On the ability of Moho reflections to affect the ground motion in northeastern Italy: a case study of the 2012 Emilia seismic sequence

It has been observed that post-critically reflected S-waves and multiples from theMoho discontinuity could play a relevant role on the ground motion due to medium to strongsize earthquakes away from the source. Although some studies investigated the correlationbetween the Moho reflections amplitudes and the damage in the far field, little attention wasgiven to the frequency content of these specific phases and their scaling with magnitude. The2012 Emilia seismic sequence in northern Italy, recorded by velocimetric and accelerometricnetworks, is here exploited to investigate Moho reflections and multiples (SmSM). A singlestation method for group velocity-period estimation, based on the multiple filter technique,is applied to strong motion data to detect SmSM. Amplitude and frequency scaling withmagnitude is defined for earthquakes from Mw = 3.9 to Mw = 5.9. Finally, the ability ofSmSM to affect the ground motion for a maximum credible earthquake within the Po plain isinvestigated by extrapolating observed engineering parameters. Data analysis shows that highamplitude SmSM can be recognized within the Po plain, and at the boundaries between thePo plain and the Alpine chain, at epicentral distances larger than 80 km, in the period rangefrom 0.25 to 3 s and in the group velocity window from about 2.6 to 3.2 km/s. 5% dampedpseudo-spectral accelerations at different periods (0.3, 1.0 and 2.0 s), and Housner intensities,are obtained from data characterized by large amplitude SmSM. A scaling relationship forboth pseudo-spectral accelerations and Housner intensities is found for the earthquakes ofthe 2012 Emilia seismic sequence. IMCS from VII to VIII is estimated, as a result of SmSMamplitude enhancement, at about 100km for a maximum credible earthquake (Mw = 6.7)in the Po plain, showing that moderate to high damage cloud be caused by these specific phases.