HR: 09:15h
AN: S11D-06 INVITED    [Abstracts]
TI: Five Years of Data at the Monterey Ocean Bottom Broadband Seismic Station (MOBB)
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: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039, United States
AU: Neuhauser, D
EM: doug@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720, United States
AU: Uhrhammer, R
EM: bob@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720, United States
AB: We present an overview of the results obtained at MOBB in the past 5.5 years of its continuous operation. In particular we focus on the observations of the long-period ocean surface gravity waves (infragravity waves; 0.002 to 0.05 Hz) and different methods to remove the long-period background and signal-generated noise from the seismic observations. MOBB was installed 40 km offshore in the Monterey Bay at a water depth of 1000 m in April 2002 in collaboration between Berkeley Seismological Laboratory and Monterey Bay Aquarium Research Institute (MBARI). It is located west of the San Gregorio Fault and represents the first step towards extending the onshore broadband seismic network in northern California westward of the Pacific-North America plate boundary. MOBB comprises a three- component broadband seismometer Guralp CMG-1T, sensitive over a wide frequency range, from 50 Hz to 2.8 mHz (360 s), a water current meter measuring current speed and direction, and a differential pressure gauge. At present, the station is autonomous and the data are on average retrieved every 4 months using MBARI's remotely operated vehicle Ventana. Work is under way to connect it to the MARS (Monterey Accelerated Research System) cable so that it will contribute continuous real time data to the northern California earthquake monitoring system. Lessons learned from the MOBB deployment as well as noise removal techniques that are specific to the ocean bottom installation will provide us reference for future installations of broadband seismic stations in the oceans. When compared to the quiet land stations, ocean bottom seismic station MOBB shows increased background noise in the band pass of interest for the study of regional and teleseismic signals. This is mainly due to deformation of the seafloor under the pressure forcing by infragravity waves. Also observed is additional signal- generated noise which is due to the reverberations in the shallow sedimentary layers as well as in the water layer. We present results of removing the long-period background noise from the seismic observations by subtracting the coherent signals derived from the pressure measurements. We also present results of the modeling of the signal-generated noise in the near surface layers and examples of removing the signal- generated noise through deconvolution. Comparison of the energy of the long-period noise (> 20 s) at MOBB to the energy of the short-period ocean waves recorded at the local buoys showed that the infragravity waves are generated locally, when the short-period ocean waves reach the shore. We have observed two types of modulation of the infragravity signal. First, the entire infragravity band signal is modulated in-phase with tides. Second, a longer-period modulation of the infragravity signal is observed and is best correlated with the energy of the 14 s period ocean waves. Understanding of the relation between the observed infragravity signal and local and distant sea conditions may contribute to unraveling the coupling mechanism responsible for the earth's low frequency hum.
DE: 7219 Seismic monitoring and test-ban treaty verification
DE: 7220 Oceanic crust
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting