Comparison of Record and Simulated Ground Motions for Tall Buildings
by Jonathan P. Stewart, Lisa M. Star and Robert W. Graves
Jonathan P. Stewart, Lisa M. Star and Robert W. Graves (2009). Comparison of Record and Simulated Ground Motions for Tall Buildings SMIP09 Seminar on Utilization of Strong-Motion Data, p. 85 - 88.
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Seismic hazard for tall buildings in California is often dominated by large magnitude earthquakes for which few recorded accelerograms are available for response history analysis. In several recent manuscripts, we compare motions for an Mw 7.8 event on the southern San Andreas fault (known as the ShakeOut event), two ShakeOut permutations with different hypocenter locations, and a Mw 7.15 Puente Hills blind thrust event beneath downtown Los Angeles, to median and dispersion predictions from the empirical NGA ground motion prediction equations. The dispersion is represented by an intra-event standard deviation term, which is lower than NGA values at low periods and abruptly increases at 1.0 sec due to different simulation procedures at low and high periods. The simulated motions attenuate faster with distance than is predicted by the NGA models for periods under approximately 5.0 sec. This suggests ground motions away from the fault rupture are under-predicted by the simulation. After removing distance attenuation bias, we have found average residuals of the simulated events (i.e., event terms) are generally within the scatter of empirical event terms, indicating that the ShakeOut event is not unusually energetic for its magnitude. The simulated motions have a depth-dependent basin response similar to the NGA models, but also show complex effects in which stronger basin response occurs when the fault rupture transmits energy into a basin at low angle. The motions also indicate rupture directivity effects that scale with the isochrone parameter.