Analysis of earthquake recordings obtained from the Seafloor Earthquake Measurement System (SEMS) instruments deployed off the coast of southern California

For more than 20 years, a program has been underway to obtain records of ea rthquake shaking on the seafloor at sires offshore of southern California, near oil platforms. The primary goal of the program. is to obtain data that can help determine if ground motions at offshore sites are significantly d ifferent than those at onshore sites; if so, caution may be necessary in us ing onshore motions as the basis for the seismic design of ail platforms, W e analyze data from eight earthquakes recorded at six offshore sites; these are the most important data recorded on these stations to date. Seven of t he earthquakes were recorded at only one offshore station; the eighth event was recorded at two sites. The earthquakes range in magnitude from 4.7 to 6.1, Because of the scarcity of multiple recordings from any one event, mos t of the analysis is based on the ratio of spectra from vertical and horizo ntal components of motion, The results clearly show that the offshore motio ns have very low vertical motions compared to those from an average onshore site, particularly at short periods. Theoretical calculations find, that t he water layer has little effect on the horizontal components of motion but that it produces a strong spectral null on the vertical component at the r esonant frequency of P waves in the water layer. The vertical-to-horizontal ratios for a few selected onshore sites underlain by relatively low shear- wave velocities are similar to the ratios from offshore sites for frequenci es less than about one-half the water layer P-wave resonant frequency, sugg esting that the shear-wave velocities beneath a site are more important tha n the water layer in determining the. character of the ground motions at lo wer frequencies.