Moho Reflection Effects in the Po Plain (Northern Italy) Observed from Instrumental and Intensity Data

Recent moderate-sized earthquakes (M(L) <= 5.2) that occurred in northern Italy demonstrate that the currently available ground-motion prediction equations (GMPEs) largely underestimate shaking in the Po Plain region at hypocentral distances greater than 70 km. In order to investigate this phenomenon, we collected a set of peak ground acceleration (PGA) observations for weak and moderate earthquakes in the area. Nonparametric regression analysis of PGA observations as a function of magnitude and hypocentral distance shows that PGA is systematically enhanced for distances between 70 and 200 km. An indirect estimation through the analysis of the attenuation of macroseismic intensities suggests that the effect also applies to strong earthquakes (M(L) > 5.5). We performed numerical modeling experiments to investigate the cause of the phenomenon. The characteristics of the computed synthetic seismograms indicate that the enhancement of ground motion is mainly an effect of the reflection of S waves at the Moho (SmS phase). The analysis of both real and synthetic data shows that the Moho reflection effect is maximized at hypocentral distances between 90 and 150 km, where the PGA increases by a factor larger than 2.4.