NEWS RELEASE, 6/19/97
Robot places UC Berkeley/French seismometer on ocean floor to "listen" to earthquake noise
Berkeley -- An international team of scientists and technicians working on the Monterey Bay Ocean Bottom International Seismic Experiment (MOISE) have succeeded in installing a unique combination of scientific instruments on the seafloor.
The first-ever deployment of instruments of this size and type by a remotely operated vehicle, MOISE is part of a global effort to eventually set up seafloor observatories for continuous monitoring of geological activity such as earthquakes.
"The long term goal is to put these packages all over the ocean floor, to obtain a better understanding of global seismic events and seismic activity at plate boundaries like that off the California coast," says Barbara Romanowicz, professor of geology and geophysics at UC Berkeley and director of the campus's Seismographic Station.
Romanowicz originally conceived the idea of placing seismometers on the ocean floor more than a decade ago while head of a French government project to study the Earth's interior. Her former colleagues there are now her collaborators on the MOISE project, from the Institut de Physique du Globe and Division Technique of the Institut National des Sciences de l'Univers.
"If we are ever going to realize the goal of establishing observatories to provide continuous access to the remote ocean floor, then we must learn to deploy these instruments from ROVs," says Debra Stakes, geologist and chief scientist from the Monterey Bay Aquarium Research Institute.
Using the Monterey institute's research vessel R/V Point Lobos and its tethered remotely operated vehicle, Ventana, the team deployed the instruments at a site about 1,015 meters deep in Monterey Bay, 40 km offshore and 10 km west of the San Gregorio fault, which runs from south of Point Reyes to Big Sur. The equipment, which can register both global and local seismic events, will be left at the site to record data until it is recovered in September by Ventana. The centerpiece of the MOISE instrument group, a broadband seismometer similar to ones that serve as the backbone for the global seismic network, was made available by the French collaborators.
About 20 broadband seismometers, which can register sound waves with periods from one-tenth of a second to 100 seconds, are located around Northern California. All but one -- an instrument in the Farallon Islands -- are at land-based seismic stations on the eastern side of the boundary zone between the Pacific and North American tectonic plates.
A record of seismic signals from the MOISE site, which lies on the western side of the plate boundary zone, will complement data from existing earthquake monitoring stations, allowing scientists to determine more accurately the locations of seismic events offshore of Central California.
The MOISE instrument data will add to the Monterey Bay Aquarium Research Institute's ongoing study of offshore faults that slice across Monterey Bay and comprise the westernmost branches of the San Andreas System. To monitor activity at faults such as San Gregorio, institute researchers are using conventional ocean-bottom seismometers as well as new instruments developed at the institute for deployment by remotely operated vehicles. These latter instruments are installed in small-diameter holes excavated by geological drill in the granite rock of Monterey Canyon.
The scientific goal of all this effort is to better locate the large and small earthquakes that occur off the Central California coast and to compare the usefulness of the data derived from each type of instrument. The resulting new technology could be used anywhere in the world.
To assure that the MOISE seismometer, which measures 38 cm by 76 cm, is firmly entrenched in the ocean floor, Monterey institute technicians used Ventana to excavate a hole and sink a caisson into it. The seismometer was then lowered into the caisson, so that it is buried about halfway up its length.
"A lot of the data we're going to collect down there is seismic background noise, produced mostly by waves and currents, and good coupling with the ground is a challenge, especially where you have sediments that vibrate at different frequencies," Romanowicz says. Without adequate coupling of the instrument to the ground, she says, "you get a blurry picture of the seismic activity, like looking through grainy glass."
Because the seismometer is so sensitive that it can register earth vibrations caused by tides, additional instruments measuring seawater pressure and local currents have been moored on the seafloor nearby to measure tidal effects, which will be "subtracted" from the gross seismic signals to obtain more accurate information.
A magnetometer provided by the French Laboratoire de Geophysique at Universite de Bretagne Occidentale was also installed at the MOISE site to measure local changes in Earth's magnetic field in correlation with seismic signals. Some studies have suggested that there are electromagnetic signals associated with seismic activity, due perhaps to the movement of water through underlying rocks.
In addition to the scientific objectives, MOISE adds up to a series of accomplishments for the Monterey Bay Aquarium Research Institute.
"This experiment represents many firsts for Ventana," says senior pilot T.Craig Dawe. "We've deployed instruments before, but nothing of this size and weight -- more than 400 pounds."
Battling strong bottom currents to install the instruments and make complex connections between them underwater also involved ground-breaking tasks for Ventana, which has logged more than 1,200 successful underwater missions since 1988. While a handful of experiments in ocean-bottom monitoring have been carried out around the world, not even manned submersibles have attempted such a complex deployment.
"Setting up MOISE took two-and-a-half years of planning and six ship-days at sea," notes Stakes. "Now we'll have to be patient and wait for the results."
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