The seismic potential of the Insubria Region (Southern Alps): insights from topographic and rheological modelling

The active tectonics of Northern Italy is characterized by the growth of the two thrust belts surrounding the Po Plain, the Southern Alps and the Apennines (Figure 1). As widely documented by regional geological data (e.g., Bigi et al., 1990; Doglioni, 1993), along the Apennines foredeep Quaternary deformation rates decrease westward from the Ferrara, to the Emilia and Monferrato Arcs; also, ongoing regional crustal movements as described by geodetic measurements seems to indicate a similar pattern. Most important for our study, this trend is in good agreement with the westward-decreasing level of seismicity shown by the historical catalog (e.g., Boschi et al., 2000; Camassi and Stucchi, 1997; Figure 1); moderate earthquakes (M 5 to 5.5) are more frequent in the east (Ferrara) than in the west (Monferrato). This is also true for the southern margin of the Southern Alps between Friuli and western Lombardia, notwithstanding the obvious difference in the structural evolution of the two thrust belts, which implies also a different seismic style in terms of maximum earthquake magnitude and recurrence behaviour, (e.g., Serva, 1990; Doglioni, 1993; Castellarin and Cantelli, 2000). In particular, based on the available observations during the historical time window, earthquakes in the Southern Alps show higher maximum magnitudes (in the order of M 6.5) then in the Northern Apenninic Arcs, as clearly illustrated by the Friuli (e.g., Slejko et al., 1987; Barbano, 1993) and Garda areas (e.g., Magri and Molin, 1986; Serva, 1990; Curzi et al., 1992; Sauro and Zampieri, 2001; Galadini et al., 2001; Valensise et al., 2003); these strong historical seismic events have longer return periods in the west (Garda) then in east (Friuli). We argue, therefore, that the understanding of Quaternary tectonics and accompanying seismic potential around the Po Plain must be framed in a common regional geodynamic setting.