Initial overview of the San Francisco Bay and Santa Cruz mountains ground motion

The strong-motion accelerograms from the Loma Prieta earthquake are analyzed for their long-period content in order to obtain a clearer picture of the long-period wave propogation details. Shear waves having periods in the 3.5 to 4 sec, and 5 to 7 sec ranges travel across four groups of stations with satisfactory coherency. Displacement accuracies are of the order of 0.5 cm for most of this data, with signal amplitudes an order of magnitude higher than the noise. Resonances associated with shear waves of 1.5 sec period are responsible for about 3/4 of the differential displacement necessary to unseat the 15 m section of the Bay Bridge.