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Spectral attenuation of SH waves along the Imperial fault

INSTITUTOS DE GEOFISICA E INGENIERIA UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO, MEXICO 20, D.F. Mexico
INSTITUTE OF GEOPHYSICS AND PLANETARY PHYSICS SCRIPPS INSTITUTION OF OCEANOGRAPHY UNIVERSITY OF CALIFORNIA, SAN DIEGO, LA JOLLA, CALIFORNIA 92093
DEL MAR TECHNICAL ASSOCIATES, P.O. BOX 1083, DEL MAR, CALIFORNIA 92014

Abstract

Spectral attenuation of SH waves has been studied to infer Q along the Imperial fault. The data set consists of six aftershocks of the Imperial Valley earthquake (15 October 1979, M_L = 6.6) digitally recorded up to a distance of 51 km. Although there is large variance inQ^–1 due to scatter in the data, Q below 3.75 km appears to be a function of frequency (increasing from about 60 at 3 Hz to 500 at 25 Hz). High Q values obtained at high frequencies strongly suggest that scattering has not removed a significant amount of energy from the signals and thus, the observed result, Q varying with frequency, is not due to scattering. For sources below 4 km the observed average SH-wave spectral amplitudes, A(f, R) along the fault can be fit by

where f is frequency, R is hypocentral distance, S(f) is source factor, Q(f) is quality factor below about 3.75 km, and t is travel time up to 3.75 km below the surface. The value of t^* for the upper 3.75 km is probably between 0.027 and 0.047 (average Q between 100 and 60) depending upon the falloff of S(f) with f(f^–3 or f^–2) beyond the corner frequency.

Footnotes

^* Present address: Sierra Geophysics, 15446 Bell-Red Road, Suite 400, Redmond, Washington 98052.

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