First level seismic microzoning of Sulmona (central Italy): major results and evidences of 2D/3D amplification.

1. Introduction The city of Sulmona (AQ, Italy) lies above a deep sediment-filled active tectonic basin, flanked by steep uplifted carbonate blocks. It is characterized by a series of complex geological, geomorphological and tectonic features which can determine important seismic amplifications due to the local stratigraphic effects, buried topography and to instability phenomena like those linked to active and capable normal faults and landslides. Here we show the results of the first level Seismic Microzoning (MS) and we discuss the possible implication for 2D or 3D amplification effects from earthquake records. The first level of MS study in the Sulmona city represents the first step of a “pilot study” that the Abruzzo Region, referring to the OPCM 3907 and the LR n. 28 of the 11.08.2011, has committed to the Engineering and Geology Department (InGEO) of the University “G. d’Annunzio” of Chieti-Pescara in collaboration with the INGV (branch of Milan for the HVSR measurement and branch of Rome for the study of the historic seismicity), with the purpose of complete the MS third level of the Sulmona city. The Sulmona basin is an intramontane depression developed with a NW-SE trend in the more external portion of the Abruzzi Central Apennines. It extends with width between 4 and 7 km, for a length of 21 km, it is limited to the east by the Mt. Morrone active normal faults system (Miccadei et al., 1999; Vittori et al., 1995; Gori et al., 2014 with references) and it is filled by a thick sequence of fluvio-lacustrine Quaternary continental deposits. The available field geological-structural studies and few subsurface geophysical data (eg., Di Filippo e Miccadei, 1997) suggest a half-graben-like geometry of the basin with a wedge pattern of the some hundreds of meters thick infilling Quaternary deposits. The late-Quaternary activity of the Mt. Morrone fault has been documented by several authors and, on the base of empiric criteria, its capability has been evaluated to produce a Mmax equal to 6.5-6.7 (eg., Barchi et al., 2000; Pizzi et al., 2002). Despite the seismic history of Sulmona is particularly rich of seismic events with effects above the damage threshold, there are no certain information about the activation of this fault in historic times. Also the earthquake of the third of November 1706, in fact, which produced an intensity (I) of 9-10 MCS in the area of Sulmona (Locati et al., 2011), has been attributed, on the base of the damage distribution, to a different seismogenic source. On the base of archeoseismological studies (Galadini e Galli, 2001; Ceccaroni et al., 2009), instead, the major probability of the last activation of the Mt. Morrone fault could be attributed to a seismic event happened in the II century A.D., responsible of important collapses diffused in the Sulmona area, and that maybe could represents the most strong earthquake of the local seismic history. However, following this hypothesis we need to consider that about two thousand years of inactivity represent a long recurrence interval for typical Apennine normal fault, highlighting the high seismic hazard for the Mt. Morrone fault. Moreover, the comparison between the seismic intensities of the Sulmona surrounding area with regard of several historical earthquakes (eg., 1349, 1456, 1706, 1915, 1933, 1984 and 2009) highlights that, in some cases, the city of Sulmona has suffered the most heavy damages (DBMI11 database, http://emidius.mi.ingv.it/DBMI11/). This could suggest that, for this city, the consequences of an earthquake are also dependent from site effects.