BROAD-BAND SEISMOLOGY AND NOISE UNDER THE OCEAN

Most of our understanding of the Earth's interior has been derived fro m measurements from global seismic networks, although no network has e ver been truly ''global'' because some 71% of the Earth's surface is u nderwater. The resulting gaps in coverage produce a biased and incompl ete image of the Earth. Work has begun toward establishing permanent o bservatories on the deep seafloor, although the technical difficulties remain severe. Will these stations be useful, and where and how shall they be established? Data from seafloor observatories will be of poor er quality than continental site data because the sea surface is an im portant and local source of broadband noise. This noise is derived fro m wind and waves through direct forcing at long periods and by nonline ar coupling to elastic waves at short periods. Our understanding of th e generation and propagation of seismic noise and of wind and wave cli matology can be used to predict the temporal and geographical variabil ity of the noise spectrum and to assess likely sites for permanent sea floor observatories. High noise levels near 1 Hz may raise detection l imits for short-period, teleseismic arrivals above m(b) = 7.5, limitin g the usefulness of many seafloor sites. Noise levels in deep borehole s will be 10 dB quieter than those at the seafloor, but sensors buried short distances below the seafloor may also provide comparable noise levels and fidelity. The retrieval of data from permanent seafloor obs ervatories remains an unsolved problem, but longterm temporary arrays of ocean bottom seismometers are now being used in regional scale expe riments using earthquakes as sources. Such experiments are likely to b e less successful in the Pacific basin than in either the Indian Ocean or North Atlantic Ocean because of higher noise levels.