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Estimating Bodywave Arrivals and Attenuation from Seismic Noise
CALIFORNIA UNIV SAN DIEGO LA JOLLA
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This paper investigates the utility of computing Time-Domain Greens Functions TDGF to be used for estimating velocity and attenuation structure for the purposes of nuclear explosion monitoring over local and near-regional distances. We have focused on two topics Earths background vibrations at frequencies below about 0.5 Hz have been attributed to ocean-wave energy coupling into the ground and propagating as surface waves and P waves compressional waves deep within the Earth. However, the origin and nature of seismic noise on land at frequencies around 1 Hz has not yet been well studied. Using array beamforming, we analyze the seismic noise fields at two remote sites Parkfield and Mojave Deserts in California, for durations of one and six months respectively. We find that 1 the seismic background noise at about 0.6-2 Hz consists of a significant amount of continuous P waves originating offshore, and 2 the power of the f-wave noise is highly correlated with the offshore wind speed, demonstrating that these high-frequency P waves are excited by distant ocean winds. We present a methodology to obtain frequency-dependent relative site amplification factors using ambient seismic noise. We treat a seismic network or array as a forced damped harmonic oscillator system where each station responds to a forcing function obtained from frequency-wavenumber beams of the ambient noise field. Taken over long time periods, each station responds to the forcing function showing a frequency-dependent resonance peak whose amplitude and spectral width depends upon the elastic and an elastic properties of the underlying medium. Our results are encouraging in that hard rock sites generally show narrower resonance peaks with reduced amplitudes relative to soft rock sites in sedimentary basins. There is also a tendency for spectral peaks to shift to higher frequencies and become more asymmetric as the site amplification increases.
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