The simulations provide the first quantitative approach to estimating the ground motions from this earthquake, which ruptured over 300 km of the San Andreas fault. The researchers produced ground motion "snapshots" of low-frequency displacements and velocities for an extended area covering central and northern California and for 15 selected locations in the vicinity of the earthquake, including downtown San Francisco, Oakland, the Golden Gate Bridge, Fort Bragg and San Jose. These "snapshots" give an idea of the intensity of ground motion that was experienced during the earthquake. 

The simulations were based on accurate slip models recently developed by USGS researchers. The slip models, which describe how much one side of the fault slipped in relation to the other, provided the necessary input to reproduce the ground motion generated by the 1906 earthquake and to quantitatively reconstruct the long-period displacement and velocity field experienced by central and northern California. 

The research focused on long-period motions, which are ground motions that are slower and consist of seismic waves in which there are fewer oscillations per minute. Large earthquakes release a lot of energy in the long-period range, so this range is important. At the same time, certain structures, such as high-rise buildings and suspension bridges, are considered long-period structures that are expected to suffer or to be more severely tested during large events. The researchers plan to expand the models to simulate ground motions over the entire frequency range and produce input motions and spectra appropriate for engineering design and applications. 

More information about this research is available from the MCEER Users Network, at http://civil.eng.buffalo.edu/users_ntwk/index.htm and/or http://civil.eng.buffalo.edu/engseislab/.