Site Effects in the Eastern Po Plain by Mean of Weak and Strong Earthquakes

Abstract

We present the activities carried out within the S2 2012-2013 Project, funded in the frame of DPCINGV Agreement, that concerns the mid-long term Seismic Hazard Assessment in Italy on two priority areas, the Po Plain and the Southern Italy. The Po Valley, an area hitherto considered of low seismological interest, has attracted the attention of the seismological community following the events of May 20, 2012. The day after the main shock that stuck eastern Emilia in 2012, the OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale deployed a temporary seismographic network in the Ferrara area. All the investigated locations were set on soft soils. The large amount of collected data allowed comparison between observed PGA and theoretical predictions, ShakeMaps and attenuation laws, but the lack of a reference site during the 2012 recordings did not allow for estimating the amplification factor. In order to accomplish this task, in February 2013, a new 5 stations recording array acquired data at four of the 2012 network sites and at the Casaglia reference site, north of Ferrara, where a borehole Very Broad-Band station is coupled with a midperiod sensor at the surface. This borehole reference station made possible the estimation of site amplification of the 2013 array sites. The reference methods allowed an easier identification of the resonance frequency, which peaks appear sharper than what displayed by single-station methods (both H/V on ambient noise and earthquakes), and relative soil amplification. A large amount of original seismological data has been recorded in a poorly instrumented area, including significant events of the 2012 Emilia and 2013 Lunigiana sequences. The entire dataset of continuous waveforms has been made available on the OGS web based OASIS Database, from the earliest stages of the project. For the largest events strong-motion parameters were calculated and published on the OASIS database. Event time series and metadata (site monographs) are available similarly to the ITACA Database.