HR: 10:50h
AN: S12B-03    [Abstracts]
TI: Seasonal Variations in Particle Motion of Microseisms and monitoring of water content at shallow depths
AU: * Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AB: There is a long history of study on microseisms, historically regarded as an annoying seismic noise for frequencies between about 0.05 and 0.3 Hz. With the emergence of dense, broadband seismic networks in the world, however, there is clearly an opportunity to carefully analyze microseisms and learn about their sources and the crustal structure in which they travel. The main point of this paper is our discovery of seasonal variations in particle motion of microseisms. We report this result from our analysis of seismic data from California Integrated Seismic Network (CISN) that has more than 150 broadband stations. But we also show some examples from other networks that seem to possess similar characteristics. It was pointed out more than 40 years ago that microseisms consist of Rayleigh waves (e.g. Haubrich et al., 1963). We first confirmed this feature by checking the phase shift between the maximum horizontal motion and the vertical motion. Indeed, we can identify Rayleigh-wave signal by performing this analysis. There exist some other type of energy in microseisms but, in this paper, we focus on the Rayleigh-wave type energy for further analyses. Once we identify the Rayleigh wave signals in microseisms, we can easily determine the direction(s) of energy propagation at a given station. We confirmed that the sources, determined at each station, point toward coasts, which was certainly expected and not surprising. We then discovered, to our surprise, that the ratio of the horizontal amplitude to the vertical (hereafter referred as HZ-ratio) displays seasonal variations. Particle motion is relatively flat in winter (northern hemisphere) and becomes closer to a circle in July. This feature is found at basically all stations with good signal-to-noise ratio that we analyzed. The manner they change with season, especially their frequency dependence, differ from one station to another. This feature is NOT related to the changes in sources of excitation because the HZ-ratio of Rayleigh waves, as it is the ratio for the horizontal and vertical amplitude of the eigenfunction of a local seismic structure, should not change with variations of excitation sources. We argue that this is caused by the changes in water content below each seismic station, especially through changes in groundwater level and the water content in the vadose zone. We have developed theoretical modeling technique and confirmed that it is possible to match data and theory using reasonable numbers for porosity for shallow crust. Apparently, variations of water content at shallow depths, typically within 10-50 m from the surface, amount to changing of the surface boundary conditions for Rayleigh waves. If this model is correct, this feature in microseisms can be used to monitor water content in the shallow crust.
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
DE: 7205 Continental crust (1242)
DE: 1829 Groundwater hydrology
DE: 1655 Water cycles (1836)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting