Assessing Current Seismic Hazards in Irpinia Forty Years After the 1980 Earthquake: Merging Historical Seismicity and Satellite Data About Recent Ground Movements
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geosciences Article Assessing Current Seismic Hazards in Irpinia Forty Years after the 1980 Earthquake: Merging Historical Seismicity and Satellite Data about Recent Ground Movements Aldo Piombino 1,*, Filippo Bernardini 2 and Gregorio Farolfi 1 1 Department of Earth Sciences, University of Florence, 50125 Florence, Italy; gregorio.farolfi@unifi.it 2 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Bologna, 40128 Bologna, Italy; fi[email protected] * Correspondence: aldo.piombino@unifi.it Abstract: Recently, a new strain rate map of Italy and the surrounding areas has been obtained by processing data acquired by the persistent scatterers (PS) of the synthetic aperture radar interferome- try (InSAR) satellites—ERS and ENVISAT—between 1990 and 2012. This map clearly shows that there is a link between the strain rate and all the shallow earthquakes (less than 15 km deep) that occurred from 1990 to today, with their epicenters being placed only in high strain rate areas (e.g., Emilia plain, NW Tuscany, Central Apennines). However, the map also presents various regions with high strain rates but in which no damaging earthquakes have occurred since 1990. One of these Citation: Piombino, A.; Bernardini, regions is the Apennine sector, formed by Sannio and Irpinia. This area represents one of the most F.; Farolfi, G. Assessing Current important seismic districts with a well-known and recorded seismicity from Roman times up to the Seismic Hazards in Irpinia Forty present day. In our study, we merged historical records with new satellite techniques that allow for Years after the 1980 Earthquake: the precise determination of ground movements, and then derived physical dimensions, such as Merging Historical Seismicity and strain rate. In this way, we verified that in Irpinia, the occurrence of new strong shocks—forty years Satellite Data about Recent Ground after one of the strongest known seismic events in the district that occurred on the 23 November 1980, Movements. Geosciences 2021, 11, 168. measuring Mw 6.8—is still a realistic possibility. The reason for this is that, from 1990, only areas https://doi.org/10.3390/ characterized by high strain rates have hosted significant earthquakes. This picture has been also geosciences11040168 confirmed by analyzing the historical catalog of events with seismic completeness for magnitude Academic Editors: Sabina Porfido, M ≥ 6 over the last four centuries. It is easy to see that strong seismic events with magnitude M ≥ 6 Giuliana Alessio, Germana Gaudiosi, generally occurred at a relatively short time distance between one another, with a period of 200 years Rosa Nappi, Alessandro Maria without strong earthquakes between the years 1732 and 1930. This aspect must be considered as Michetti and Jesus Martinez-Frias very important from various points of view, particularly for civil protection plans, as well as civil engineering and urban planning development. Received: 26 October 2020 Accepted: 30 March 2021 Keywords: Irpinia; seismic hazard; earthquake; strain rate; GNSS; InSAR Published: 7 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. This study is based on the analysis of a fine-scale ground velocity map of Italy de- termined by the fusion of Global Navigation Satellite Systems (GNSS) with synthetic aperture radar interferometry (InSAR) data derived from satellites [1]. The dataset derives from a period of observation between 1990 and 2012. The InSAR dataset is part of the “Piano Straordinario di Telerilevamento” (Special program for Remote Sensing, promoted Copyright: © 2021 by the authors. by the Italian Ministry of Environment). Due to the quasi-polar orbit of the satellites, Licensee MDPI, Basel, Switzerland. space-borne InSAR observations can only determine the East–West (E–W) and Up–Down This article is an open access article (U–D) components of the movement of persistent scatterers. However, there are millions distributed under the terms and of scatterers that are unreachable, due to the fact that only a few hundred GNSS stations conditions of the Creative Commons 2 Attribution (CC BY) license (https:// exist. The North–South (N–S) component is provided by a C continuous bi-cubic inter- creativecommons.org/licenses/by/ polation function that is well suited to interpolate sparse GNSS stations displaced inside 4.0/). Geosciences 2021, 11, 168. https://doi.org/10.3390/geosciences11040168 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 168 2 of 14 the study site and surrounding areas. To do this it uses a hierarchical structure at different refinement levels. The fusion of GNSS with InSAR is a method based on the calibration of InSAR with GNSS measures derived from permanent stations and survey campaigns [2,3]. The results are a coherent fine-scale ground velocity map with a spatial resolution that is unreachable using the previous velocity field maps determined with the GNSS technique alone. By using this technique, Farolfi, Piombino, and