Seismic attenuation above 10 Hz in southern California from coda waves recorded in the Cajon Pass borehole

Abstract

We estimate the attenuation around Cajon Pass, southern California, for frequencies above 10 Hz, and we find total Q to exhibit only weak frequency dependence, ranging from ∼800 at 10 Hz to ∼1500 at 100 Hz. The intrinsic attenuation is approximately twice the level of the scattering attenuation. Measurements are made using earthquake seismograms recorded at 0–3 km depth using the multiple lapse time window method. The results are not dependent on receiver depth and are consistent with previous estimates of Q made from direct waves recorded at 2.5 km depth. Our Q values are therefore thought to be representative of the seismogenic crust, and the technique used is uncontaminated by the highly attenuating near surface, at least above ∼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 ∼10 Hz. Q exhibits strong frequency dependence below 10 Hz ( ∝ƒ1.8), consistent with previous studies in active tectonic regions, and only weak frequency dependence at higher frequencies ( ∝ƒ0.34). This change in behavior renders it unwise to extrapolate Q measurements outside the frequency range from which they were derived, for example, in earthquake‐source studies. Possible factors responsible for the apparent change in frequency dependence of Q are considered. Further work is required, however, to resolve the causes of this change.