Moho reflection effects in the Po plain (northern Italy) observed from instrumental and intensity data

Recent moderate-sized earthquakes (ML ≤ 5:2) that occurred in northernItaly demonstrate that the currently available ground-motion prediction equations(GMPEs) largely underestimate shaking in the Po Plain region at hypocentral distancesgreater than 70 km. In order to investigate this phenomenon, we collected a set of peakground acceleration (PGA) observations for weak and moderate earthquakes in thearea. Nonparametric regression analysis of PGA observations as a function of magnitudeand hypocentral distance shows that PGA is systematically enhanced for distancesbetween 70 and 200 km. An indirect estimation through the analysis of theattenuation of macroseismic intensities suggests that the effect also applies to strongearthquakes (ML > 5:5). We performed numerical modeling experiments to investigatethe cause of the phenomenon. The characteristics of the computed synthetic seismogramsindicate that the enhancement of ground motion is mainly an effect of thereflection of S waves at the Moho (SmS phase). The analysis of both real and syntheticdata shows that the Moho reflection effect is maximized at hypocentral distancesbetween 90 and 150 km, where the PGA increases by a factor larger than 2.4.