Geological Evidence of Quaternary Tectonic Activity as a Tool for the Evaluation of the Seismic Potential of a Region: Some Preliminary Results on Castenedolo, Ciliverghe and Capriano Folds (BS)-(Northern Po Plain, Italy)
by
Franz Livio
Università degli Studi dell'Insubria, Como
Coauthors: Francesca Giardina, Cipriano Carcano, Alessandro M. Michetti, Andrea Piccin, Sergio Rogledi, Giancanio Sileo

Prevention and mitigation of the effects of natural catastrophic events, which are able to cause serious social and economic consequences, like earthquakes, is one of the goals of the Dark Nature Project.

The recent catastrophic Sumatra seismic event (Dec. 26, 2004, Mw 9.2) has redrawn the media's and the international scientific community's attention on the importance of Seismic Hazard Assessment (SHA).

We present some preliminary results on the localization and characterization, through geological and geophysical analyses, of some important structures buried beneath the Po Plain and located along the Southern Alps foothills west of Lake Garda.

As well described in literature (Serva, 1990; Doglioni, 1993; Castellarin & Cantelli, 2000; Boccaletti & Martelli, 2004; Fantoni et al., 2004), the Northern Italy structural framework is characterized by the growth of two thrust belts, surrounding the Po Plain: the Apennines and the Southern Alps. Structurally the external margin of the Southern Alps is constituted by folds and thrusts, buried below the Po Plain and mostly parallel to the Southern margin of the chain, which are generally tilted to the south and sealed by a Messinian unconformity; this has commonly been interpreted as the deactivation of the Southern Alpine deformation in Lombardy since the Upper Miocene. However, as reminded by the recent Mw 5.0, Nov 24, 2004, Salò earthquake, geological and seismological evidence suggest that the thrust belt beneath the Lombardian Southern Alps foothills should be better evaluated in terms of seismic hazard.

Our study area is located south of the piedmont belt of the central eastern Southern Alps, ca. 10 Km south-west of Lake Garda. Here, literature data (Desio, 1965) underlie the occurrence of some isolated relieves (Castenedolo, Ciliverghe and Capriano hills) whose presence cannot be explained by glacial or fluvioglacial morphogenic processes.

Each of these hills, in fact, has been interpreted as the culmination of a young anticline structure. At Castenedolo and Capriano Ridge Early Pleistocene marine deposits were uplifted more than 200 m; to the east, the Middle Pleistocene continental sequence constituting the Ciliverghe Hill was faulted, uplifted and gently tilted (Baroni & Cremaschi, 1986; Castaldini e Panizza, 1991).

Our work, aimed to a better characterization of these structures, is based on a) literature data compilation; b) geological and geomorphological field survey; c) DTM interpretation of the morphology of this sector of the Po Plain; d) subsurface data compilation (seismic lines and exploration drillings, courtesy released by ENI E&P; and water wells stratigraphic data, collected and courtesy released by Regione Lombardia); e) 3D geological model reconstruction.

Our work allowed us to identify some evidence of Quaternary compressive tectonics in the study area. In particular the seismic profiles clearly show a series of fault propagation folds, ca. 10 Km wide, controlled by the Plio-Quaternary growth of several out-of-sequence north verging thrusts. A regional marker horizon, represented by a sequence boundary termed "R surface" by Muttoni et al. (2003), and dated at ca. 0.9 Ma, has been used to identify and map these structures in detail. Preliminary results show a strong correspondence between structural and geomorphological high, confirming the tectonic origin of these isolated relieves and suggesting a recent activity of these structures. In particular, the observed deformations clearly affect the basin's quaternary filling up to at least the upper part of Middle Pleistocene.

It is important to note that this region overlaps with the epicentral area of the Dec 25th, 1222, Brescia earthquake, (Io=IX MCS; Magri & Molin, 1986; Serva, 1990; Guidoboni, 1986; Guidoboni, 2002) one of the stronger Alpine historical earthquakes that struck the Po Plain.

A reconstruction of the recent movements of these folds, through geometric and kinematic models (Zehnder et al., 2000; Johnson et al., 2002) applied to reflection seismic profiles, coupled with paleoseismological analyses, will allow us to correctly evaluate the seismic potential of the underlying associated faults, in terms of maximum expected earthquake magnitude and recurrence rates.

Date received: July 14, 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-34.