geosciences Article Seismic and Geodetic Imaging (DInSAR) Investigation of the March 2021 Strong Earthquake Sequence in Thessaly, Central Greece Gerassimos A. Papadopoulos 1,*, Apostolos Agalos 1 , Andreas Karavias 2, Ioanna Triantafyllou 3, Issaak Parcharidis 2 and Efthymios Lekkas 3 1 International Society for the Prevention & Mitigation of Natural Hazards, 10681 Athens, Greece; [email protected] 2 Department of Geography, Harokopio University, 17671 Athens, Greece; [email protected] (A.K.); [email protected] (I.P.) 3 Department of Dynamic Tectonic Applied Geology, Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, 15784 Athens, Greece; [email protected] (I.T.); [email protected] (E.L.) * Correspondence: [email protected] Abstract: Three strong earthquakes ruptured the northwest Thessaly area, Central Greece, on the 3, 4 and 12 March 2021. Since the area did not rupture by strong earthquakes in the instrumental period of seismicity, it is of great interest to understand the seismotectonics and source properties Citation: Papadopoulos, G.A.; of these earthquakes. We combined relocated hypocenters, inversions of teleseismic P-waveforms Agalos, A.; Karavias, A.; and of InSAR data, and moment tensor solutions to produce three fault models. The first shock Triantafyllou, I.; Parcharidis, I.; ◦ ◦ (Mw = 6.3) occurred in a fault segment of strike 314 and dip NE41 . It caused surface subsidence Lekkas, E. Seismic and Geodetic −40 cm and seismic slip 1.2–1.5 m at depth ~10 km. The second earthquake (Mw = 6.2) occurred to Imaging (DInSAR) Investigation of the NW on an antithetic subparallel fault segment (strike 123◦, dip SW44◦). Seismic slip of 1.2 m the March 2021 Strong Earthquake − Sequence in Thessaly, Central Greece. occurred at depth of ~7 km, while surface subsidence 10 cm was determined. Possibly the same ◦ ◦ Geosciences 2021, 11, 311. https:// fault was ruptured further to the NW on 12 March (Mw = 5.7, strike 112 , dip SSW42 ) that caused doi.org/10.3390/geosciences11080311 ground subsidence −5 cm and seismic slip of 1.0 m at depth ~10 km. We concluded that three blind, unknown and unmapped so far normal fault segments were activated, the entire system of which Academic Editors: forms a graben-like structure in the area of northwest Thessaly. Ioannis Koukouvelas, Riccardo Caputo, Tejpal Singh Keywords: March 2021 earthquakes; Thessaly area; Central Greece; hypocenter relocation; and Jesus Martinez-Frias teleseismic P-waveforms inversion; InSAR data inversion; Okada displacement; blind normal faults; antithetic faults; graben-like structure Received: 7 June 2021 Accepted: 21 July 2021 Published: 25 July 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in During March 2021 the area of northwest Thessaly, Central Greece, was ruptured published maps and institutional affil- by a sequence of three strong earthquakes [1] (Figure1) that occurred on the 3 (10:16:08 iations. UTC), 4 (18:38:19 UTC) and 12 (12:57:50 UTC) of the month. Source parameters of these earthquakes, determined by the National Observatory of Athens (NOA) as well as from other national and international seismological centers, are listed in Table S1. It is noteworthy that for the first and third strong earthquakes consistent moment magnitudes, Mw~6.3 and M ~5.7, respectively, have been determined by the various centers. However, magnitudes Copyright: © 2021 by the authors. w Licensee MDPI, Basel, Switzerland. determined for the second strong earthquake vary from Mw = 5.9 to Mw = 6.3, an issue This article is an open access article examined further later. On the other hand, the various fault-plane solutions produced distributed under the terms and by seismological centers are consistent in that the three strong earthquakes shared focal conditions of the Creative Commons mechanisms of normal faulting striking roughly NW–SE. Attribution (CC BY) license (https:// This is also consistent with the active tectonics of the area, which is dominated by a creativecommons.org/licenses/by/ NE–SW to N–S active extensional field [2–8]. The main faults that dominate the area are the 4.0/). Tyrnavos and Larissa Faults, which dip to N–NE, and the Rodia Fault, which dips to S–SW Geosciences 2021, 11, 311. https://doi.org/10.3390/geosciences11080311 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 311 2 of 24 Geosciences 2021, 11, 311 NE–SW to N–S active extensional field [2–8]. The main faults that dominate the area2 of are 24 the Tyrnavos and Larissa Faults, which dip to N–NE, and the Rodia Fault, which dips to S–SW (Figure 1). The Tyrnavos-Larissa Basin is a tectonic structure which is bounded by the Tyrnavos and Larissa Faults to the south and by the Rodia Fault to the north. The (Figure1). The Tyrnavos-Larissa Basin is a tectonic structure which is bounded by the TyrnavosTyrnavos and Fault Larissa is one Faults of the to major the south active and structures by the Rodia in the Fault study to area. the north. It mainly The affects Tyrnavos the FaultTriassic is one crystalline of the major limestone active of structures the Pelagonian in the studybasement area. as It mainlywell as affectsPliocene the and Triassic Qua- crystallineternary deposits limestone [2]. The of the general Pelagonian trend of basement the Tyrnavos as well Fault as Plioceneis about andE–W, Quaternary although a depositsslightly right [2]. The-bending general geometry trend of is the evident. Tyrnavos The Faultlength is of about the well E–W,-defined although and a slightlymapped right-bendingfault trace exceeds geometry 12 km is evident.