HR: 0800h
AN: S31A-0280    [Abstracts]
TI: Excitation and Transmission of Microseisms Over Oceanic Paths
AU: * Lee, W
EM: udongi@chonbuk.ac.kr
AF: Chonbuk National University, 664-14 Deokjin-Dong, Chonju, 561-756 Korea, Republic of
AU: Schwab, F A
EM: schwab@igpp.ucla.edu
AF: University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1567 United States
AB: As originally defined, microseisms were the seismic background noise of period 4-10 seconds that limited the usefulness of recorded seismograms. Microseisms are generated by storms at sea and affect areas of continental dimensions. Pressure fluctuations are transmitted to the bottom of the ocean, where for a realistic model of the oceanic structure, we model this excitation with a point-source, vertical singlet; thus generating P-SV modes propagating outward from the source. In the period range of interest: (1) close to the source---within a couple of 100 km---shallow penetrating modes 1, 2, and 3 dominate the theoretically computed seismograms; (2) introduction of appropriate Q values in the oceanic sediments, causes these shallow penetrating modes to lose dominance at greater distances, in favor of more deeply penetrating higher modes; (3) then including also the appropriate Q values in the oceanic low-velocity zone in the upper mantle, at these greater distances we lose those of these higher modes that have relatively large displacements in the low-velocity zone. We are left with the dominant higher modes that must explain excitation and transmission of 4-10 second microseisms over oceanic paths. Inspection of the eigenfunctions, and depth dependence of energy density E(z) of these modes, show them to be mode numbers from 18 up, with E(z) very roughly constant from the sea surface to the 450-km discontinuity, and having a significant increase to another roughly constant value between the 450- and 670-km discontinuities. The principle microseisms at around 5 seconds penetrate to about 700 km with most of the energy between the two discontinuities; the secondary microseisms at around 10 seconds have another strong increase in E(z) at about 700 km and penetrate to as much as 1400 km.
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: Fall Meeting 2005