The M-w 5.4 Reggio Emilia 1996 earthquake: active compressional tectonics in the Po Plain, Italy

We have analysed the seismic sequence that occurred in October 1996 near the town of Reggio Emilia on the southern edge of the Po Plain. The onset of the sequence was marked by a 5.4 moment magnitude main shock, located at 15 km depth. The main-shock focal mechanism is a reverse solution with a strike-slip component and the scalar moment is 1.46 x 10(17) N m. We used broad-band digital recordings from a borehole station, located at about 70 km from the epicentre, for a spectral analysis in order to estimate attenuation and source parameters for the main shock. In addition, the empirical Green's function method has been applied to evaluate the source time function in terms of both moment rate and stress rate. We infer an asperity-like rupture process for the main shock, as suggested by the short duration of the stress release with respect to the overall duration of the moment rate function. This analysis also allows us to estimate the average dynamic stress drop of the main shock (600 bar). We analysed the digital recordings of the temporary local seismic network deployed after the main shock and of a permanent local network maintained by the Italian Petroleum Agency (AGIP). During 15 days of field experiments, we recorded more than 800 aftershocks, which delineate a 9 km long, NE-elongated distribution, confined between 12 and 15 km depth, suggesting that the basement is involved in the deformation processes. 102 focal mechanism of aftershocks have been computed from P-wave polarities, showing mainly pure reverse solutions. We calculate the principal stress axes from a selected population of earthquakes providing a constraint on the stress regime of this part of the Po Plain. The focal mechanisms are consistent with a N-S subhorizontal al. All the seismological data we have analysed confirm that this region is undergoing active compressional tectonics, as already inferred from recent earthquakes, geomorphological data and other stress indicators. Moreover, the elongation of the Reggio Emilia aftershock sequence is consistent with the regional direction of the thrust fronts cropping out in the area. suggesting that they are still active.