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

Abstract

SUMMARY 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 × 1017 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 σ1. 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.

Heterogeneous strength and fault zone complexity of carbonate-bearing thrusts with possible implications for seismicity

“…In this area, the integration of seismic reflection profiles with seismological data shows that the strongest seismic sequences, such as the M W = 5.4 Reggio Emilia sequence in 1996 (Selvaggi et al, 2001) and the M L = 5.9 Emilia sequence in 2012 (Govoni et al, 2014), nucleate within the same sedimentary sequence exposed in the Umbria-Marche Apennines (Pieri and Groppi, 1981). Seismic profiles crossing the active compressional front highlight compressional structures with the same geometry, similar wavelength and fault dimensions of the fault mapped in the Umbria-Marche Apennines (Massoli et al, 2006;Carminati et al, 2010).…”

Active tectonics of the outer northern Apennines: Adriatic vs. Po Plain seismicity and stress fields

“…There, focal mechanisms for events preceding the 2013 earthquakes were characterized by a wide range of orientation of P and T axes, and included thrust, strike-slip and extensional fault plane solutions (e.g. Gasparini et al, 1985;Zollo et al, 1995;Amato, 1997, 2000;Mariucci et al, 1999;Selvaggi et al, 2001;Santini et al, 2011;Macchiavelli et al, 2012). Furthermore, thrusts buried beneath the Adriatic Sea appear to be sealed by undeformed Middle-Upper Pleistocene deposits (e.g.…”

Recent and active tectonics of the external zone of the Northern Apennines (Italy)

“…Consequently, normal faulting is the dominant active deformation style in the Tyrrhenian area, whereas the active tectonics of the Adriatic sector is accommodated by folding and thrusting (Benedetti et al 2000(Benedetti et al , 2003Burrato et al 2003;Vannoli et al 2004;Boccaletti et al 2004;Basili and Barba 2007;DISS Working Group 2007;Picotti and Pazzaglia 2008;Wilson et al 2009). Active shortening is well constrained by the analysis of earthquake fault plane solutions (e.g., Gasparini et al 1985;Boccaletti et al 1985;Anderson and Jackson 1987;Frepoli and Amato 1997;Selvaggi et al 2001;Pondrelli et al , 2004Lavecchia et al 2004;Chiarabba et al 2005;Piccinini et al 2006;Boncio and Bracone 2009), stress analysis (e.g., Mariucci et al 1999;Mariucci and Muller 2003;Montone et al 2004), and geodetic measurements (e.g., Battaglia et al 2004;Serpelloni et al 2005;D'Anastasio et al 2006). However, the knowledge of the main recent and active structures remains limited to scattered areas (e.g., Benedetti et al 2003;Burrato et al 2003;Vannoli et al 2004;Piccinini et al 2006;Picotti and Pazzaglia 2008;Picotti et al 2009;Wilson et al 2009).…”