Body-wave spectra of central California earthquakes

abstract PZ and SH spectra (0.5 to 20 + Hz) of 26 events (0.9 ≦ M ≦ 4.1) in the January 15, 1973, earthquake sequence on the San Andreas fault near Bear Valley have been obtained from seismograms written at a three-component short-period ( T o = 1 sec) station (Δ = 12 to 18 km) located 4 km west of the San Andreas fault trace in the Gabilan Range. For each of the larger events ( M ≧ 3.3), the observed SH spectrum has greater high-frequency to low-frequency content than the observed PZ spectrum. For the smaller events, the observed SH and PZ spectra are similar. At high frequencies ( ƒ ⪞ 10 Hz), the observed SH -displacement spectra decrease more rapidly with frequency than do the observed PZ displacement spectra—indicating more attenuation for SH than for PZ for these propagation paths. For any reasonable propagation-path correction, SH source displacement spectra corner frequencies are systematically larger than PZ source displacement corner frequencies for these events. Ratios of spectra of large-to-small events in the sequence imply an equivalent total path Q α of 175 to 250 and an equivalent total path Q β of 100 to 150 for these paths.