Preliminary results from EMERSITO, the rapid response network for site effect studies

Abstract

On May 20, 2012, at 02:03 UTC, a ML 5.9 reverse-fault earthquake occurred in the Emilia-Romagna region, northern Italy, at a hypocentral depth of 6.3 km (http://iside.rm. ingv.it/), close to the cities of Modena and Ferrara in the plain of the Po River. The epicenter was near the village of Finale Emilia where macroseismic intensity was assessed at 7 EMS98 [Tertulliani et al. 2012, this issue], while the closest accelerometric station, MRN, located less than 20 km west-ward at Mirandola (Figure 1) recorded peaks of ground accelerations of about 300 cm/s2 (www.protezionecivile.gov.it/resources/ cms/documents/Report_DPC_1_Emilia_EQSd.pdf ). The mainshock triggered liquefaction phenomena a few kilometers eastwards of the epicenter, around the village of San Carlo. On the same day, two other shocks of ML 5.1 followed (02:07, 13:18 GMT; http://iside.rm.ingv.it/). On May 29, 2012, at 07:00 UTC another ML 5.8 earthquake hit the region (http://iside.rm.ingv.it/), with the epicenter close to the village of Mirandola (Figure 1). Three other strong aftershocks occurred afterwards, of ML 5.3 (May 29, at 10:55), ML 5.2 (May 29, at 11:00) and ML 5.1 ( June 3, at 19:20). For a detailed description of the seismic sequence, see Moretti et al. [2012], Scognamiglio et al. [2012], and Massa et al. [2012], in this issue. The Emilia seismic sequence resulted in 25 casualties, several of whom were among the workers in the many factories that collapsed during working hours, and there was extensive damage to monuments, public buildings, industrial sites, and private homes. The Po Plain region that was struck by the 2012 Emilia seismic sequence is a very large E-W trending syntectonic alluvial basin, which covers about 45,000 km2. It is surrounded by the Alps to the north and the Apennines to the south, and it is filled with Plio-Pleistocene terrigenous sediments and Holocene deposits, with depths varying from a few hundred meters up to several kilometers. The epicentral area was located south of the Po River, corresponding to the active front of the northern Apennines thrust belt (north-vergence), which is composed of buried folds and thrust faults that locally produce structural highs (Figure 1), and are known as Pieghe Emiliane and Ferraresi [Pieri and Groppi 1981]. The top of this limestone and marl bedrock rises to ca. 100 m from the surface and has been derived locally from borehole logs. The seismic response of this ca. 150-m-deep soft cover was investigated using weak-motion events and microtremors recorded in a borehole by Margheriti et al. [2000]. The occurrence of the May 2012 seismic sequence made it possible to study the seismic response under near-field conditions. These studies are aimed at providing tools to reduce the impact of future earthquakes on the local communities. In addition to the amplification due to one-dimensional (1D) resonance, it is well known that seismic responses of deep sedimentary basins are affected by 2D and 3D effects (e.g., wave diffraction, conversion at the basin edges, trapping and focusing of energy within the soil volume). Evidence of basin-induced surface waves and edge effects have been observed in many basins worldwide; e.g., the Osaka basin in Japan [e.g., Kawase 1996, Pitarka et al. 1998], various southern California basins [Graves et al. 1998, Day et al. 2008], and the Parkway basin in New Zealand [Chávez-García et al. 1999]. In Italy, good examples of site amplification in alluvial basins can be found for the Gubbio, Città di Castello, L'Aquila and Fucino basins [ Bordoni et al. 2003, Bindi et al. 2009, Cara et al. 2011, Milana et al. 2011]. Therefore, the day after the mainshock, the INGV rapidresponse network for site effects, called EMERSITO, planned the experiments presented in this report. EMERSITO put together independent research groups from several territorial centers of the Istituto Nazionale di Geofisica e Vulcanologia (INGV; National Institute of Geophysics and Volcanology) who agree to collaborate spontaneously and on the basis of a data archiving and sharing policy. They then deployed their seismic equipment in the epicentral area, building on the experience of the 2009 L'Aquila earthquake [Di Giulio et al. 2011, Margheriti et al. 2011, Milana et al. 2011]. The deployment was planned also in collaboration with the geological survey of the Regione Emilia Romagna (Servizio Geologico e Sismico e dei Suoli) and the University of Modena, as well as being in the framework of SISMIKO [Moretti et al. 2012]. As a result of this effort, since May 22, three linear arrays have been deployed (Figure 1), with a total of 22 sites instrumented, 16 of them equipped with both velocimeters and accelerometers. These arrays recorded most of the aftershock sequence, including the MW 5.8 May 29, 2012, 07:00 earthquake. The continuous recordings will be archived into the EIDA database (http://eida.rm.ingv.it/) under restricted access. The aim of this report is to describe the experiments performed by the EMERSITO team, as well as the main features of the recorded earthquakes. A preliminary insight in the site response of the investigated area within the context of the geological structure of the Po Plain is also given.