Spectral Comparison of Vertical and Horizontal Seismic Strong Ground Motions in Alluvial Basins

Abstract

We analyze observations from the SMART2 array and the 1994 Northridge, California earthquake of spectral differences between vertical and horizontal strong seismic motions in alluvial basins. Our explanation is that the most energetic of such high-frequency vertical ground accelerations are generated by S-to-P seismic wave conversion within the transition zone between the underlying bedrock and the overlying sedimentary layers. The differences in combined scattering and anelastic attenuation for P and S waves lead to the observed spectral differences of the vertical motions between rock and deep alluvium sites. This model also accounts for the frequency content differences between the vertical and horizontal motions at sites in alluvial basins than at rock sites at similar distance ranges. The high-frequency cutoff of the acceleration power spectrum, fmax, is a useful comparison parameter. The results help in computing matched sets of synthetic ground motions above 2 Hz at alluvial sites.