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Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting

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Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting acoustics Article Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting Anica Otilia Placinta 1, Felix Borleanu 2,* , Emilia Popescu 1, Mircea Radulian 1,3 and Ioan Munteanu 4 1 Department of Research and Development Innovation in Earth Sciences, National Institute for Earth Physics, 12 Calugareni Street, P.O. Box MG2, Magurele, 077125 Ilfov, Romania; [email protected] (A.O.P.); [email protected] (E.P.); [email protected] (M.R.) 2 National Data Center, National Institute for Earth Physics, 12 Calugareni Street, P.O. Box MG2, Magurele, 077125 Ilfov, Romania 3 Academy of Romanian Scientists, 54 Splaiul Independentei, 050094 Bucharest, Romania 4 Faculty of Geology and Geophysics, University of Bucharest, 6 Traian Vuia Street, 020956 Bucharest, Romania; [email protected] * Correspondence: [email protected] Abstract: Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of Citation: Placinta, A.O.; Borleanu, F.; magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower Popescu, E.; Radulian, M.; Munteanu, I. Earthquake Source Properties of a crust. The mainshock triggered a significant increase in the number of small-magnitude events spread Lower Crust Sequence and over an unusually large area in the ECAB. The paper’s goal is to compute the source parameters Associated Seismicity Perturbation in of the earthquakes that occurred during the aforementioned sequence, by empirical application of the SE Carpathians, Romania, Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple Collisional Setting. Acoustics 2021, 3, methods and seismicity evolution at regional scale is investigated. Our results highlight a still active 270–296. https://doi.org/10.3390/ deformation regime at the edge of the EE Craton, while the source parameters reveal a complex acoustics3020019 fracture of the mainshock and a very high-stress drop. Academic Editor: C. W. Lim Keywords: earthquake sequence; source parameters; seismic source scaling; SE Carpathians foredeep Received: 12 March 2021 Accepted: 8 April 2021 Published: 19 April 2021 1. Introduction Publisher’s Note: MDPI stays neutral The crustal seismicity developed in front of the SE Carpathians bending zone (SEC) is with regard to jurisdictional claims in spread over the entire area between the Neogene Trotu¸sand Intra-Moesian faults (Figure1) . published maps and institutional affil- Crustal earthquakes are generated into a complex structure with significant lateral inhomo- iations. geneities involving different kinematics [1]. This is also reflected in the variability of the focal mechanisms that show consistent stress regime with different localization: normal faulting on the eastern and southern flank of the Foc¸saniBasin (FB), thrust faulting in the area adjacent to the Vrancea subcrustal earthquakes and strike-slip solutions preferably Copyright: © 2021 by the authors. observed along the Peceneaga-Camena Fault [2]. The crustal seismicity in the SEC partly Licensee MDPI, Basel, Switzerland. overlaps with the intermediate-depth earthquakes generated in a depth range of 60–180 km, This article is an open access article beneath the Vrancea Zone (Figures1 and2). distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Acoustics 2021, 3, 270–296. https://doi.org/10.3390/acoustics3020019 https://www.mdpi.com/journal/acoustics Acoustics 2021, 3 271 Acoustics 2021, 3 FOR PEER REVIEW 2 FigureFigure 1.1. (a) Main geological units within the epicentral area (modified(modified afterafter [[3])3]) andand epicentersepicenters distributiondistribution (blue(blue dots)dots) ofof thethe seismicseismic events events associated associated with with the the 2014 2014 M ăMrăă¸se¸stisequence.răşeşti sequence. The The inset inset map map shows shows the study the study area (pinkarea (pink square) square) displayed dis‐ played at the regional scale. The red line represents three vertical cross‐sections crossing the epicentral area. (b) Seismic at the regional scale. The red line represents three vertical cross-sections crossing the epicentral area. (b) Seismic activity activity at the SE Carpathians Arc bend according to the ROMPLUS catalogue (between 2010 and 2014, [4]) colored func‐ at the SE Carpathians Arc bend according to the ROMPLUS catalogue (between 2010 and 2014, [4]) colored function of tion of depth (intermediate‐depth events in black