Accession Number : ADA309655
Title : Attenuation and Source Studies in Northern Eurasia.
Descriptive Note : Semi-Annual technical rept. 9 Jan-8 Jul 92,
Corporate Author : SAINT LOUIS UNIV MO DEPT OF EARTH AND ATMOSPHERIC SCIENCES
Personal Author(s) : Mitchell, B. J. ; Xie, J. K.
PDF Url : ADA309655
Report Date : 30 DEC 1992
Pagination or Media Count : 46
Abstract : A new inversion method, in which the frequency dependence of shear wave internal friction (Q) is allowed to vary with depth, was developed and applied to selected Rayleigh wave attenuation data in the Basin and Range Province of the southwestern United States. Models were further constrained to satisfy observed values of Q and the frequency dependence of Lg waves in that region. Many models can explain those two data sets within their uncertainties, but at 1 Hz most have low values of Q (50-80) in the upper 8 km of the crust, rapidly increasing values to about 1000 at mid-crustal depths, and decreasing values at greater depths. Models for which the frequency dependence is low (0.0-0.1) in the upper crust best explain the available data. In the lower crust that frequency dependence may be substantial and leads to models in which may be an order of magnitude lower (= 100) at a period of 100 s than it is a 1 s (= 1000). The low Q values, and their constancy with frequency, in the uppermost crust can be explained by fluid flow in a network of cracks in brittle rock. Increasing Q with depth to 10-15 km is caused by the increased closing of those cracks due to pressure and their enhanced healing with increasing temperature. Viscous flow at greater depths contributes further to the dissipation of cracks and to further increases in Q. Decreasing Qp values at greater depths can be explained as the result of increasing temperature, increasing content of partial melt, enhanced dislocation motion, or some combination of these effects. An Lg coda Q map has been derived for nearly all of Eurasia. Q values range from 1000 or more in stable region to 200 or less in regions having undergone recent tectonic activity.
Descriptors : *SEISMIC DATA, *EURASIA, *TECTONICS, SHEAR PROPERTIES, DATA BASES, CRACKS, DISSIPATION, ROCK, BRITTLENESS, DISLOCATIONS, EARTH CRUST, ATTENUATION, INVERSION, VISCOUS FLOW, RAYLEIGH WAVES, ELASTIC WAVES, INTERNAL FRICTION.
Subject Categories : Seismology
Distribution Statement : APPROVED FOR PUBLIC RELEASE