Moment magnitudes differing by up to 0.5 units have been published for the same events of the 2012 Ferrara seismic sequence. With respect to the mainshock that occurred on 20 May 2012, results by Malagnini et al. (2012) and Pondrelli et al. (2012) represent opposite extremes: although the former used model Padania, a region‐specific velocity structure based on all the available geological and geophysical information from local studies, the latter used a global crustal model with a set of phase corrections calibrated over the central Apennines by Ekström et al. (1998). Model Padania well reproduces the observed dispersion of surface‐wave group velocities in a band of shorter periods, between 33 and 100 s, whereas Pondrelli et al. (2012) performed their inversions in the 50–150 s period band. Here, we show that because surface waves generated within the thick sediments of the Po river floodplain dominated the seismograms, the source excitation terms that came out of a regression scheme performed on the ground motions recorded during the sequence were systematically affected by a broadband increase of the spectral amplitudes at frequencies below 0.4 Hz (frequency range of the regressions: from 0.1 to 22.5 Hz). As a consequence, the two largest events of the sequence share a common true moment magnitude Mw∼5.6, even though their enhanced spectral level from 0.1 to 0.4 Hz is consistent with Mw∼6.0.
Electronic Supplement:Figures of hypocentral distances, moment tensor (MT) solutions, and site terms from the ground‐motion regression; and tables of velocity models and MT solutions.