Seismic Hazard Assessment for a High Populated and Industrialized Area: The Case of the Insubria Region (Lombardian Southern Alps, Italy)
by
Francesca Giardina
Università degli Studi dell'Insubria, Como
Coauthors: F. Livio, Università dell'Insubria, Como, Italy; G. Sileo, Università dell'Insubria, Como, Italy; K. Chunga, Università dell'Insubria, Como, Italy; K. Mueller, University of Colorado, Boulder, CO, USA; A.M. Michetti, Università dell'Insubria, Como, Italy

One of the purposes of the Dark Nature Project is the characterization of natural risks for densely populated areas; the importance of this issue has been reminded to us in the last years by numerous catastrophic events. Regarding Northern Italy, after the recent Salò earthquake (Nov 24, 2004; Mw 5.0), a new attention has been paid to the seismic hazard assessment in Lombardy. This research is focused on characterizing the maximum magnitude and recurrence intervals of future potential earthquakes in the Insubria region between Western Lombardy and Ticino, at the Italy-Switzerland border, a densely populated and high industrialized area. Based on available historical and instrumental data, the seismicity of this region is characterized by modest magnitude events with relatively long recurrence intervals. The present seismic code, which is derived from the earthquake catalogue, considers most of this area at one of the lowest level of seismic hazard of the whole Italian peninsula. However, geological and geomorphological evidence suggests that Quaternary structures within the thrust belt beneath the Lombardian Southern Alps foothills are active and should be better evaluated in terms of an ungraded seismic hazard. A review of literature, field mapping and geomorphic study, along the Lombardian foothills has allowed us to identify evidence for active Late Quaternary compressive tectonics along the Lombardian South Alpine piedmont belt. This includes: a) in the Insubria region the presence of a sequence of glacial and fluvioglacial Middle Pleistocene deposits which are uplifted ca. 200 m along the foothills between Como and Lecco across the forelimb of an active anticline, Albese con Cassano; b) in the piedmont area between Brescia and Lake Garda, uplifted fragments of now largely eroded and buried fault-related folds are preserved at Castenedolo and Ciliverghe, as firstly interpreted by Desio (1956). Notably, however, western and eastern Lombardy are characterized by very different levels of historical seismicity. To the east, near Lake Garda, the Castenedolo and Ciliverghe structures coincide with the epicentral zone of the Dec 25, 1222, Brescia earthquake, one of the stronger Alpine earthquakes that struck the Po Plain (Io = IX MCS, macroseismically derived magnitude = 6.2; Magri & Molin, 1986; Serva, 1990; Guidoboni, 2002). Therefore, near Brescia the evidence of recent folding is associated with a significant earthquake hazard (see Livio et al., this volume). In contrary, in the Insubria region, the evidence of Late Quaternary shortening has not yet been related to strong historical earthquakes. In order to better understand the differences in historical seismicity along the Lombardian piedmont belt, we investigated relationships between surface geomorphology and associated deep structures in both the Insubria region and the area near of Lake Garda. We modeled geological, morphological and hydrological features, using the Albese con Cassano ridge (East of Como) as a type example for the western part of the belt. Orombelli (1976) previously described the Albese con Cassano area as a tectonic structure which 1) uplifted a sequence of glacial to fluvioglacial Middle Pleistocene deposits, 2) produced a secondary compressive fault within this sequence, 3) tilted more recent pro-glacial lake deposits, and 4) diverted the local drainage network. Our investigations allowed to confirm these data, and to propose that the presence of uplifted Middle Pleistocene deposits provides evidence for the possibly active nature of a growing anticline which accumulated ca. 200 m of vertical displacement during the Middle Pleistocene to Present (?). Comparing structures in the Insubria region with the thrusts near Lake Garda, the geometries and the fault length or segmentation appears similar, although slip rates are likely to be lower towards the west. If this interpretation is correct, the seismic potential of the Insubria region could be considered comparable to that one near Lake Garda, although faults there (Insubria) likely have longer recurrence periods. Future research will be aimed at validating this hypothesis in a search for additional evidence of active Holocene shortening and paleoseismicity to better define the maximum magnitude and recurrence intervals of past earthquakes in this high populated area.

Date received: July 25, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-66.