Seasonal variations of observed noise amplitudes at 2-18 Hz in southern California

P>We show that noise amplitudes at frequencies above 1 Hz exhibit strong seasonal variations in a broad southern California region. The results are based on 3-component seismic data recorded between 2002 and 2009 by 30 stations. Focusing on continuous 6-hr night-time segments, the seismograms are bandpass-filtered in nine frequency bands between 2 and 18 Hz. Squared amplitudes are median-filtered to reduce the influence of earthquake signals and integrated to yield half-hourly noise energy estimates. The 6-hr minimum energy values are converted back to ground velocity and used as representative daily noise level amplitudes. Notwithstanding various trends, drifts and other transient complexities, a common feature of the resulting time series in both the horizontal and vertical components are annual amplitude changes at all examined frequencies and all stations. The strength of amplitude variations shows no correlation with distance from the coast and some particularly clear seasonal changes are seen near topographic features in arid uninhabited areas. Comparison to meteorological data suggests that the main sources for the high-frequency noise field are variations of temperature and wind at the surface. In addition to acting directly on topographic irregularities and other surface features, these sources (and especially temperature changes) may also generate high-frequency noise by inducing multitudinous small-scale failures in the shallow crust.