A coupled seismic-geotechnical approach to site-specific strong motion

At Lawrence Livermore National Laboratory (LLNL) we have developed a new approach to provide estimates of incident strong ground-motion time-histories at bedrock, which are specific to a site. The estimates involve the recording of small earthquakes at the site, both at bedrock and at the surface, from specific faults in the region. The method also involves the definition of rupture scenarios for those faults. The estimates are obtained for earthquakes of specified moment magnitude (energy release). The new approach is a coupled seismic-geotechnical method. Besides its advantage of giving site-specific time-histories, this approach has the added benefit of providing an optimized definition of the dynamic geotechnical site properties which will be used to calculate shear wave propagation in the soils, from the estimated bedrock motions. This method is intended to replace state-ofpractice approaches in which the site motion estimation may involve data from distant locations and faults having nothing to do with the specific site. The procedure is demonstrated at the Painter Street bridge site in Rio Dell, CA, for which we provide a range of surface motions corresponding to an earthquake of magnitude 7 on the subducting plate underlying this region. These calculated motions bracket the records of the Petrolia event ( M = 7) measured near the site.