Il terremoto aquilano del 6 aprile 2009 rilievo macrosismico, effetti di superficie ed implicazioni sismotettoniche

Abstract

Galli P. et al., April 6, 2009 L’Aquila earthquake: macroseismic survey, surficial effects and seismotectonic implications.
(IT ISSN 0394-3356, 2009).
In the night of April 6, 2009, a frightful earthquake (Mw=6.3) struck the L’Aquila region (central Apennines, Italy), awaking also million of citizens in the 100-km-far city of Rome. In the epicentral region the death toll reached 308, most in the town of L’Aquila. In only 90
minutes, the first experts of the Quick Earthquake Survey Team (QUEST) started the macroseismic survey, providing data on 40 localities within the first 16 hours, 70 on April 7, 130 on April 9, up to 316 at the end of the in situ survey (July; Tabs.1-2. GALLI & CAMASSI, 2009). The maximum intensity was given to Onna and Castelnuovo (9-10 MCS, Mercalli-Cancani-Sieberg scale), which were almost totally razed to the ground, while others four villages reached 9 MCS and three 8-9 MCS, amongst which L’Aquila (i.e., old town sector intramoenia). Most of the damage and collapses affect rubble-stone and/or masonry buildings, especially those which have been overloaded with not-collaborative R.C. roofs (Figs.1-3). The R.C. buildings generally experienced few structural damage, and only a dozen were involved in partial collapses (e.g., soft-first story; Fig.3A).
The mesoseismic area (7-8 MCS) is strongly NW-SE elongated, from the hypocentre zone (L’Aquila southwestern outskirts) toward the
Aterno Valley (SE; Fig.4). This fact reflects the geometry, kinematics and rupture dynamics of the seismogenic fault, which has been
recognized to be a N135° normal fault (dip at depth 55°), the surficial expression of which fits with the so-called Paganica-San
Demetrio Fault System (PSDFS in Fig.5). In facts, we followed surficial breaks along ~19 km of this structure, from the village of
Collebrincioni to the NW toward the one of San Demetrio ne’ Vestini to the SE. In some places, we measured a maximum 15 cm offset
across the fault, both on bedrock (carbonate and conglomerate) fault scarps, and in unconsolidated talus deposits (Fig.6).
The macroseismic effects distribution fits quite well with the few intensity datapoints of the 1461 event, that could be thus considered
a historical “twin” of the 2009 earthquake. However, preliminary paleoseismological data show a more complex history of the Paganica
faults, with coseismic offsets consistent with events much stronger than the 1461 and 2009 events. Therefore, we hypothesise that the
PSDFS might generate both earthquakes with magnitude similar to the one of 1461 and 2009 (Mw~6.3), but also stronger events, rupturing together with adjacent structures, such as the Middle Aterno Fault System (MAFS in Fig.5). In this case, the length of the entire fault system (~35 km) would suggest the occurrence of Mw~6.9 earthquake, which is the magnitude class of many other Apennines events, like the 1915 (Fucino) and 1980 (Irpinia) ones.