HR: 1330h
AN: S52D-0160    [PDF]
TI: Nature of Microseism and its Use for Shallow Structure Study
AU: * Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AB: Microseism is a well known ambient seismic noise that peaks around 0.14 Hz and 0.07 Hz. It has been studied for over half a century (e.g., Gutenberg, 1936; Longuet-Higgins, 1950; Hasselman, 1963; Haubrich, 1967) but there are many features that are still unclear, including the locations of excitation. With emergence of dense, broadband seismic networks as well as availability of ocean wave data, there exist some opportunities to make progress in our understanding of microseism and its potential use for shallow seismic structure. In this talk, we will focus on two features of observation that we have confirmed so far, using microseism observation by a network in Southern California (TriNet) and the buoy data off the coast of Southern California (NDBC). The first point will be on the correlation between ocean wave behaviors, observed by buoys and its relation to microseismic activity. Examination of both types of data over three years shows that correlation is very clear; an interesting aspect of correlation is that there is better correlation between microseisms at, for example, Pasadena and buoy data in the outer Pacific ocean. Ocean wave behaviors just off the coast of Los Angeles, such as those in the Santa Monica basin, do not correlate well. Excitation of microseism seems to be mostly occurring along the outer ocean of the Pacific coast. The second observation is related to particle motion of microseisms which are mostly Rayleigh waves. The ratio of the horizontal to the vertical amplitudes indicate clear seasonal variations that reach its maximum in February (winter) and its minimum in August (summer). Sensitivity of this ratio (horizontal amp./vertical amp.) with respect to seismic parameters inside the Earth show that it is sensitive to seismic structure in the upper 10km, especially in the top-most 1 km. The result suggests an existence of seasonal variations in seismic velocity (as well as density change to some extent) at shallow depths and points to a possibility that temporal changes of shallow seismic structure can be monitored by analyzing microseisms.
DE: 7200 SEISMOLOGY
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2003 Fall Meeting