HR: 08:20h
AN: S11C-02 INVITED     [Abstracts]
TI: Mid-Ocean Microseisms: Coastal Source Areas and Historical Wave Climate Implications
AU: * Bromirski, P D
EM: pbromirski@ucsd.edu
AF: Scripps Institution of Oceanography, Integrative Oceanography Division 9500 Gilman Drive, La Jolla, CA 92093-0209 United States
AU: Duennebier, F K
EM: fkd@hawaii.edu
AF: U. of Hawaii, Dept. of Geology & Geophysics 2525 Correa Rd., Honolulu, HI 96822 United States
AU: Stephen, R
EM: rstephen@whoi.edu
AF: Wood Hole Oceanographic Institution, 360 Woods Hole Rd (MS24), Woods Hole, MA 02543-1542 United States
AB: The Hawaii-2 Observatory (H2O), located about half way between Hawaii and California, is an excellent site for studying the origin and propagation of microseisms since it is located in the mid-ocean far from shorelines and shallow water. During the period between Dec 26, 2001 and January 24, 2002 on Leg 200 of the Ocean Drilling Program, the bridge crew of the JOIDES Resolution took environmental measurements (wind speed and direction, wave height, etc.) for comparison with the data collected by the H2O seismic system nearby. Comparison of the ship's weather log with the seismic data at frequencies from about 0.2 to 0.5 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor (4977 m below) is generated locally by ocean gravity waves at the sea surface. These signals result from the well-known double-frequency (DF) microseism wave-wave interaction mechanism. Near-shore seismic stations on the U.S. West Coast see similar short period DF (SPDF) spectra, also generated locally by wind seas. SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period, most likely associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with microseism energy at lower frequencies, 0.1- 0.2 Hz, is poor, implying that the long period DF (LPDF) signals have their origin at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of these 0.1 - 0.2 Hz signals. Most of the microseism energy at H2O between 0.08 and 0.2 Hz appears to be generated by high amplitude storm waves impacting long stretches of coastline nearly simultaneously. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines. Thus wave climate reconstructions from archived seismograms at near-coastal stations along the West Coast, e.g. during the 1940-41 strong El Ni\~{n}o winter, predominately reflect local ocean wave variability.
UR: http://www.soest.hawaii.edu/h2o/
DE: 7255 Surface waves and free oscillations
DE: 7200 SEISMOLOGY
DE: 4259 Ocean acoustics
DE: 3025 Marine seismics (0935)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting