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, ML = 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.