Physics-Based Long-Period Ground Motion Scenarios in and Around the Po Plain Sedimentary Basin (Northern Italy)

Unexpected large and prolonged shaking (> 80s) associated with long-period ground motion has been observed inside the Po Plain sedimentary basin (Northern Italy) during the two M~6, May 20-29, 2012, earthquakes. Long-period ground motion impacts on the seismic response of taller structures. It is hence important to understand the characteristics of long-period ground motion associated with the 3D structure and finite fault properties, in particular in those regions with deep sedimentary basins and a complex geological context.

We implement a recent high resolution model of the Po basin (MAMBo), derived from geological constraints, in spectral-element code SPECFEM3D_cartesian (Peter et al., 2012). The simulations are numerically accurate for periods of 2 sec and longer, and incorporate complex 3D basin structure and topography as well as the spatial and temporal heterogeneity of source rupture. The response of our basin model has been evaluated for several instrumental earthquakes. Synthetics seismograms reproduce well amplitude and long duration, as well as envelope and coda, observed in paths that travel through sediments.

We also evaluate ground motion produced by plausible earthquakes inferred from historical data, such as the Modena (1501) and Verona (1117) events that caused well-documented strong effects in a unusually wide areas with lengths of hundreds of kilometers. We test different representations of the seismic source, from point source to finite sources with different rupture histories, evaluating the impact on shaking amplitude. We compare our results with damage maps (when available) and with the GMPEs currently adopted for this area, evaluating the effects of finite fault and 3D propagation on ground shaking. We show that deterministic ground motion calculation can indeed provide information to be actively used to mitigate the effects of destructive earthquakes on critical infrastructures.