HR: 1340h
AN: S13C-1073    [Abstracts]
TI: Locating Sources of the Earth's "hum" using an array-based Method
AU: * Rhie, J
EM: rhie@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall UC Berkeley, Berkeley, CA 94720 United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall UC Berkeley, Berkeley, CA 94720 United States
AB: Until now, the continuous background free oscillations of the earth have been studied mainly using normal mode approaches, which do not have the temporal and spatial resolution needed to investigate the geographical distribution of the sources of excitation of this weak signal. For this, a propagating wave approach is needed, to capture the directional dependence of the long period Rayleigh wave energy that contributes to the "hum". We have developed an array-based method which utilizes two (or three) large aperture regional arrays of very long period seismometers, and combines a traditional beam-forming approach with an anti-dispersion filtering technique. This method is designed to enhance the consistent surface wave energies over the array and measure their direction of arrival. The two main arrays, equipped with STS-1 seismometers are in California (Berkeley Digital Seismic Network) and in Japan (F-net). A third array can be formed using STS-1 equipped stations in Europe, but provides weaker directional constraints. Data for intervals of time free of earthquakes of $M_w$ 5.5 or larger are either band-pass filtered between 100-400 sec, or filtered with a gaussian filtered centered at 240 sec. Dispersion corrections are applied, following the reference spherically symmetric PREM model. We analyze maximum stack amplitude as a function of back-azimuth, taking into account the distortion due to non-uniform array responses. Our results show that the Rayleigh wave background energy is generated primarily over the oceans and that the location of the sources shift seasonally, correlating with maxima in significant wave height. We perform forward modelling experiments which confirm that a uniform distribution of sources or a distribution of sources over continental areas does not fit the observed patterns. In order to further investigate the atmosphere/ocean/sea floor coupling mechanism inferred for the source of excitation of the "hum". we need to expand the available seismic time series, which is limited by rigorous selection criteria to avoid contamination by earthquake signals. For this purpose we have developed a filtering method which allows us to remove significant contributions from moderate size earthquakes ($M_w$ $<=$ 6) and further assess the space/time correlations with ocean wave data.
DE: 7255 Surface waves and free oscillations
DE: 7294 Instruments and techniques
DE: 7200 SEISMOLOGY
DE: 4504 Air/sea interactions (0312)
DE: 4560 Surface waves and tides (1255)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting