HR: 1340h
AN: S13C-1071    [Abstracts]
TI: Regional and Global Variations of Microseisms
AU: * Alvizuri, C
EM: rodricels@hotmail.com
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AU: Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AB: We developed a methodology to analyze microseisms using seismic data from Southern California. Our basic approach consists of (1) selection of Rayleigh wave signals using three-component seismograms, (2) determination of the direction of propagation (excitation source), and (3) measurement of seasonal variations in the horizontal to vertical amplitudes that we refer to as the HZ ratio. We have now collected data from seismic stations in North America extensively. The total number of stations is over 200 and covers various climatic regions in North America. Based on this extensive data set, we report on comparisons of various characteristics of microseisms such as source locations and their seasonal variations (changes in source azimuth), and seasonal variations in the HZ ratios. There are some interesting differences between microseisms on the Pacific coast side and those on the Atlantic coast side. The first obvious difference is in the peak frequencies. At stations close to the Pacific ocean, the predominant frequencies of microseisms are about 0.15 Hz. At stations close to the Atlantic oceans, on the other hand, is about 0.2 Hz. While substantial energy is found over an interval of 0.1 Hz and thus there exist overlapping frequency ranges, the systematic difference of the peak frequency is unmistakable. We speculate that this feature is related to the ocean depth differences in the source regions of microseisms, meaning the microseisms are excited at shallower source region (ocean depth region) on the Atlantic side. However, there still remains a speculation. The second difference is found in the source locations. For stations in Southern California, the source locations were (surprisingly) concentrated within narrow angles; the microseism energy propagated through a station almost in the same direction throughout the year. On the Atlantic side, there seems to be large variations in azimuth, including some indications of seasonal changes in azimuth. The HZ ratios on the Atlantic ocean side display similar seasonal variations with stations on the Pacific side, meaning that they show the minimum in July and the maximum in January. We postulate that these variations are caused by changes of water-content in the vadose zone and thus expect different behaviors for different climatic regions; however, we have not found significant differences in the HZ ratios among different climatic regions yet. We will report on our comparisons between relatively ?wetŚ region and ?dryŚ region and discuss the effects on the HZ ratios.
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
DE: 7205 Continental crust (1242)
DE: 1655 Water cycles (1836)
DE: 1829 Groundwater hydrology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting