred along and across the main geotectonic units of the island. The best-fit fault model has a two-segment listric fault plane (average slip 1.76 m) of normal type that lies adjacent to the northern coastline of Samos. This fault plane is 35 km long, extends to 15 km depth, and dips to the north at 60◦ and 40◦ angles for the upper and lower parts, respectively. A predominant dip-slip component and a substantial lateral one were modelled. Citation: Sakkas, V. Ground Keywords: 2020 Samos earthquake; SAR interferometry; GNSS; fault modelling; slip distribution Deformation Modelling of the 2020 Mw6.9 Samos Earthquake (Greece) Based on InSAR and GNSS Data. Remote Sens. 2021, 13, 1665. https://doi.org/10.3390/rs13091665 1. Introduction The Northern Aegean Sea (Greece) is characterized by a complex geotectonic setting Academic Editors: Magaly Koch and with intense seismic activity (Figure1). The area that is bordered by the Northern Anatolian Mimmo Palano Fault (NAF) zone to the north and the Hellenic Trench to the south exhibits a strong extensional regime consistent with major continental extension [1,2]. The kinematic and Received: 19 March 2021 dynamic models of the area are compatible with plate tectonic motions [3,4] and highlight Accepted: 22 April 2021 the occurrence of strong ground deformation and intense seismicity. The NE–SW trending, Published: 24 April 2021 dextral, strike-slip faulting in the northern Aegean Sea is associated with and linked to the NAF, which trends parallel to the North Aegean Trough [5–7]. However, the eastern part of Publisher’s Note: MDPI stays neutral the Aegean Sea has diverse fault trends and character. This region accommodates several with regard to jurisdictional claims in E–W trending fault zones that exhibit normal-type motions, which are consistent with the published maps and institutional affil- extensional stress field [3,8]. iations. The study area of Samos Island—located in the eastern part of the northern Aegean Sea—is subject to extensional forces. It is situated to the south of a NE–SW trending, dextral, strike-slip transfer zone called the Izmir Balikesir Transfer Zone (IBTZ) [1,9], as well as a zone of E–W normal faulting near Izmir Bay (Turkey). East of the IBTZ is the Sakarya Copyright: © 2021 by the author. Tectonic Unit (STU), which includes Chios Island [10], while in the south the islands of Licensee MDPI, Basel, Switzerland. Samos and Ikaria are part of the tectono-metamorphic belt of the Hellenides Tectonic Unit This article is an open access article (HTU) [10]. This broad region represents the transition between western Turkey and the distributed under the terms and Aegean domains [10–12]. conditions of the Creative Commons The geology of the island consists of four main tectono-metamorphic units that are Attribution (CC BY) license (https:// mainly covered by Mio-Pliocene sedimentary basins [11–14]. Based on geological and creativecommons.org/licenses/by/ seismological studies, there are five active fault zones that have the potential to generate 4.0/). Remote Sens. 2021, 13, 1665. https://doi.org/10.3390/rs13091665 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 24 Remote Sens. 2021, 13, 1665 The geology of the island consists of four main tectono-metamorphic2 of 24 units that are mainly covered by Mio-Pliocene sedimentary basins [11–14]. Based on geological and seis- mological studies, there are five active fault zones that have the potential to generate large- large-magnitudemagnitude earthquakes earthquakes [15]. These [15]. zones These are zones oriented are oriented approximately approximately E–W. Two E–W. of Two of them them are locatedare in located the northern in the coastal northern part, coastal and two part, in theand southern two in centralthe southern part of central Samos. part of Samos. Their dip anglesTheir vary dip between angles 50vary and between 75◦. The 50 longest and 75°. fault The zone longest (~27 fault km) occurszone (~27 to thekm) occurs to the north and offshorenorth of and the offshore island; it of is the a normal island; fault it is thata normal dips tofault the that north dips at to an the angle north of at an angle of 40◦. This fault zone40°. This is located fault zone in the is southern located in part the of southern the Samos part Basin of the (SB) Samos [14]. Basin This basin(SB) [14]. This basin extends westwardextends towards westward the deeper towards (1600 the m deeper depth) (1600 North m Ikarian depth) Basin North (NIB). Ikarian Basin (NIB). Figure 1. (aFigure) Index 1. map(a) Indexand (b map) relief and map (b) reliefof the map broader of the area broader of Samos, area ofwhich Samos, shows which the showslocation the of location continuous Global Navigationof Satellite continuous System Global (GNSS) Navigation sites that Satellitewere used System in this (GNSS) study sites(red triangles), that were together used in thiswith study the focal (red mechanism solutions oftriangles), the Mw6.9 together earthquake with and the its focal major mechanism aftershock. solutions Faults (black of the lines) Mw6.9 were earthquake taken from and[15–18]. its majorRed dashed lines define the Izmiraftershock. Balikesir Faults Transfer (black Zone lines) (IBTZ); were taken yellow from dash [15ed–18 line]. Red marks dashed the Sakarya lines define Tectonic the Izmir Unit Balikesir(STU); blue dashed line definesTransfer the Hellenides Zone (IBTZ); Tectonic yellow Unit dashed (HTU); line NIB: marks North the Ikaria Sakarya Basin; Tectonic SB: Samos Unit Basin; (STU); NAF: blue dashedNorth Anatolian line Fault zone. defines the Hellenides Tectonic Unit (HTU); NIB: North Ikaria Basin; SB: Samos Basin; NAF: North Anatolian Fault zone. Several large-magnitude earthquakes have occurred over the last few years in this Several large-magnitudepart of the Aegean earthquakes [19,20]. haveThe most occurred recent over on thee occurred last few to years the innorth this of part Samos on 30 Oc- of the Aegean [tober19,20 ].2020. The This most earthquake recent one occurredcaused extensiv to the northe damage of Samos on the on island 30 October and the surrounding 2020. This earthquakeareas, as caused well as extensive significant damage co-seismic on the islanddisplacements. and the surrounding Combined areas,GNSS asand InSAR tech- well as significantniques co-seismic have been displacements. proved effective Combined in prec GNSSisely and measuring InSAR techniques the ground have deformation. The been proved effectiveGNSS intechnique precisely can measuring provide the an groundabsolute deformation. 3D vector of The ground GNSS techniquedisplacement (estimated can provide anerrors absolute ~2–3 3D mm vector and of~5–8 ground mm for displacement the horizontal (estimated and vertical errors component, ~2–3 mm respectively), and ~5–8 mm forbut the it is horizontal limited to and point-wise vertical component,coverage. The respectively), InSAR provides but it spatial is limited coverage, to but the in- point-wise coverage.formation The is InSAR in the providesline-of-sight spatial (LOS) coverage, direction, but and the further information multi-geometrical is in the analysis is line-of-sight (LOS)required direction, to obtain and furtherthe true multi-geometrical ground motion components. analysis is required Moreover, to obtainthe conventional dif- the true groundferential motion components. interferometry Moreover, may detect the conventional displacements differential of at least interferometry ~28 mm, which is half the may detect displacementswavelength ofof atthe least radar ~28 signal. mm, Thus, which joint is half application the wavelength of GNSS of and the InSAR radar data can effec- signal. Thus, jointtively application determine of the GNSS realand ground InSAR displacement data can effectively field. determine the real ground displacementThe field. purpose of the present work is to study the ground deformation associated with The purposethe ofMw6.9 the present earthquake work isby to combining study the groundGNSS and deformation InSAR data associated for analysis. with The methodol- the Mw6.9 earthquakeogy has been by combining previously GNSS applied and to InSAR other tect dataonically for analysis. active areas The method-of Greece [21–23]. This ology has been previously applied to other tectonically active areas of Greece [21–23].