The 2012 Ferrara seismic sequence: Regional crustal structure, earthquake sources, and seismic hazard

Abstract

[1] Inadequate seismic design codes can be dangerous, particularly when they underestimate the true hazard. In this study we use data from a sequence of moderate-sized earthquakes in northeast Italy to validate and test a regional wave propagation model which, in turn, is used to understand some weaknesses of the current design spectra. Our velocity model, while regionalized and somewhat ad hoc, is consistent with geophysical observations and the local geology. In the 0.02-0.1 Hz band, this model is validated by using it to calculate moment tensor solutions of 20 earthquakes (5.6 ≥ M W ≥ 3.2) in the 2012 Ferrara, Italy, seismic sequence. The seismic spectra observed for the relatively small main shock significantly exceeded the design spectra to be used in the area for critical structures. Observations and synthetics reveal that the ground motions are dominated by long-duration surface waves, which, apparently, the design codes do not adequately anticipate. In light of our results, the present seismic hazard assessment in the entire Pianura Padana, including the city of Milan, needs to be re-evaluated.

Frontal compression along the Apennines thrust system: The Emilia 2012 example from seismicity to crustal structure

“…This lateral change in dip between the two activated fault segments is not revealed by fault planes solution derived from published centroid moment tensors (Cesca et al, 2013;Malagnini et al, 2012;Pondrelli et al, 2012;Saraò and Peruzza, 2012;Scognamiglio et al, 2012), which do not show appreciable change of dip between the two large events. For example the S-dipping plane of focal solutions have dip of 46 • and 38 • in Scognamiglio et al (2012;see also http://cnt.rm.ingv.it/tdmt.html), and of 45 • in Malagnini et al (2012). Our results suggest that there might be a potential large uncertainty (10-15 • ) on fault dip from centroid moment tensor, probably due to the strong complexity of the medium around the seismic source.…”

“…2a). The Mw 6.0 earthquake has a purely inverse focal solution (Malagnini et al, 2012;Pondrelli et al, 2012;Scognamiglio et al, 2012), with planes dipping around 45 • (Fig. 1a).…”

The May 20 (MW 6.1) and 29 (MW 6.0), 2012, Emilia (Po Plain, northern Italy) earthquakes: New seismotectonic implications from subsurface geology and high-quality hypocenter location

“…This has resulted in several studies based on comparisons between the new seismic data collected during the Emilia sequence (mainshocks and aftershocks) and the sub-surface geological structures and geomorphological evidence (e.g., Bonini et al, 2014;Burrato et al, 2012;Malagnini et al, 2012;Ventura and Digiovambattista, 2013). Other studies have provided new models of faults and displacement patterns through inversion of geodetic data (Pezzo et al, 2013;Tizzani et al, 2013) and through tomographic inversion methods and/or relocated seismicity .…”

Site response estimation at Mirandola by virtual reference station

“…Moreover, for the area affected by the 2012 seismic sequence, some studies provide regional velocity models (for example Malagnini et al 2012;Vuan et al 2011;Govoni et al 2014), while others give evidence of the seismic response at some sites within the epicentral area or compare the ground-motion observation with prediction equations for soft soils (Barnaba et al 2014).…”