Synthesis of regional ground motion from western US earthquakes

Synthetic displacement, velocity and acceleration seismograms and the resulting response spectra are computed and analyzed for earthquake ground motions in the western United States. A source model for the 1975 Pocatello Valley earthquake is used with plane layered earth models to estimate the ground motions at Minuteman Wing V which is at a range of about 700 km. The 1975 Yellowstone earthquake was a similar event at the same range from Wing V. However, the ground motions are estimated to be an order to magnitude smaller due to three effects: the two earthquakes were the same size at long periods but the Yellowstone event appears to be a factor of 2 to 4 smaller at short periods; the sources are oriented such that radiation pattern differences between the two events are nearly maximum; the Yellowstone caldera and, perhaps, other highly attenuating structures to the south cause the Wing V ground motions from the Yellowstone event to be much smaller than otherwise expected.