Seismic Moments for Episodic Tremor near Cholame Estimated from Spectra of Ground Motion at UPSAR
Abstract
Episodic tremor is thought to be due to shear slip at depths below the seismogenic zone; that is, small seismogenic patches, within broader zones of aseismic slip, rupture to produce seismic radiation. We investigated the spectral nature of tremor radiation near Parkfield by comparing spectra of tremor recorded at UPSAR to spectra from nearby microearthquakes. For this purpose, we analyzed a tremor episode that was particularly well recorded on Jan. 21, 2005. At that time, the UPSAR array consisted of 12 working stations each with three components sampled continuously at 40 sps. Sensors were Mark products geophones with natural periods of 2 Hz. Ground displacement spectra of tremor at UPSAR show two prominent peaks, one at 2-3 Hz and the other at just over 4 Hz. In the frequency range below about 1.5 Hz, the spectra rise monotonically with decreasing frequency. At frequencies greater than 5 Hz, the tremor spectra show a high-frequency decay typical of earthquakes, decreasing as frequency squared. By comparison, a small earthquake recorded at the UPSAR array was found to have a typical source spectrum with a well-defined low-frequency plateau and the same high-frequency decay. For the tremor spectra, the amplitudes are quite low and so it appears that low-frequency noise dominates at the lowest frequencies. Accordingly, we have assumed that the spectral levels of tremor between 2 and 5 Hz approximately define low-frequency plateaus of the shear source. If so, then we can estimate tremor moments and magnitudes from these levels. Whereas the nearby earthquake had a moment magnitude of 2.4, the moment magnitudes for the well-recorded tremors are 1.7 to 1.8.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.S23A2096F
- Keywords:
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- 7209 SEISMOLOGY / Earthquake dynamics;
- 7215 SEISMOLOGY / Earthquake source observations;
- 7299 SEISMOLOGY / General or miscellaneous