The May-June 2012 Ferrara Arc earthquakes (northern Italy): structural control of the spatial evolution of the seismic sequence and of the surface pattern of coseismic fractures

The Ferrara 2012 seismic sequence was characterized by two main compressional events, which occurred on May 20 and 29, 2012, with MW 6.1 and MW 6.0, respectively (quick Regional Centroid Moment Tensor [RCMT] at http:// autorcmt.bo.ingv.it/quicks.html). These events were followed by five events with MW >5.0 (two on May 20 and three on May 29, 2012) and by hundreds of events of lower magnitudes distributed along a WNW-ESE-elongated area of ca. 500 km2 (ISIDe database at http://iside.rm.ingv.it/ iside/standard/index.jsp.). The ongoing activity of the northward- verging fold-and-thrust structures of the Ferrara-Romagna Arc (Figure 1A) and the eastward-verging Coastal Adriatic Arc (referred to as the Outer Thrust System [OTS] in Lavecchia et al. 2003) has been a debated topic in the Italian literature. Some have considered the OTS to be inactive, on the basis of seismic reflection evidence of undeformed Middle Pleistocene-Holocene deposits sealing the buried thrust fronts [Argnani et al. 1997, Bertotti et al. 1997, Di Bucci and Mazzoli 2002, Picotti et al. 2009]. Others have considered the OTS to be active and possibly seismogenic, as they recognize an associated pattern of compressional and strike slip earthquakes with P-axes mainly radial to the thrust-front direction [Lavecchia et al. 2003, Lavecchia et al. 2004, Scrocca et al. 2007, Boncio and Bracone 2009, Toscani et al. 2009, Carminati et al. 2010, Boccaletti et al. 2011, Sgroi et al. 2012]. Recent geodetic data also support the ongoing NNE motion of the Apennine compressional units above the Adriatic foreland [Serpelloni et al. 2005, Devoti et al. 2011]. The Ferrara 2012 seismic sequence activated the central, nearly E-W striking, sector of the broad Ferrara Arc. Over the past 1000 years, there have been no strong (M ≥5.0) earthquakes associated with this portion of the Arc (Figure 1B,C). Apart from minor events (MW <4.5) in the epicentral area of the 2012 seismic sequence (Figure 1B, dotted line), the historical and instrumental catalogs report five earthquakes with MW between 4.5 and 4.7 (in 1561, 1574, 1978, 1986, 1987) [Castello et al. 2005, CPTI11 catalogue at http://emidius.mi. ingv.it/CPTI; ISIDe database]. Conversely, the eastern, NWSE striking segment of the Ferrara Arc has been activated by several historical earthquakes: 14 events localized around Ferrara of 4.5 ≤ MW ≤ 5.5, including the highly damaging Ferrara 1570 earthquake (MW 5.5, Io VII-VIII) and the Argenta 1624 earthquake (MW 5.5, Io VII-VIII). The western, WSW-ENE-striking segment of the Ferrara Arc was activated at its southern end by two earthquakes of MW 5.5 in 1831 and 1832, and more recently by the Reggio-Emilia 1996 earthquake (MW 5.4). Since instrumental measurements have been used, a number of seismic sequences that originated at mid-to-lower crustal depths (of ca. 20-25 km) have been reported, which activated the basal thrust of the Ferrara Arc (e.g., 2008, MW 5.5) [Bragato et al. 2012] or its antithetic splays (e.g., Faenza 2000, MW 4.8; Monghidoro 2003, MW 5.3) [Ciaccio and Chiarabba 2002, Lavecchia et al. 2003, Piccinini et al. 2006]. The focal mechanisms of all of the instrumental events possibly associated with the Ferrara Arc show prevalent reverse and reverseoblique kinematics (Figure 1C), with P-axes nearly perpendicular to the average structural trend [Boncio and Bracone 2009, Montone et al. 2012]. This study aims to provide a preliminary interpretation of the earthquake-fault association(s) of the Ferrara 2012 seismic sequence. First, on the basis of the literature data, a simplification of the complex pattern of possibly active and interconnected fold-and-thrust structures, which characterizes the Ferrara Arc, is attempted. This simplification is achieved through a geometric hierarchization among the various structures involved. Secondly, the most likely seismogenic sources are preliminarily identified. Finally, the coseismic ground ruptures and the possible driving factors are discussed.