ER3D: a structural and geophysical 3D model of central Emilia-Romagna (Northern Italy) for numerical simulation of earthquake ground motion

Abstract. During the 2012 seismic sequence of Emilia region (Northern Italy), the earthquake ground motion in the epicentral area featured longer duration and higher velocity than those estimated by empirical-based prediction equations typically adopted in Italy. In order to explain these anomalies, we (1) build up a structural and geophysical 3D digital model of the crustal sector involved in the sequence, (2) reproduce the earthquake ground motion at some seismological stations through physics-based numerical simulations and (3) compare the observed recordings with the simulated ones. In this way we investigate how the earthquake ground motion in the epicentral area is influenced by local stratigraphy and geological structure buried under the Po Plain alluvium. Our study area covers approximately 5000 km 2 and extends from the Po river right bank to the Northern Apennines morphological margin in N-S direction, and between the two chief towns of Reggio Emilia and Ferrara in W-E direction, involving a crustal volume with 20 km of thickness. We set up the 3D model by using already published geological and geophysical data, with a detail corresponding to a map at scale 1:250 000. The model depicts the stratigraphic and tectonic relationships of the main geological formations, the known faults and the spatial pattern of the seismic properties. Being a digital vector structure, the 3D model can be easily modified or refined locally for future improvements or applications. We exploited high performance computing to perform numerical simulations of the seismic wave propagation in the frequency range up to 2 Hz. In order to get rid of the finite source effects and validate the model response, we choose to reproduce the ground motion related to two moderate-size aftershocks of the 2012 Emilia sequence that were recorded by a large number of stations. The obtained solutions compare very well to the recordings available at about 30 stations, in terms of peak ground velocity and signal duration. Snapshots of the simulated wavefield allow us to explain the exceptional length of the observed ground motion as due to surface waves overtones that are excited in the alluvial basin by the buried ridge of the Mirandola anticline. Physics-based simulations using realistic 3D geo-models show eventually to be effective for assessing the local seismic response and the seismic hazard in geologically complex areas.