[3]. Earthquake The length parameters of the well-defined determined and by mappedNOA showed fault trace that exceedsthe strong 12 kmseismic [3]. Earthquakeactivity was parameters preceded by determined a series of by about NOA 30 showed foreshocks, that thewith strong local magnitude, ML, ranging from 0.9 to 2.9, occurring in an area of radius of ~30 km around seismic activity was preceded by a series of about 30 foreshocks, with local magnitude, ML, rangingthe epicenter from 0.9of the to 2.9, first occurring shock from in an 28 area February of radius up ofto ~303 March km around (Figure the 1). epicenterThe first strong of the firstearthquake shock from caused 28 February damage up mostly to 3 March in old (Figure unreinforced1). The first houses strong and earthquake other structures caused damagemainly in mostly the villages in old unreinforced of Damasi, housesMesochori and and other Vlachogianni structures mainly [1] (Figure in the villages1). Further of Damasi,damage Mesochoriwas noted andtowards Vlachogianni NW after [the1] (Figure second1 strong). Further earthquake damage mainly was noted in the towards village NWDomeniko. after the second strong earthquake mainly in the village Domeniko. FigureFigure 1.1. The Thessaly Thessaly area, area, Central Central Greece, Greece, that that ruptured ruptured by by three three strong strong earthquakes earthquakes on on3, 4 3,and 4 12 March 2021 with magnitudes, Mw, of 6.3, 6.2 and 5.7, respectively, as determined in this study; and 12 March 2021 with magnitudes, Mw, of 6.3, 6.2 and 5.7, respectively, as determined in this study;stars show stars strong show strongearthquake earthquake epicenters epicenters relocated relocated in the present in the present study. study.Blue and Blue purple and purplecolored circles illustrate foreshocks and aftershocks occurring from 28 February to 3 March and from 3 to 15 colored circles illustrate foreshocks and aftershocks occurring from 28 February to 3 March and March, respectively (epicentral determinations by NOA, http://www. fromgein.noa.gr/en/seismicity/earthquake 3 to 15 March, respectively (epicentral-catalogs determinations, last access 25 by May NOA, 2021 http://www.gein.noa.gr/en/). Black lines and black tri- seismicity/earthquake-catalogsangles show the main normal ,faults last access in the 25 area May [2 2021).–8] and Black settlements lines and reported black triangles in the main show text, the mainrespectively; normal faultsfault name in the codes: area [ 2L–F8 ]= and Larissa settlements Fault, T reportedF = Tyrnavos in the Fault, main R text,F = R respectively;odia Fault. In fault the nameinset map, codes: arrowsLF = Larissaillustrate Fault directio, TF =ns Tyrnavos of main lithospheric Fault, RF = motions; Rodia Fault. thick In and the thin inset rectangles map, arrows show illustratethe study directions area and ofthe main east lithosphericCorinth Gulf motions; area, respectively, thick and thin the last rectangles one discussed show the in study the main area text. and the east Corinth Gulf area, respectively, the last one discussed in the main text. The March 2021 earthquake sequence is quite challenging for a number of reasons. First,The no Marchstrong earthquake 2021 earthquake has occurred sequence in isthe quite entire challenging Thessaly area for asince number 1980, of therefore, reasons. First,the last no strongsequence earthquake is the first has recorded occurred by in themodern entire seismograph Thessaly area and since other 1980, networks, therefore, e.g. the , lastCGPS. sequence Besides, is thethe firstMarch recorded 2021 earthquakes by modern seismographruptured an area and otherwhich networks, has been e.g.,considered CGPS. Besides,as a long the-term March seismic 2021 earthquakesgap identified ruptured on the anbasis area of which the historical has been seismicity considered of as the a long-termThessaly area. seismic The gap discussion identified on on thethis basisissue of opened the historical after the seismicity observation of the that Thessaly the north area. Theside discussion of Thessaly, on including this issue the opened area of after the theMarch observation 2021 activity, that thehas northnot rupture side ofd Thessaly,by strong includingearthquakes the since area of AD the 1781, March while 2021 the activity, south hasside not ruptured ruptured by by several strong strong earthquakes earthquakes since ADfrom 1781, 1954 while up to the 1980, south after side an rupturedapparent byquiescence severalstrong since AD earthquakes 1773 [9,10 from] (Figure 1954 2).