Combined effect of basin resonance and source directivity: implications for long period hazard in the Apennines, central Italy

Abstract

During the three strongest shocks of the 1997 Umbria-Marche, central Italy, seismic sequence, long-period (T > 1 s) spectral ordinates showed large variations in intermountain basins in the Apennines. In particular, at a strong-motion station in the Rieti plain, about 65 km south of epicenters, long-period response spectra during the Mw 5.6 and 5.7 shocks had larger amplitudes than the Mw 6.0 ones; in contrast, in the Gubbio basin, about 40 km northwest of the epicenters, the Mw 6.0 shock had spectral ordinates exceeding those of the Mw 5.6 and 5.7 shocks by more than a factor of 10 at long periods. Since focal mechanisms were similar for the causative earthquakes and the difference in magnitude and source-to-receiver-path is small, these observations can only be explained in terms of a different source directivity. The availability of a rock station on the Gubbio basin edge and other moderate-magnitude earthquakes of the same seismic sequence allows us to separate the local amplification term from the varying contribution of source directivity for the different shocks. Their combination is responsible for long-period ground displacements significantly larger than 10 cm at Mw 6.0 and 40-km source distance. Since source directivity is a very recurrent feature for normal-faulting earthquakes of the Apennines (evident during the recent L’Aquila earthquakes even at magnitudes as small as 3), these results arise a concern for many intermountain basins located in a geographical position favorable to a significant hazard increase due to source directivity. Furthermore, the performance of different Ground Motion Prediction Equations has been explored as well as of corrections based on various source directivity models.