HR: 1340h
AN: S13C-1070    [Abstracts]
TI: Characterization of the Late 2003 Microseismic Signal Offshore Southern California
AU: * Allmann, B P
EM: ballmann@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0225
AU: Laske, G
EM: gabi@mahi.ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0225
AB: The Scripps OBS (ocean Bottom seismometer) group that provides instruments for the national instrument pool has recently test--deployed their new passive seismic instruments. These instruments have a LC2000 data logger system, a Nanometrics Trillium 40 3--component seismometer and a Cox--Webb differential pressure gauge. The test deployment took place between November 6th, 2003 and January 8th, 2004 and included five units operating at a water depth of about 1000~m, 40~km offshore from La Jolla, CA. Data were recorded continuously, at a sampling rate of 31.25~Hz. During the deployment, we recorded numerous local, regional and teleseismic earthquakes but this study focuses on the observation of signal in the frequency band 0.01--0.5~Hz that is not related to earthquakes. We determine the dominant bands of the infragravity and microseismic noise recorded on the ocean floor and compare its amplitudes with those recorded on land on broad--band seismometers of the Trinet, Anza and GSN networks. We also compare our seismic data with oceanographic and weather data. We identify four time periods of high microseismic activity that occurred in December 2003. Two of these are associated with major storm events that exhibit a marked increase in wave height and wind speed. These events correlate well with the microseism recorded on the ocean floor but are recorded only poorly on land. The sharp onset of one of the events allows us to track its propagation. The microseism signal increases first at station CIA (Catalina Island), then one hour later at a buoy at the La Jolla Scripps Pier, and another hour later at the OBSs, which are actually closer to CIA than the pier is. The two other events are somewhat puzzling: the dominant frequency is about 1.5 times lower than for the two storms and the signals show no obvious dispersion. They are extremely well recorded on the OBSs as well as on land but do not correlate with any regional weather or wave height data. The fact that these events are also clearly seen at other stations in the Pacific suggests a powerful source and/or favorable, nondispersive propagation conditions.
DE: 7294 Instruments and techniques
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting