GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.2 239 A large scale ambient vibration survey in the area damaged by May-June 2012 seismic sequence in Emilia Romagna, Italy E. Paolucci 1 , D. Albarello 1 , S. D’Amico 2 , E. Lunedei 1 , L. Martelli 3 , M. Mucciarelli 4 , D. Pileggi 5 1 Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente, University of Siena, Italy 2 Department of Physics, University of Malta, Msida, Malta 3 Servizio Geologico, Sismico e dei Suoli, Regione Emilia Romagna, Bologna, Italy 4 Ist. Naz. Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy 5 Consultant geologist of Regione Emilia Romagna, Siena, Italy Introduction. Starting from May 2012 until the end of June 2012, the Po Plain region (northern Italy) has been affected by a seismic sequence characterized by two mainshocks, the first on May 20 (M W = 5.86) and the second (located about 12 km west of the former) on May 29 (M W = 5.66). The result after this last strong seismic event has been 17 victims, serious damages in the buildings and in the infrastructure of the struck municipalities and, moreover, severe losses to the economy of the region. With the aim of assessing the possible site effects related to the amplification of the ground motion, an intensive ambient vibration surveywas carried out by teams fromdifferent institutions (Universities of Siena, Basilicata and Malta, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale and Regione Emilia-Romagna) in the area damaged in Emilia-Romagna during the seismic sequence� �� � ������ ��� ������ ������� ������������ ��������� ���� ���������� . As a whole, 204 single station acquisitions [analysed with Horizontal to Vertical Spectral Ratio technique or HVSR: SESAME (2004)] and 34 multiple station measurements (seismic arrays) were performed: most these measurements concerned areas where seismic effects ���� �� ����� �� ��� �������� ���� ����������� ������� ���� ��� ��� were at least VI MCS (Fig.1). ���� ����������� ������� ���� ��� ��� Some preliminary results from OGS and UNIBAS measurements have been described by Priolo et al . (2012). The purpose of the present work is describing general outcomes of the above survey focusing on some experimental features of passive seismic measurements in the context of a sedimentary basin characterized by geological outcropping and geomorphological settings approximately homogeneous. Geological settings. The local seismicity is mainly due the compressional tectonic activity of the faults belonging to Ferrara Arc (Pieri and Groppi, 1981). This structure is a fold-and- thrust system which has given rise to two buried ridges which together constitute a N-verging arc extending between Reggio Emilia and theAdriatic coast, reaching northwards the zones near and over the Po River (Bigi et al. , 1990). In particular it is possible to distinguish an internal ridge, with its maximum uplift between Novi di Modena and Medolla, known as Mirandola’s structure (or Mirandola’s ridge), and a more external one, that has the most high point between Bondeno and Ferrara. The study area is a deep sedimentary basin characterized by alluvial Holocene sediments, particularly alternances of sand, silt and clay of different environments, like river channels and levees, meander and floodplain deposits (Martelli et al. , 2014). The whole alluvial succession, belonging to Pleistocene-Holocene age, is attributable to two main depositional cycles: Emiliano-Romagnolo Lower Synthem (AEI), included between 700,000-650,000 and 450,000-400,000 years (Middle Pleistocene), and Emiliano-Romagnolo Upper Synthem (AES), included between 450,000/400,000 years and the present (RER & ENI-Agip, 1998). This continental succession lies on a Lower-Middle Pleistocene deposits (mainly sands) of marine and transitional origin (RER & ENI-Agip, 1998; Martelli et. al , 2014). The depth of the bottom of the alluvial succession changes due to Ferrara Arc configuration: in synclinal the continental deposits thickness exceeds 500 m, instead in the anticlinal areas it reaches about 100 m deep (RER & ENI-Agip, 1998). Single-station measurements (HVSR). The most part of single station acquisitions was performed by � ����������������� ������� ���������� ������� �������� ��������������� � a three-directional digital tromograph Tromino Micromed ( www.tromino.eu) with a sampling frequency of 128 Hz and an acquisition time of 15-20 minutes� ��� ���� ; the HVSR curve was obtained f�������� ��� ��������� ��������� �� ������� ollowing the procedure described in Picozzi et al. (2005). In some sites,