Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; February 1999; v. 89; no. 1; p. 215-238
© 1999 Seismological Society of America
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High-frequency attenuation in the crust and upper mantle of the northern Tien Shan

Vladislav G. Martynov, Frank L. Vernon, Robert J. Mellors and Gary L. Pavlis

The Institute of the Physics of the Earth Russian Academy of Sciences, 10 B. Gruzinskaya St., Moscow 123810, Russia
Institute of Geophysics and Planetary Physics University of California at San Diego, La Jolla, California 92093
Department of Geological Science San Diego State University, 550 Campanile Drive, San Diego, California 92182-1020
Department of Geological Sciences Indiana University, 1005 10th St., Bloomington, Indiana 47405

Abstract

We analyze 243 three-component broadband digital seismograms of aftershocks from the Ms = 7.3 Suusamyr, Kyrgyzstan, earthquake to determine seismic attenuation in the northern Tien Shan. Pg, Sg, and SmS body waves and S coda waves were used to estimate the frequency-dependent (f = 1.2-30 Hz) attenuation and scattering parameters of the crust and upper mantle as a function of depth. Using the equation Q(f) = Q0fn for multiple layers, we find that Q0 increases with depth from 76 (upper crust) to 1072 (upper mantle), and the value of n decreases from 0.99 to 0.29 over the same range. The Q coda results also demonstrated an azimuthal dependence: Q0 = 736 in the north-south direction and Q0 = 494 in the east-west direction. There is a strong 2{phi} dependence on azimuth for high frequencies (> 1.2 Hz). The depth and azimuthal dependence of the quality factor show that the Q is complicated and three dimensional. Estimates of the inhomogeneity scale a show two types of multiple scatterers (Wu and Aki, 1985): a velocity perturbation with a = 0.5-3.0 km and an impedance perturbation with a = 0.052-0.413 km. It appears that a increases with depth. The coefficient of scattering decreases from <g0>f = 0.0055 km–1 at h = 0-6 km to 0.0020 km–1 at h = 6-11 km and 0.0048 km–1 at h = 11-15 for forward scattering and <g{pi}>f = 0.0006 km–1 at depth of low crust and upper mantle for backscattering. Estimates of seismic albedo B0 show that the primary source of attenuation is intrinisc anelasticity, and the variation of Q0 and n indicates that the level of heterogeneity varies with depth, possibly due to pressure. The lateral variation of Q0 can be connected with azimuthal anisotropy in the upper mantle related to current deformation under the Tien Shan.




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V. G. Martynov, F. L. Vernon, D. L. Kilb, and S. W. Roecker
Directional Variations in Travel-Time Residuals of Teleseismic P Waves in the Crust and Mantle beneath Northern Tien Shan
Bulletin of the Seismological Society of America, April 1, 2004; 94(2): 650 - 664.
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Seismic Attenuation and Shallow Velocity Structures at Stromboli Volcano, Italy
Bulletin of the Seismological Society of America, April 1, 2002; 92(3): 1102 - 1116.



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