Stability of coda wave attenuation during the Loma Prieta, California, earthquake sequence

Abstract

The Loma Prieta, California, earthquake occurred in a densely instrumented region with a history of microearthquake recording beginning more than a decade before the October 1989 mainshock. This affords an unprecedented opportunity to detect changes in seismic wave propagation in the Earth's crust associated with a major earthquake. In this study we use pairs of nearly identical earthquakes (doublets) to search for temporal changes of coda attenuation in the vicinity of the Loma Prieta earthquake. We analyze 21 earthquake doublets recorded from 1978 to 1991 that span the preseismic, coseismic, and postseismic intervals and measure the change in coda Q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 Hz. This method provides an estimate of changes in coda Q that is insensitive to other factors that influence coda amplitudes. Our observations place an upper bound of about 5% on preseismic, coseismic, and postseismic changes of coda Q in the epicentral region of the Loma Prieta earthquake. Even at this low level, the changes are neither spatially coherent nor correlated between adjacent frequency bands. The only hint of a signal is in the preseismic data where there is a possible precursory increase in coda Q of approximately 5% in the two years before the mainshock. The stability of coda Q throughout the Loma Prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda Q for other earthquakes.