Shake Map Methodology for Intermediate-Depth Vrancea (Romania) Earthquakes

We establish and test a shake map methodology for intermediate-depth Vrancea earthquakes, based on seismological information gathered in Romania during recent years. We use region- (azimuth-) dependent attenuation relations derived from stochastic simulations of ground motions using spectral models of Vrancea earthquakes. Both region boundaries and Fourier amplification spectra for the characterization of seismic site effects are based on several hundred weak, moderate and strong-motion records and macroseismic intensity maps. We determine region-specific, magnitude- and distance-dependent amplification factors of peak values and instrumental intensity relative to rock. We interpolate recorded ground motions and ground motion estimates from the obtained amplification factors and attenuation relations for rock conditions. The resulting shake maps show a good agreement with macroseismic descriptions of moderate-sized and large Vrancea earthquakes, demonstrating the feasibility of a seismological approach to shake map generation. Unlike previous methodologies, this approach requires neither expensive assessments of geology-dependent site amplification factors, nor large numbers of strong-motion records. Our results are in good agreement with empirical topographic slope-site amplification relations, but give a better reflection of the abnormal attenuation of seismic waves in the Transylvanian region and the strong amplification in the Focsani basin.