Local Seismic Response of Sites with Pronounced Topography: Observations & Modeling

Abstract:

Many cases of pronounced amplification evident in strong ground motion recordings have been reported at sites with specific topographic features in last four decades. Although attempts have been made to explain such observations with the effect of the terrain geometry, the strong levels of observed amplification remain unexplained most of the time. However, the recent growth of seismic monitoring networks with appropriate site characterization allows for more systematic studies. We present results obtained for a number of elevated sites in Switzerland and Japan. A detailed site characterization, including measured S-wave velocity profiles down to 30-200 m, is available for these sites, as well as empirical amplification functions relative to a derived reference rock profile. On the one hand, we have not identified any general link between the geometrical features of the surface topography and the observed site response. On the other hand, the subsurface structure presents seismic velocity gradients even at outcropping rock sites. The studied rock sites (Vs30 > 800 m/s), on average, do not exhibit any systematic amplification, while the rest of the sites (Vs30 < 800 m/s) present frequency dependent amplifications with respect to reference rock condition. The observed amplification seems to be controlled in first place by the subsurface velocity structure. Finally, detailed 3D numerical modeling of seismic response was performed for a number of these sites. The joint effects of the terrain geometry and available velocity gradients are investigated. The synthetic amplification functions are compared with the empirical ones, and limits of the adopted models are discussed.