Amplification of Ground Motion Rotations by Local Geology

There is no doubt that local geology can modify seismic ground motion to a great extent. During the last three decades important advances have been achieved on the understanding of site effects and its quantitative evaluation. The pioneering analytical solutions by Trifunac (1971, 1973) for semi cylindrical alluvial basins and canyons under antiplane SH waves mark the start of a series of developments. The bulk of the contributions dealt with ground motion amplification but little was done to characterize seismic rotational motion. The record of spatial gradients of motion has been scarce. The use of dense arrays is partially filling the gap but the lack of extensive measurements can be mitigated using mathematical models as the mentioned above. The evaluation of rotational ground motion relies essentially on the computation of spatial gradients and this requires improved numerical approaches. In recent years the Method of Fundamental Solutions (MFS) has been found to be a powerful approach to solve wave propagation problems. In this work we examine the relationship between the MFS and the indirect BEM and use them to compute the rotational response at various local geology features. Results are displayed as rotational spectra which, used together the usual response spectra, may provide useful, thrustworty descriptions of seismic ground shaking for engineering purposes.