Dark Nature and Paleoseismology: Understanding the Seismic Landscape of the Southern Alps, Italy
by
Alessandro M. Michetti
Università degli Studi dell'Insubria, Como

The knowledge of the seismicity level of a region is one of the key elements for evaluating the impact of natural processes on the society. Instrumental and historical data might be inadequate to reach this goal. For instance, historical seismic events having magnitude in the range of 6.0 to 6.5 have generated "catastrophic" effects in Northern Italy (e.g., Jan. 3, 1117, Verona; Dec. 25, 1222, Brescia; and Jan. 25, 1348, Friuli, events). Due to the dramatically increased vulnerability of the area (i.e., high density of both population and critical industrial facilities in the Po Plain), nowadays their recurrence would probably be much more damaging, in terms of both victims and economic loss.

The return period of such strong earthquakes, however, could be longer than the time window of ca. 1000 yr for which the Italian seismic catalogue (arguably one of the best in the world) is regarded as complete. Therefore, earthquake hazard studies in this region must be based on paleoseismological evidence.

In this paper I summarize the preliminary findings of a joint research project conducted by University of Insubria, APAT, ENI E&P, and University of Colorado, aimed at A) the seismotectonic characterization of the Southern Alps and B) the paleoseismic analysis along selected local active tectonic structures. This research is particularly focused on the sector at the border between the Po Plain and the Lombardia alpine foothills, an area where the relations between active tectonics and seismicity are still poorly recognized.

The first results show that the fold and thrust belt along the Southern Alps-Po Plain margin is characterized by significant Quaternary activity not only in the eastern sector (Friuli, Veneto; Slejko et al., 1987; Benedetti et al., 2000; Galadini et al., 2001) but also in the area between Lake Garda and Lake Maggiore. The most relevant geologic and geomorphic signature of this activity is the occurrence within the piedmont belt of isolated hills, which corresponds to the culmination of young anticlines and locally to the epicentral area of strong historical earthquakes (i.e., near Castenedolo, just west of Lake Garda, epicentral area of the Dec. 25, 1222, I = IX MCS, earthquake; e.g., Guidoboni, 1986; Serva, 1990). This feature was already noted by Desio (1965), but still very little is known about the seismic potential associated to these recent folds.

The large amount of paleoseismological data collected in recent years shows that each earthquake source creates a signature on the geology and the geomorphology of an area that is unequivocally related with the order of magnitude of its earthquake potential. This signature is defined as the seismic landscape of the area (e.g., Serva et al., 1997; Michetti and Hancock, 1997; Michetti et al., 2005).

We argue that the structural and geomorphic characteristics of these young anticlines can be used for the definition of the seismic landscape in the Southern Alps, following the same methodological approach that has been adopted for the intermountain basins in the extensional setting of the Apennines (Serva et al., 2002).

The preliminary results and methodological approach of these active tectonics and paleoseismological studies in the Central and Western Southern Alps might be relevant for several other alpine areas, showing similar seismotectonic features and poor seismic hazard characterization from the geological point of view.

References

Benedetti L., Tapponnier P., King G. C. P., Meyer B., Manighetti I. (2000), "Growth folding and active thrusting in the Montello region, Veneto, northern Italy", Journal of Geophysical Research, B, Solid Earth and Planets, vol.105, no.1, pp.739-766.

Desio A. (1965), "I Rilevi Isolati della Pianura Lombarda ed i movimenti tettonici del Quaternario", Istituto Lombardo (Rend. Sci.) A 99, pp. 881-894 (1965).

Galadini, F., Galli P., Molin D., Ciurletti G. (2001) "Searching for the source of the 1117 earthquake in northern Italy: a multidisciplinary approach", In: T. Glade et al. (eds.), "The use of historical data in natural hazard assessments", Kluwer Academic Publisher, 3-27.

Guidoboni, E. (1986) "The earthquake of December 25, 1222; analysis of a myth", Geologia Applicata e Idrogeologia, vol.21, Part 3, pp.413-424.

Michetti, A.M & Hancock, P.L., (1997), "Paleoseismology: Understanding Past Earthquakes Using Quaternary geology". Journal of Geodynamics, 24(1-4): 3-10.

Michetti A.M., F. Audemard, & S. Marco (2005), "Future trends in paleoseismology: Integrated study of the seismic landscape as a vital tool in seismic hazard analyses", Special Issue "Paleoseismology, integrated study of the Quaternary geological record for earthquake deformation and faulting", Tectonophysics, in press.

Serva, L. (1990), "Il ruolo delle scienze della terra nelle analisi di sicurezza di un sito per alcune tipologie di impianti industriali; il terremoto di riferimento per il sito di Viadana (MN). Role of Earth sciences in the safety analysis of the site of a particular typology of an industrial plant; the reference earthquake at Viadana", Boll. Soc. Geol. It., 109, 2, 375-411.

Serva, L., Vittori, E., Ferreli, L., & Michetti, A.M., (1997), "Geology and Seismic Hazard". In: B. Grellet, B. Mohammadioun and W. Hays, eds., "Proceedings of the Second France-United States Workshop on Earthquake Hazard Assessment in Intraplate Regions: Central and Eastern United States and Western Europe", October 16, 1995, Nice, France, 20-24, Ouest Editions, Nantes, France.

Serva, L., Blumetti, A.M., Guerrieri, L. and Michetti, A.M. (2002), "The Apennine intermountain basins: the result of repeated strong earthquakes over a geological time interval". Boll. Soc. Geol. It., Spec. Vol. 1 (2001): 939-946.

Slejko D., Carraro F., Carulli G.B., Castaldini D., Cavallin A., Doglioni C., Nicolich R., Rebez G., Semenza E., & Zanferrari A. (1987), "Modello sismotettonico dell'Italia nordorientale". CNR, Rendiconto n.1, 1-82.

Date received: July 28, 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-71.