High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations

Abstract

Abstract Two borehole seismometer arrays (KNW-BH and PFO-BH) have been established in the Southern California Batholith region of the San Jacinto Fault zone by the U.S. Geological Survey. The sites are within 0.4 km of Anza network surface stations and have three-component seismometers deployed at 300 m depth, at 150 m depth, and at the surface. Downhole horizontal seismometers can be oriented to an accuracy of about 5° using regional and near-regional initial P-wave particle motions. Shear waves recorded downhole at the KNW-BH indicate that the strong alignment of initial S-wave particle motions previously observed at the (surface) KNW Anza site (KNW-AZ) is not generated in the near-surface weathered layer. The KNW-BH surface instrument, which sits atop a highly weathered zone, displays a significantly different (≈ 20°) initial S-wave polarization direction from that observed downhole and at KNW-AZ, which is bolted to an outcrop. Although downhole initial shear-wave particle motion directions are consistent with a shear-wave splitting hypothesis, observations of orthogonally polarized slow shear waves are generally elusive, even in seismograms recorded at 300 m. A cross-correlation measure of the apparent relative velocities of Sfast and Sslow horizontally polarized S waves suggests shallow shear-wave anisotropy, consistent with the observed initial S-wave particle motion direction, of 2.3 ± 1.7 per cent between 300 and 150 m and 7.5 ± 3.5 per cent between 150 and 0 m.