Catani [1] provided new information about the complex geodynamics of the Italian peninsula and thanks to the high spatial resolution of the ground movements map, identified interesting patterns of small areas with respect to the surrounding ones. Moreover, their work confirmed the division of peninsular Italy into two sectors, with opposed E–W components of movement in the Stable Europe Frame (Figure1). This has been depicted by older studies based only on GNSS station movements ([4] and references therein): the western block (Tyrrhenian) is moving westward, while the eastern one (Adriatic) shows an eastward movement. The relative motion of these blocks implies their divergence; the effect of which is represented by the Geosciences 2021, 11, x FOR PEER REVIEW 3 of 15 numerous currently active normal fault systems along the central and southern Apennines— which are close to the border between these two sectors—and the associated seismicity. FigureFigure 1. 1. MapMap of of the the East East component component of of the the ground ground velocity velocity field field of of the the Italian Italian Peninsula, Peninsula, derived derived fromfrom Global Global Navigation SatelliteSatellite SystemsSystems (GNSS) (GNSS) and and synthetic synthetic aperture aperture radar radar interferometry interferometry (InSAR) (InSAR)during moreduring than more two than decades two decades of observation of observation (1990–2017). (1990–2017). The The area area of the of the Central Central Apennines Apenninespresents major presents earthquakes major earthquakes from 1990 from to present 1990 to day. present From day. the From figure the above, figure itabove, is clear it is that clear the thatmain the seismogenic main seismogenic areas are areas linked are tolinked the boundary to the boundary that divides that divides the two the blocks two withblocks opposite with E–W oppositecomponents E–W of components velocity. of velocity. TheThe Apenninevertical component chain, an approximatelyof the InSAR lineardata highlights belt hosting the the current most rapidly general slipping uplift occurringnormal faults, in most and theof mostSouthern damaging Italy, earthquakes,even if this areuplift coincident is lower with than the in areas the inCentral which Apenninesthe morphological (especially surface in the height, “Abruzzo when Dome” averaged [1 on]), confirming a horizontal a scale wealth of tensof the of kilometers,geological literature.is greatest [Conversely,5]. In this area, few the firstareas studies show based subsidence, on the relative mainly movements because ofof the human GNSS groundwaterstations have exploitation. already determined In this aframe, medium the valuehighest of auplift ca. 3 values mm/a extension,of the whole linked Southern to the Apennines—exceedingdifferential movements 1.8 between mm/a—are the two present blocks. in This the alsochain allows segment the emplacementbetween Benevento of melt andintrusions Potenza. along This deep-rooted area of higher-than-surro faults [6]—the lastundings occurrence uplift of roughly this kind corresponds probably triggered to the Irpiniathe 2013/2014 sector, Matesein a belt seismic just west swarm of the [7]—and Campania–Puglia the widespread border. emission Thus, of it deep-originated is possible to callCO 2this[8]. area This the regime “Irpinian is dissecting Dome” (Figure the former 2). The Cenozoic Ufita and east-verging Marzano faults thrust represent belt related the surface traces of the two different patterns of the East–West ground velocity component (Figure 3 (top)). The uplifting area is divided into two different parts and, between them, exists a narrow corridor of lower uplift <1 mm/a, (Figure 3 (bottom)). It is interesting to note that this corridor is placed near the epicentral areas of the 1930 and 1980 earthquakes. The geographical axis of the Irpinian dome is placed east of the main NE-dipping faults, on the surface projection of the hanging wall. Any useful information of the N–S component of the ground movement can be detected by InSAR satellites because their quasi-polar orbits only make the detection of vertical and E–W velocity components possible. Geosciences 2021, 11, 168 3 of 14 to the west-dipping subduction of the Apulian lithosphere [9]. This compressive regime ended at 650 ka in the middle Pleistocene [10]. The E–W component of InSAR movements [1] has also confirmed the frame depicted by [11], in which the Ortona–Roccamonfina is not a single lineament, but a 30 km wide deformation channel: this channel is characterized by prevalent west-directed velocities in the stable Europe frame, nested in the Adriatic eastward-moving block. The vertical component of the InSAR data highlights the current general uplift occur- ring in most of Southern Italy, even if this uplift is lower than in the Central Apennines (especially in the “Abruzzo Dome” [1]), confirming a wealth of the geological literature. Conversely, few areas show subsidence, mainly because of human groundwater exploita- tion. In this frame, the highest uplift values of the whole Southern Apennines—exceeding 1.8 mm/a—are present in the chain segment between Benevento and Potenza.