Ground-motion simulation in the Calabrian accretionary prism (Southern Italy) using a 3D geologic-based velocity model

Ground motion prediction is one of the main goals in seismic hazard assessment. Empirical ground motion prediction equations may fail to reproduce the complexity of ground shaking in complex 3D media and therefore the use of full waveform modelling is increasingly adopted to model ground shaking. The knowledge of the 3D crustal structure in terms of geometries of the main discontinuities and velocities is fundamental to model wave propagation. However, we often lack detailed geological and geophysical information to build reliable models.

We exploit here a large set composed of high-resolution 2D and 3D seismic data and of about 40 wells with stratigraphic and velocity information, both onshore and offshore, to constrain a 3D crustal velocity model in a sector of the Calabrian accretionary prism (southern Italy). We interpret the main reflection discontinuities and constrain their depth at all available wells in the study area and we use well’s check-shots and velocity data to estimate interval-velocities of the main stratigraphic units. We then combine all depth and velocity information into a regional 3D crustal velocity model of the first 8-10 km. This is subsequently extended to a depth of ~50 km using available regional crustal models to obtain the final model used for ground motion simulation.

We implement our crustal model in the spectral-element code SPECFEM3D_Cartesian to simulate wave propagation in the 3D velocity model honoring surface topography. This allows reconstructing the low-frequency part of the waveforms (up to ~1 Hz), which is then combined with high-frequency seismograms obtained with a stochastic method following the hybrid broadband simulation approach by Graves and Pitarka (2010).

We evaluate the goodness of our model by simulating real earthquakes and comparing simulated and recorded waveforms at the available seismic stations in the area. We compare the results from our 3D model with the ones obtained using a local tomography model and the European crust model EPcrust. The maps of ground motion obtained from the simulated broadband waveforms are then compared with empirical ShakeMaps. These results will also be useful for earthquake scenario calculations, by simulating potential seismic sources identified from structural analysis of geological and seismic data.