HR: 1340h
AN: S23A-1104    [Abstracts]
TI: Mechanism of Excitation and Propagation of the Longest-period Microseisms on Oceanic structures.
AU: * Lee, W
EM: udongi@chonbuk.ac.kr
AF: Dept. of Earth and Environmental Science, Chonbuk National University, Deokjin-dong, Chonju, 561756, Korea, Republic of
AU: Gurung, G
EM: moostang@chonbuk.ac.kr
AF: Dept. of Earth and Environmental Science, Chonbuk National University, Deokjin-dong, Chonju, 561756, Korea, Republic of
AU: Schwab, F
EM: schwab@igpp.ucla.edu
AF: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, LA, CA 90095-1567, United States
AU: Jo, B
EM: bgjo@chonbuk.ac.kr
AF: Dept. of Earth and Environmental Science, Chonbuk National University, Deokjin-dong, Chonju, 561756, Korea, Republic of
AB: Microseisms are generated by storms at sea. We have recently shown that in the period range 4-20 seconds, the explicit mechanism of excitation is the associated cloud-to- "ground" (CG) lightning strikes on the sea surface. The mechanism of propagation is high-order P-SV (Rayleigh) modes whose principal energy is in the mid- to lower-earth mantle. The greatest microseism amplitudes are those at the longest periods---up to at least 2560 seconds---and these were not in evidence from our initial computations with an earth model lacking a core. We have therefore extended the earlier work to treat a complete oceanic structure: a standard oceanic model down to 1115 km of depth, with the recent full-earth model AK135F used from this depth down to the center of the earth. The addition of the core completely changes the modal characteristics at all periods, but the change is especially striking at long periods. Adding just the (liquid) outer section of the core introduces the mantle-(outer) core interface wave/mode that (1) is composed of all usual P-SV modes, and (2) concentrates the wave's/mode's energy more and more dramatically in the region of the interface as period decreases. Then, adding the (solid) inner section of the core introduces a whole new suite of inner-core modes. In our spectral range of current interest---periods in excess of 100 seconds---the complicated interaction of the "usual" P-SV modes, the mantle- (outer) core interface wave/mode, and the suite of inner-core modes is demonstrated with the multimode phase- velocity, group-velocity, and energy-density-integral spectra. Using these spectral results, the quantitative evaluation of CG lightning strikes from storms at sea, as the mechanism for explaining the longest-period microseismic excitation, is then presented graphically: as a function of CG lightning strikes per day, the upper envelope of the computed surface excitation (Fourier-amplitude) spectra, from this full, modal representation is compared with the experimental maximum and minimum amplitude measurements.
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory
DE: 7290 Computational seismology
SC: Seismology [S]
MN: 2007 Fall Meeting