The 2009 L'Aquila earthquake (Central Italy): Seismic microzonation of a severely hit area

The April 6th, 2009 L’Aquila earthquake (5.8 Richter magnitude) caused severe damage to many ancient hill towns in the region, killing 308 residents, injuring about 11,600 and leaving about 65,000 homeless. Owing to an agreement between the Italian Civil Protection National Service, the Tuscany Regional Service for Seismic Prevention and the Earth Sciences Department of the Pisa University, the seismic microzonation of an area south-west of L’Aquila was performed. The study focused on the area including some of the most damaged villages, such as Bagno, Civita di Bagno, San Benedetto. Following the standards and guidelines for the seismic microzonation issued by the Italian Civil Protection Department (GRUPPO DI LAVORO MS, 2008) many investigation were carried out: geologic and geo-engineering survey, geophysical prospecting, instrumental registration, and geotechnical analyses. Thus an underground model was defined and depicted by means of representative geological-technical sections, suitable for the subsequent modelling phase (FIANCHISTI et alii, 2009). In the study area geological formations and Plio-Quaternary continental deposits crop out. - The former belong to the M. Ocre-Valle Aterno-M. Camarda Succession and include (CENTAMORE et alii, 2006): Calcareniti a Briozoi member: it crops out close to the Civita Lake and is mainly formed of medium-thick strata of white or grey calcarenite and calcirudite with Bryozoa (Langhian p.p. - Serravallian). The rock is mainly strong and intersected by joint sets with moderate spacing, high persistence, moderate to high aperture and high roughness. Sandstone-pelite turbidite succession: it crops out at Bagno Grande and is formed of medium-thick, frequently massive yellow sandstone with medium-thin pelitic interbeds (sandstone/pelite > 2) (Lower Messinian p.p.). The rock is medium strong and intersected by closely spaced, very persistent, open, smooth or slightly rough, weathered and damp joints. - The Plio-Quaternary continental deposits include the Aielli- Pescina Supersynthem (Pliocene (?) - Middle Pleistocene) gathering most of them, and the Holocene and Present deposits (landslide bodies, debris, colluvium and alluvial deposits, etc.). The following ones are most represented: Fluvial-lacustrine deposits: they crop out close to the S. Maniero lake and are formed by massive sand with pelitic interbeds and lens-shaped conglomerate interbeds; these deposits are very consistent and intersected by two wide-spaced joint sets. Landslides: many landslides involve the slopes between Bagno, Civita di Bagno and San Benedetto. Most of them are dormant. Two coseismic landslides were identified: one on the slope south-west of the San Maniero lake, the other in the upper part of Bagno. Subsequently the underground model was defined and depicted by means of representative geological-technical sections, based on geological, geotechnical, geophysical and seismological data. Fig. 2 shows the example of the San Benedetto - San Maniero Lake area, where the Plio-Quaternary deposits attain a great thickness and the bedrock is intersected by normal faults. Finally, 1st level and 3rd level microzonation maps were performed. The 1st level map classifies the study area in three categories: stable zones, stable but amplification subject zones, and instability subject zones. This map roughly and qualitatively identifies the areas that could be characterized by specific seismic local effects. The 3rd level map, based on the knowledge of several data (seismic input, geological-technical model and cross sections, in situ and laboratory geotechnical data, geophysical and seismological data), identifies homogeneous geological- geophysical units and the related stratigraphic columns, on which the modelling was performed. Fig. 3 shows the 3rd level map of the San Benedetto area.