Abu Zeid, Nasser

On May 20th and 29th, 2012 two earthquakes (ML 5.9 with hypocenter depth at 6.3 km and ML 5.8 with hypocenter depth at 10.2 km, respectively; ISIDe Database) struck the Emilia area. The epicentre was located in the vicinity of Finale Emilia and Medolla (Modena). Co-seismic effects exemplified by liquefactions and surface ruptures occurred in the surrounding area (Provinces of Bologna, Ferrara, Modena, Reggio Emilia, Mantova and Rovigo). The maximum effects where concentrated along the towns located 15–25 km from the epicentre (SW portion of Ferrara Province). Soon after the main events, several geochemical and geophysical surveys were carried out in different sites at Modena and Ferrara Provinces, where surface rupture and liquefaction effects were most evident. Results gained from soil and dissolved gases and geoelectrical-geophysical surveys evidenced that the main liquefied layer is related to a medium coarse-grained sand saturated aquifer located at 8–12 m b.g.l. On the other hand, superficial unsaturated sediments underwent liquefaction represented by densification and lateral spreading. As a consequence, liquefied soil caused ground failures and damages to the built environment. The extent of the liquefaction phenomena, its concentration along the Reno paleo-river ridge and the building damage, has highlighted the need to further characterized the possible rule of lithology and natural gas content on the outset of liquefaction.

During the seismic crisis of May–June 2012, that strongly affected the central sector of the Ferrara Arc, relevant coseismic effects were observed, such as ground deformations and amplification phenomena due to low quality mechanical characteristics of the shallow subsurface (i.e. few hundreds of meters). This portion of the subsurface is not investigated by neither hydrocarbon explorations (too deep) nor geotechnical surveys (too shallow). Furthermore, direct analysis is not cost effective to carry out over such a wide area. To overcome these limitations, we exploited seismic noise-based strategies, which are not invasive and do not require expensive equipment. We carried out several single-station and array measurements (i.e. ESAC, Re-Mi, and HVSR), across some of the major tectonic structures of the eastern Po Plain, belonging to the most advanced buried sector of the Northern Apennines. Such investigations were performed along two profiles, about 27 km-long and oriented SSW–NNE, i.e. almost perpendicular to the regional trend of the Ferrara Arc structures. Our results clearly document lateral shear wave velocity variations and the occurrence of resonance phenomena between 0.52 and 0.85 Hz. Additionally, based on inversion procedures, we were able to infer the depth of the resonant interface(s) and we associated such interface(s) to the major known stratigraphic discontinuities, thus emphasizing the recent tectonic activity of the blind thrusts affecting this sector of the Ferrara Arc.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is the Italian agency devoted to monitor in real time the seismicity on the Italian territory. The seismicity in Italy is of course variable in time and space, being also very much dependant on local noise conditions. Specifically, monitoring seismicity in an alluvial basin like the Po one is a challenge, due to consistent site effects induced by soft alluvial deposits and bad coupling with the deep bedrock (Steidl et al., 1996). This problem was tackled by INGV first with the Cavola experiment (Bordoni et al., 2007), where a landslide was seismically characterized using a seismic array and also down-hole logging of P- and S-wave travel times at a borehole drilled within the array; later, with an ad hoc project in 2000-2001, with the first installation of a broad band seismic station nearby Ferrara in a borehole of 135 meters depth. Comparison of recordings with a surface seismic station indicated a noise reduction of 2 decades in power spectral density at frequencies larger than 1.0 Hz (Cocco et al., 2001). The instrumentation in Ferrara has been working for several months but after that the seismic station was discontinued due to lack of maintenance manpower. The Centro di Ricerche Sismologiche (CRS, Seismological Research Center) of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS, Italian National Institute for Oceanography and Experimental Geophysics) in Udine (Italy) after the strong earthquake of magnitude M=6.4 occurred in 1976 in the Italian Friuli-Venezia Giulia region, started to operate the Northeastern Italy (NI) Seismic Network: it currently consists of 15 very sensitive broad band and 21 simpler short period seismic stations, all telemetered to and acquired in real time at the OGS-CRS data center in Udine (Fig. 1). Real time data exchange agreements in place with other Italian, Slovenian, Austrian and Swiss seismological institutes lead to a total number of about 100 seismic stations acquired in real time, which makes the OGS the reference institute for seismic monitoring of Northeastern Italy. Since 2002 OGS-CRS is using the Antelope software suite on several workstations plus a SUN cluster as the main tool for collecting, analyzing, archiving and exchanging seismic data, initially in the framework of the EU Interreg IIIA project “Trans-national seismological networks in the South-Eastern Alps”. SeisComP is also used as a real time data exchange server tool (Bragato et al., 2011). Among the various Italian institution with which OGS is cooperating for real time monitoring of local seismicity there is the Regione Veneto (Barnaba et al., 2012). The Southern part of the Veneto Region stands on the Po alluvial basin: earthquake localization and characterization is here again affected in this area by the presence of soft alluvial deposits. OGS ha already experience in running a local seismic network in difficult noise conditions making use of borehole installations (Priolo et al., 2012) in the case of the monitoring of a local storage site for the Italian national electricity company ENEL. Following the ML=5.9 earthquake that struck the Emilia region around Ferrara in Northern Italy on May 20, 2012 at 02:03:53 UTC, a cooperation of INGV, OGS, the Comune di Ferrara and the University of Ferrara lead to the reinstallation of the very broad band borehole seismic station in Ferrara. The aim of the OGS intervention was on one hand to extend its real time seismic monitoring capabilities toward South-East (Fig. 1), including Ferrara and its surroundings, and on the other hand to evaluate the seismic response at the site. As concerns the superficial geology of the area where the borehole seismic station has been installed, the outcropping materials are represented by alluvial deposits of different environments, like channel and proximal levee, inter-fluvial, meander and swamps deposits. As a consequence, the outcropping deposits are everywhere Holocene in age substantially loose or poorly compacted in the first meters-decameters and granulometrically could vary from clay to coarse sand. Two preliminary reports prepared by the Italian Department of Civil Defense (Dipartimento Nazionale di Protezione Civile) in collaboration with other institutions describe the data recorded by the national accelerometric network and complemented by additional data recorded by a number of temporary stations (Dolce et al., 2012a; Dolce et al., 2012b). These reports bear witness of strong ground motion values with an acceleration peak of about 0.9 g in the vertical component recorded during the ML=5.8 earthquake of May 29, 2012 by the Mirandola station, located at about 2 km from the epicentre. The analysis of the seismic noise recorded at some stations shows a quite pronounced peak of the horizontal-to-vertical spectral ratio (H/V) in the frequency range of 0.6 – 0.9 Hz common to all stations. Finally, strong evidence of liquefaction phenomena are reported at several sites (e.g.: S. Carlo, S. Agostino and Mirabello), most of which have been attributed to the occurrence of saturated sandy layer(s) at shallow depth deposited along an abandoned reach of the Reno River (Papathanassiou et al., 2012). Details of the station configuration and installation will be outlined, with first results.