Ground-motion simulations within ShakeMap methodology: application to the 2008 Iwate-Miyagi Nairiku (Japan) and 1980 Irpinia (Italy) earthquakes

ShakeMap package uses empirical Ground Motion Prediction Equations (GMPEs) to estimate the ground motion where recorded data are not available. Recorded and estimated values are then interpolated in order to produce a shaking map associated with the seismic event of interest. The ShakeMap approach better works in regions with dense stations coverage, where the observed ground motions adequately constrain the interpolation. On the other hand, in poorly instrumented regions, the ground motion estimate mainly relies on the GMPEs that account only for average characteristics of source and wave propagation processes. In this study we investigated the improvement of ShakeMap in the near-fault area accounting for source effects, at different level of approximation, and including synthetic estimates at sites where recordings are not available. We focus on the 2008, Mw 7.0, Iwate-Miyagi Nairiku (Japan) earthquake as a case study because of the large amount of recorded data. As first step, we calculated the shakemaps to be used as reference using all station recordings and simple source information such as the earthquake magnitude and fault geometry. We then removed several subsets of stations from the original data-set and computed the shakemaps (i) using the estimations of the ground motion obtained by GMPE and (ii) replacing them with the peak values from synthetic time-histories. Simulated seismograms are computed by a hybrid deterministic-stochastic method for extended fault, using rupture fault models with different degree of approximation of source properties. In fact, within few hours, a preliminary rupture model can be obtained from the inversion of teleseismic data, and after 24 hours a kinematic source inversion of strong-motion records can be available. We evaluate the deviations from the reference map and the sensitivity to the number of sites where recordings are not available. Our results show that shakemaps are more and more reliable as the coverage of stations is uniformly distributed in the near-source area. Moreover, the shakemaps obtained by the use of combination of slip inversion model and strong-motion simulations are more reliable than those obtained by the use of GMPEs. The accuracy of source model does not significantly affect the results, making the proposed methodology applicable to regions with sparse station coverage. As an example, we apply it to a poorly instrumented earthquake of similar magnitude, the 1980, Ms 6.9, Irpinia (Southern Italy) earthquake. When the peak motions inferred from synthetic seismograms are included in the database, the fit with respect to the observed Mercalli–Cancani–Sieberg intensities improves.