Berkeley - Twenty-five miles off the Monterey coast and 3,000 feet down, a remotely operated vehicle, the Ventana, this week placed the first permanent broadband earthquake monitor on the California seafloor.
The instrument will help University of California, Berkeley, seismologists and Monterey Bay Aquarium Research Institute (MBARI) geologists measure earthquake activity from the ocean side of the fractured fault zone running up and down the coast, and hopefully reveal new information about seismic activity on the Pacific Plate, which slides past the North American Plate and generates periodic earthquakes.
"It's hard to look at the plate boundary in detail, and in particular at the San Andreas and San Gregorio faults, without instruments on the other side - the ocean side - of the fault zone," said UC Berkeley project leader Barbara Romanowicz, a professor of earth and planetary science and director of the Berkeley Seismological Laboratory. "Without these types of seafloor monitors, we don't have good information on the location and depth of earthquakes, and the type of faulting involved, as well as the level of seismic activity on these faults."
"We have learned from our temporary seismometer deployments that many earthquakes are either poorly located or completely overlooked on the offshore segments of these major faults" added Debra Stakes, an MBARI geologist and co-principal scientist for the project.
The oceans, in general, represent a big gap in the global network of seismic monitoring stations, limiting the ability of geophysicists to create three-dimensional pictures of the interior of the Earth.
"An off-shore component to our regional broadband network would help us understand better the deep crustal expression of the transition from the structure of the North American to the Pacific plate, and the associated earthquake hazards," Romanowicz said.
While Romanowicz is primarily interested in the structure of the deep Earth and global seismic studies, Stakes focuses on regional tectonics and the geologic processes occurring at mid-ocean ridges.
While scientists and engineers watched aboard MBARI's ship Point Lobos, the Ventana placed the state-of-the-art digital broadband seismometer on the ocean floor on Tuesday, April 9, and on Wednesday deployed nearby a separate recording system. By Thursday, the system was instructed to start collecting data by a command provided through a communications connection made by the remotely operated vehicle (ROV) on the seafloor.
This is the first of what Romanowicz hopes is a network of some 20 instruments comprising an undersea observatory along the coast where there are no islands. The Berkeley Seismological Laboratory already operates one instrument - part of The Berkeley Digital Seismic Network - on the edge of the Pacific Plate on the Farallon Islands, 30 miles west of San Francisco,
The project, MOBB (Monterey Ocean Bottom Broadband), is a follow-up on the Monterey International Seismic Experiment (MOISE), a collaboration between the Berkeley Seismological Laboratory, MBARI, the French Institut National Des Sciences De L'Univers, and the Institut de Physique du Globe of Paris. The procedures developed by this project have been emulated worldwide in other seafloor observatory efforts.
In 1997, MOISE collaborators plunked a broadband seismic monitor on the floor of Monterey Bay for three months to test the equipment. The current instrument is permanent, though not yet able to send data instantly to shore. The researchers must retrieve the recording package periodically, until MBARI scientists or others develop a system allowing real-time data retrieval, perhaps via radio from a buoy floating above the instruments, or through a cable connection.
A few isolated permanent ocean bottom stations already exist around the world, including one in the middle of the Pacific Ocean and connected to Hawaii by an old underwater phone line. There are no networks yet that provide comprehensive data within a single area.
For lack of permanent seafloor sensors, most seismologists today use temporary deployments of standard short-period ocean bottom seismometers to investigate earthquake activity on the ocean floor, dumping them overboard and retrieving them a few months later. Though this works with older narrow-band seismometers, newer and highly sensitive broadband seismometers must be placed more precisely. The broadband sensors, built at UC Berkeley, measure seismic vibrations ranging from a tenth of a second to 300 seconds, and must remain within five degrees of the vertical. Each package contains three components to measure movement vertically and in two horizontal directions.
The 1997 test showed that, as far as broadband seismometers are concerned, oceans are very noisy. Currents, wind-generated waves and surf produce vibrations that can drown out seismic vibrations.
MBARI scientists and engineers developed methods of placing the seismometers to reduce interference, such as burying them in the seafloor. Using the Ventana, they dug a hole in the bottom and placed in it a two-foot deep, 20-inch wide plastic pipe. The seismometer, a cylinder encased in titanium and standing 20 inches tall and 16.25 inches wide, was lowered into it this week and covered by glass beads to stabilize and protect it from water currents.
The vehicle placed the recording package in a trawl-resistant frame that had previously been lowered to the sea floor. The recording system is about 30 feet away from the seismometers to minimize the impact of its vibrations, and so as not to interfere with the seismometers during periodic recovery of the recording system.
The MBARI crew plans to check on the package on April 22.
The MOISE team includes UC Berkeley seismologist Robert Uhrhammer; Doug Neuhauser, computer resources manager for the Berkeley Seismological Laboratory; Steve Etchemendy, director of marine operations at MBARI; and electrical engineer Paul McGill, mechanical engineer Jon Erickson, mechanical technician John Ferreira, science technician Tony Ramirez and chief Ventana pilot Craig Dawe of MBARI.
The MOBB project is supported by the National Science Foundation and by funds to MBARI from the Lucille and David Packard Foundation.
Materials provided by University Of California - Berkeley. Note: Content may be edited for style and length.
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