The Origin and Propagation of Microseisms: ODP Leg 200 at the Hawaii-2 Observatory

The Hawaii-2 Observatory (H2O), located about half way between Hawaii and California, is an excellent site for study of the origin and propagation of microseisms, because it is far from shorelines and shallow water where most microseism energy is thought to be generated. H2O collected seismic data during Leg 200 of the Ocean Drilling program, when the Joides Resolution was drilling near the observatory between Dec 26, 2001 and January 24, 2002. During this period, the bridge crew took environmental measurements (wind speed and direction, wave height, etc.) for comparison with the microseismic data collected by the H2O seismic system, consisting of buried geophones and Guralp CMG-3 sensors, and a broad-band hydrophone. Comparison of the ship 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 sea height, implying that the energy reaching the ocean floor (4977 m below) is generated locally by ocean gravity waves at the sea surface. The energy is believed to be generated by non-linear wave-wave interaction of opposing wave trains, transmitting a non-attenuating pressure wave to the ocean floor at twice the frequency of the source gravity waves. This is the double-frequency microseism source mechanism in the deep ocean. Near-shore seismic stations on the U.S. west coast see similar spectra generated by waves at nearby shorelines. Correlation of local environmental data above H2O with microseisms at lower frequencies, 0.1- 0.2 Hz, is poor, implying that these signals have their origin at distant locations. Correlation of the H2O seismic data with NOAA buoy data and with seismic data from mainland and island stations does not clearly define source areas of these 0.1 - 0.2 Hz signals. There is some indication that only a small number of sources is involved, since the spectra from all of the seismic stations are very similar at these low frequencies.