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Application of the telegraph model to coda Q variations in southern California

Davis, Paul M. and Clayton, Robert W. (2007) Application of the telegraph model to coda Q variations in southern California. Journal of Geophysical Research B, 112 (B9). Art. No. B09302. ISSN 0148-0227. doi:10.1029/2006JB004542.

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We examine waveforms and data used to construct coda magnitude in southern California to estimate the spatial variation of coda Q and its dependence on frequency. Our analysis combined with independent borehole data suggests that coda is mainly generated by multiple scattering in the upper few kilometers of the crust where large impedance contrasts occur because of surface layering or fracturing. The ubiquitous observation that coda Q increases with frequency is explained as arising from multiple reverberations in the upper crust. We suggest that the telegraph model that has been successfully used to describe reflection seismogram multiples in the exploration industry may also apply to earthquakes. Under this model the apparent increase of Q with frequency is due to trapping of high-frequency energy in the upper crust. This behavior is expected if the associated reflector series has an exponential autocorrelation function, a feature of the telegraph model. At lower frequencies, trapping is less efficient. The combined effects give rise to an apparent absorption band that we suppose is superimposed on frequency-independent intrinsic attenuation. Maximum apparent attenuation occurs at wavelengths equal to the dimensions of the regions of upper crust that contain the scattering layers. At lower frequencies, trapping is less effective, and attenuation decreases as the longer-wavelength waves sample the deeper crust and upper mantle where because of overburden pressures, acoustic impedance contrasts are less extreme. By taking spectral ratios of coda waves to direct S, we estimate that intrinsic Q is high (∼3000) and that coda may be modeled as multiple scattered S waves in a region of anisotropic scattering. The exponential decay of the coda is a result of the perfectly reflecting surface of the Earth with backscattering from random near-surface layers causing progressive leakage and loss of energy downward into the more transparent lower crust and mantle.

Item Type:Article
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URLURL TypeDescription DOIArticle
Clayton, Robert W.0000-0003-3323-3508
Additional Information:© 2007 by the American Geophysical Union. Received 26 May 2006; revised 27 November 2006; accepted 3 May 2007; published 11 September 2007. This work was supported by grants from the UCLA NSF Science and Technology Center for Embedded Networked Sensing (CENS; NSF STC award CCR-0120778) and by the Southern California Earthquake Center (SCEC). SCEC is funded by NSF Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008. We thank the Southern California Earthquake Data Center (SCEDC) for supplying the coda and waveform data used in this study. This is SCEC contribution 1078. The reviewers and Associate Editor are thanked for their constructive comments. Ben Wu is thanked for help in the early part of the analysis.
Group:Seismological Laboratory
Funding AgencyGrant Number
Southern California Earthquake Center (SCEC)UNSPECIFIED
Subject Keywords:coda; Q; scattering
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Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center (SCEC)1078
Issue or Number:B9
Record Number:CaltechAUTHORS:20121001-093602285
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Official Citation:Davis, P. M., and R. W. Clayton (2007), Application of the telegraph model to coda Q variations in southern California, J. Geophys. Res., 112, B09302, doi:10.1029/2006JB004542
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:34578
Deposited By: Tony Diaz
Deposited On:01 Oct 2012 21:11
Last Modified:09 Nov 2021 23:09

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