Abstract
We report on the installation of a long-term buried ocean-floor broadband seismic station (MOBB) in Monterey Bay, California (U.S.A.), 40 km off-shore, at a water depth of 1000 m. The station was installed in April 2002 using a ship and ROV, in a collaborative effort between the Monterey Bay Aquarium Research Institute (MBARI) and the Berkeley Seismological Laboratory (BSL). The station is located on the western side of the San Gregorio Fault, a major fault in the San Andreas plate boundary fault system. In addition to a 3-component CMG-1T seismometer package, the station comprises a current meter and Differential Pressure Gauge, both sampled at high-enough frequency (1 Hz) to allow the study of relations between background noise on the seis- mometers and ocean waves and currents. The proximity of several land-based broadband seismic stations of the Berkeley Digital Seismic Network allows insightful comparisons of land/ocean background seismic noise at pe- riods relevant to regional and teleseismic studies. The station is currently autonomous. Recording and battery packages are exchanged every 3 months during scheduled one day dives. Ultimately, this station will be linked to shore using continuous telemetry (cable and/or buoy) and will contribute to the earthquake notification sys- tem in Northern California. We present examples of earthquake and noise data recorded during the first 6 months of operation of MOBB. Lessons learned from these and continued recordings will help understand the nature and character of background noise in regional off-shore environments and provide a reference for the installa- tion of future off-shore temporary and permanent broadband seismic stations.
Highlights
Two-thirds of the Earth’s surface is covered by oceans, and this represents a significant challenge for the investigation of global scale dynamic processes in the Earth’s interior, as well as tectonic processes at ocean-continent boundaries
The data comparison between the two systems seemed to indicate that the ocean-floor installation was quieter at long periods (Beauduin et al, 1996), this remained controversial, as only 10 days of data were acquired in this experiment, and water circulation may have increased the noise in the borehole
One possibility is to design an «observational» transfer-function, using data from near-by land stations that do not show the ringing. This is illustrated in fig. 13, where we show a comparison of original P-wave train at MOBB and JRSC and «cleaned» MOBB data after removal of the corresponding transfer function
Summary
Two-thirds of the Earth’s surface is covered by oceans, and this represents a significant challenge for the investigation of global scale dynamic processes in the Earth’s interior, as well as tectonic processes at ocean-continent boundaries. In the summer of 1997, the international MOISE experiment (Monterey Ocean bottom International Seismic Experiment) allowed us to collect 3 months of broadband seismic data from a seafloor system installed 40 km off-shore in Monterey Bay, in a cooperative experiment between MBARI, IPG (Paris France) and UC Berkeley (e.g., Romanowicz et al, 1998; Stakes et al, 1998) During this experiment, the feasibility of performing under-water electrical and data cable connections between instruments, using an ROV operated from a ship, were successfully illustrated for the first time. Prior to the instrumentation deployment, the MBARI team manufactured and deployed a 1181 kg galvanized steel trawl-resistant bottom mount to house the recording and power systems, and installed a 53 cm diameter by 61 cm deep cylindrical PVC caisson to house the seismometer pressure vessel. Due to multiple datalogger problems (hardware and software) encountered in the first half of 2003, the best data available so far span the time period April-December 2002, as illustrated below
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