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.