Earthquake source parameters and the frequency dependence of attenuation at Coalinga, Mammoth Lakes, and the Santa Cruz Mountains, California

Abstract

Least squares best fits were calculated to the Fourier amplitude spectra of P and S waves of locally recorded earthquakes. Attenuation, amplification, and earthquake source parameters were calculated from parameters of the fits. Approximately 8200 spectra were fit at 39 sites at Coalinga, Mammoth Lakes, and the Santa Cruz Mountains, California. Typically, the spectra were fit from 2 to 45 Hz for P waves and 2 to 20 Hz for S waves. The source spectral falloff of P waves was limited to be no greater than f −2.5 ( f is frequency) by low attenuation sites in the Sierra Nevada. Attenuation was measured through t *, the travel time divided by Q . Attenuation was considered to be frequency dependent according to t * = t o * f −α , which is equivalent to Q = Q o f α . Negative values of α were found to minimize the sum of squared residuals at most of the sites studied. Attenuation and amplification varied from site to site systematically with site geology. Source parameters were determined for individual earthquakes by averaging fit parameters from between four and 18 spectra, depending on the earthquake. Seismic moments ranged from roughly 5×10 12 to 10 15 Nm at Coalinga and Mammoth Lakes. Moments ranged from 2×10 11 to 3×10 14 Nm in the Santa Cruz Mountains. When the fits were determined using negative values of a, the seismic moments trended approximately as f c −3 ( f c is the corner frequency). For α = −0.5, stress drops varied from roughly 0.2 to 50 MPa at Coalinga and Mammoth Lakes. Stress drops were lower in the Santa Cruz Mountains, ranging from approximately 0.05 to 8 MPa.