Detecting seasonal variations in seismic velocities within Los Angeles basin from correlations of ambient seismic noise

P>We analyse 3 yr of continuous seismic records from broad-band stations of the Caltech Regional Seismic Network (CI) in vicinity of the Los Angeles basin. Using correlations of ambient seismic noise, relative velocity variations in the order of 0.1 per cent can be measured between all interstation pairs. It is the first time that such an extensive study between 861 interstation pairs over such a large area has been carried out. We perform these measurements using the 'stretching' technique, assuming that one of the two waveforms is merely a stretched version of the other. Obviously this assumption is always violated and the two waveforms are generally decorrelated due to temporal changes in the Earth crust, due to different sources or simply because the cross-correlations are not fully converged. We investigate the stability of these measurements by repeating each measurement over various time-windows of equal length. On average between all interstation pairs in the Los Angeles basin a seasonal signal in the relative velocity variation is observed, with peaks and troughs during winter and summer time, respectively. Generally the observed signal decreases with increasing interstation distance and relative traveltime perturbations can only be measured up to an interstation distance of 60 km. Furthermore, the traveltime perturbations do not depend on azimuth of station pairs, suggesting that they are not related to seasonal variations of the noise sources. Performing a simple regionalization by laterally averaging measurements over a subset of stations we found the sedimentary basin showing the most consistent signal and conclude that the observed seasonality might be induced either by changes in the ground-water aquifer or thermo-elastic strain variations that persist down to a depth of 15-22 km.