and crustal earthquakes within the color scale—right side), seismic sta‐ depthtions distribution (intermediate-depth (blue triangles) events and in blackneighboring and crustal cities. earthquakes The white dashed within lines the colorrepresent scale—right the local side),fault system; seismic the stations black distributionline shows the (blue Pericarpathian triangles) and Fault. neighboring The inset cities. map shows The white a zoom dashed of the lines epicentral represent area. the The local red fault star system;shows the the mainshock black line showsepicenter. the PericarpathianThe abbreviations Fault. are The as follows: inset map VR—Vrancea shows a zoom Zone, of the IMF—Intra epicentral‐Moesian area. The Fault, red starPCF—Peceneaga shows the mainshock‐Camena epicenter.Fault, AMRR—Amara, The abbreviations PGOR—Pogoanele, are as follows: VR—Vrancea TLB—Topalu Zone, seismic IMF—Intra-Moesian stations. Fault, PCF—Peceneaga-Camena Fault, AMRR—Amara, PGOR—Pogoanele, TLB—Topalu seismic stations. Acoustics 2021, 3 272 Acoustics 2021, 3 FOR PEER REVIEW 3 Figure 2. (a) Epicenters distribution (colored as a function of local magnitude ML) within the epicentral area (0.75 deg. Figure 2. (a) Epicenters distribution (colored as a function of local magnitude M ) within the epicentral area distance around the mainshock) for the seismic activity recorded during a time interval of L100 days (22 November 2014–2 (0.75 deg. distance around the mainshock) for the seismic activity recorded during a time interval of 100 days March 2015) from the mainshock (red star) occurrence; (b) hypocenters distribution displayed in the same area and for the (22 November 2014–2 March 2015) from the mainshock (red star) occurrence; (b) hypocenters distribution displayed in the same time interval mentioned above versus latitude and function of ML and (c) versus longitude and function of ML. same area and for the same time interval mentioned above versus latitude and function of ML and (c) versus longitude and function of ML. The intermediate‐depth seismicity is located in a narrow seismogenic volume which descendsThe intermediate-depthalmost vertically in seismicitythe mantle, is able located to generate in a narrow 2–3 seismogenicstrong events volume (Mw ≥ which7) per century.descends The almost seismicity vertically in the in theoverlying mantle, crust able is to limited generate to 2–3 moderate strong‐ eventssize events (Mw (Mw≥ 7) < per 6) andcentury. spreads The over seismicity a larger in area the overlying following crust some is specific limited alignments. to moderate-size One of events the significant (Mw < 6) seismicallyand spreads active over afaults larger placed area following in front of some the SEC specific is the alignments. Peceneaga One‐Camena of the (PCF) significant fault zoneseismically and related active deformation, faults placed inwhich front delineates of the SEC (Figure is the Peceneaga-Camena 1) the boundary between (PCF) fault the Moesianzone and Platform related deformation,(MP) and North which Dobrogea delineates Orogen (Figure (NDO,1) the[5]). boundary Along its path between towards the NWMoesian from Platform the Black (MP) Sea Basin and North to the Dobrogea SEC, the fault Orogen shows (NDO, two [segments5]). Along with its path distinct towards seis‐ micNW activities: from the Black (1) from Sea the Basin Black to the Sea SEC, Basin the to fault the shows Danube two River, segments and (2) with from distinct the Danube seismic Riveractivities: to the (1) NE from edge the of Black the Vrancea Sea Basin epicentral to the Danube area. The River, first and segment (2) from is theless Danube active with River a stressto the regime NE edge of of thrusting the Vrancea faulting epicentral [6,7], area.while The the firstsecond segment segment is less displays active witha significant a stress seismicregime ofactivity thrusting with faulting predominant [6,7], while strike the‐slip second faulting segment along displays the major a significant fault line seismicand its satellites,activity with oriented predominant SE—NW. strike-slip Towards faulting the NW along edge, the where major the fault PCF line intersects and its satellites,the Peri‐ Carpathian Fault, the seismicity configuration becomes more complex (Figure 1). Acoustics 2021, 3 273 oriented SE—NW. Towards the NW edge, where the PCF intersects the Peri-Carpathian Fault, the seismicity configuration becomes more complex (Figure1). Several studies [7–11] indicate
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