Moments, magnitudes, and radiated energies of non-volcanic tremor near Cholame, CA, from ground motion spectra at UPSAR
Geophysical Research Letters
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Abstract
By averaging the spectra of events within two episodes of tremor (on Jan. 21 and 24, 2005) across the 12 stations of UPSAR, we improved the S/N sufficiently to define source spectra. Analysis of eleven impulsive events revealed attenuation-corrected spectra of displacement similar to those of earthquakes, with a low-frequency plateau, a corner frequency, and a high frequency decay proportional to f−2. Seismic moments, M0, estimated from these spectra range from about 3 to 10 × 1011 N-m or moment magnitudes in the range 1.6 to 1.9. The corner frequencies range from 2.6 to 7.2 Hz and, if interpreted in the same way as for earthquakes, indicate low stress drops that vary from 0.001 to 0.04 MPa. Seismic energies, estimated from the ground motion spectra, vary from 0.2 × 105 to 4.4 × 105 J, or apparent stresses in the range 0.002 to 0.02 MPa. The low stress parameters are consistent with a weak fault zone in the lower crust at the depth of tremor. In contrast, the same analysis on a micro-earthquake, located near Cholame (depth = 10.3 km), revealed a stress drop of 0.5 MPa and an apparent stress of 0.02 MPa. Residual spectra from ω−2 model fits to the displacement spectra of the non-volcanic tremor events show peaks near 4 Hz that are not apparent in the spectra for the microearthquake nor for the spectrum of earth noise. These spectral peaks may indicate that tremor entails more than shear failure reminiscent of mechanisms, possibly entailing fluid flow, associated with volcanic tremor or deep volcanic earthquakes.
Suggested Citation
Fletcher, J.B., and McGarr, A., 2011, Moments, magnitudes, and radiated energies of non-volcanic tremor near Cholame, CA, from ground motion spectra at UPSAR: Geophysical Research Letters, v. 38, no. 16, https://doi.org/10.1029/2011GL048636.
| Publication type | Article |
|---|---|
| Publication Subtype | Journal Article |
| Title | Moments, magnitudes, and radiated energies of non-volcanic tremor near Cholame, CA, from ground motion spectra at UPSAR |
| Series title | Geophysical Research Letters |
| DOI | 10.1029/2011GL048636 |
| Volume | 38 |
| Issue | 16 |
| Publication Date | August 24, 2011 |
| Year Published | 2011 |
| Language | English |
| Publisher | American Geophysical Union |
| Publisher location | Washington, D.C. |
| Larger Work Type | Article |
| Larger Work Subtype | Journal Article |
| Larger Work Title | Geophysical Research Letters |