SEISMIC ATTENUATION ABOVE 10 HZ IN SOUTHERN CALIFORNIA FROM CODA WAVES RECORDED IN THE CAJON-PASS BOREHOLE

We estimate the attenuation around Cajon Pass, southern California, fo r frequencies above 10 Hz, and we find total Q to exhibit only weak fr equency dependence, ranging from similar to 800 at 10 Hz to similar to 1500 at 100 Hz. The intrinsic attenuation is approximately twice the level of the scattering attenuation, Measurements are made using earth quake seismograms recorded at 0-3 km depth using the multiple lapse ti me window method. The results are not dependent on receiver depth and are consistent with previous estimates of Q made from direct waves rec orded at 2.5 km depth. Our Q values are therefore thought to be repres entative of the seismogenic crust, and the technique used is uncontami nated by the highly attenuating near surface, at least above similar t o 10 Hz. We also calculate Q from surface data between 1 and 10 Hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at similar to 10 Hz. Q exhibits strong frequen cy dependence below 10 Hz (proportional to f(1.8)), consistent with pr evious studies in active tectonic regions, and only weak frequency dep endence at higher frequencies (proportional to f(0.34)). This change i n behavior renders it unwise to extrapolate Q measurements outside the frequency range from which they were derived, for example, in earthqu ake source studies. Possible factors responsible for the apparent chan ge in frequency dependence of Q are considered. Further work is requir ed, however, to resolve the causes of this change.