HR: 1340h
AN: S23A-1106    [Abstracts]
TI: Observations of Infragravity Waves at the Ocean-Bottom Broadband Seismic Stations Endeavour (KEBB) and Explorer (KXBB)
AU: * Dolenc, D
EM: ddolenc@d.umn.edu
AF: Large Lakes Observatory, University of Minnesota, Duluth, MN 55812, United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720, United States
AU: McGill, P
EM: mcgill@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, United States
AU: Wilcock, W
EM: wilcock@u.washington.edu
AF: School of Oceanography, University of Washington, Seattle, WA 98195, United States
AB: The long-period background noise observed at ocean-bottom seismic stations is mainly due to deformation of the seafloor under the pressure forcing by long-period ocean surface gravity waves (infragravity waves; 0.002 to 0.05 Hz). Understanding the nature and characteristics of the coupling between the infragravity waves and the solid earth is important for the study of the infragravity wave generation and dissipation as well as for the study of the earth's hum and structure using non-seismic sources. Ocean-bottom broadband stations KEBB and KXBB were deployed as part of a three-year multidisciplinary experiment funded by the W. M. Keck foundation to monitor the linkages between seismic deformation and hydrothermal fluxes on the northern Juan de Fuca plate. The seismic component of the project was a collaboration between the University of Washington, the University of Oregon, and the Monterey Bay Aquarium Research Institute. Station KEBB was installed 247 km west of Vancouver Island (VI) at a water depth of 2376 m in August 2003. A year later station KXBB was installed 105 km offshore VI at a water depth of 2370 m. Each station comprised a Guralp CMG-1T three-component broadband seismometer, sensitive over a wide frequency range from 50 Hz to 2.8 mHz (360 sec), connected to a recording and battery package. Both seismometers were completely buried in the ocean floor sediments. The two stations recorded data continuously and stored them locally until retrieval once per year. Infragravity waves can be observed at KEBB and KXBB on stormy as well as quiet days in the period band from 30 to 400 sec. When compared to the energy of short-period ocean waves recorded at local buoys, the low- frequency seismic noise is found to be mainly generated when the short-period ocean waves reach the coast, and not when the storm passes directly above the stations. Two types of modulation of the infragravity signal are observed. First, a longer-period modulation of the infragravity signal is observed and is best correlated with the energy of the 14-16 sec period ocean waves. Second, the entire infragravity band signal is modulated in phase with the tides. The observations suggest that the process which results in the tidal modulation takes place in the nearshore region, before the infragravity waves propagate back from the shelf into the deeper water. We also present a comparison of the long-period seismic noise observed on the vertical and horizontal KEBB and KXBB seismic channels to the nearshore ocean current measurements near the surface, as well as to the ocean bottom current observations from the Endeavour Ridge region only a few kilometers from KEBB.
DE: 4546 Nearshore processes
DE: 4560 Surface waves and tides (1222)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting