Velocity Change in the Zone of a Moderate Mw 5.0 Earthquake Revealed by Autocorrelations of Ambient Noise and by Event Spectra

Abstract

A moderate Mw 5.0 earthquake occurred near Mogul, Nevada (just west of Reno, Nevada), on 26 April 2008. This mainshock was surrounded by notable foreshock and aftershock sequences. Due to the long foreshock sequence, the area was well instrumented at the time of the mainshock. We investigated the foreshock and aftershock sequences for evidence of velocity changes in the structure immediate to the hypocenter and above it in the area of observed strong ground motion. Using autocorrelations of the time periods with nearly continuous foreshocks and aftershocks, we detected a nearly 1% negative change in velocity on recordings of station MOGL which was approximately over the hypocenter of the mainshock. We also observed from these recordings a shift in the spectral peaks to lower frequency following the mainshock, again indicative of a velocity decrease. Due to the different spatial sampling of the two methods, the effects could not be attributed to the same subsurface volume. However, both results are strong evidence for coseismic velocity changes accompanying an earthquake which is very much smaller than those for which previous velocity changes have been reported. We hypothesize that these changes can be observed for even smaller earthquakes, given a fortuitous placement of observing stations.