new ways to
to earthquake noise
December 10, 1997
SAN FRANCISCO, CA In an experiment that included many technical and scientific
firsts, a team headed by researchers at the Monterey Bay Aquarium Research Institute
(MBARI) has succeeded in installing a unique combination of seismic instruments on the
seafloor which yielded detailed information over a period of more than three months. The
Monterey Bay Ocean Bottom International Seismic Experiment (MOISE) was the first-ever
deployment of instruments of this size and type by a remotely operated vehicle
MOISE represents a significant step in evaluating technology and methods for setting up
seafloor observatories around the world to continuously monitor events such as earthquakes
and landslides. And, the experiment provided the kind of crucial information from the
western side of the tectonic plate boundary off Central California that can help
scientists to accurately locate sites of seismic activity in the region.
Debra Stakes, a geologist at MBARI, led the scientific effort, which included
collaborators from the University of California at Berkeley Seismographic Station, the
Institut de Physique du Globe, and the Institut National des Sciences de
Using MBARIs ship, R/V Point Lobos, and its (ROV), Ventana, in June the team
deployed the instruments at a site about 1,015 meters (3,300 feet) deep in Monterey Bay.
The MOISE site is 40 km (24 miles) offshore and 10 km (6 miles) west of the San Gregorio
fault, which runs roughly from Point Reyes south to Big Sur. Over six days, in a series of
complicated maneuvers, Ventanas operators installed instruments weighing more than
400 pounds and connected them to data-logging computers. The equipment, capable of
registering both global and local seismic events, was left on the seafloor to record data
for more than 100 days, until the ROV recovered it in September.
Conditions were far from optimal at the MOISE site. Strong currents and the vibration
of soft sediments caused background noise in data from the instruments. Nonetheless,
Stakes said, "every instrument worked for the entire duration of the experiment. The
deployment involved ground-breaking tasks for our ROVs, but everything fell into place.
The recovery of the instruments was flawless." While a handful of experiments in
ocean-bottom monitoring have been carried out around the world, not even human-occupied
submersibles have attempted such a complex deployment, Stakes added.
The centerpiece of the MOISE instrument group was a broadband seismometer so sensitive
that it can "feel" vibrations caused by tides. Instruments similar to these,
able to measure seismic waves with frequencies from .01 to 100 hertz (cycles per second),
serve as the backbone of the global seismic network. About 20 broadband seismometers are
situated around Northern California. All but onewhich is stationed at the Farallon
Islands northwest of San Franciscoare at land-based monitoring stations on the east
side of the boundary zone between the Pacific and North American tectonic plates. In
contrast, the MOISE site lies on the west side of the plate boundary zone. Thus, the
three-month record of seismic signals recovered from the experimental site complements
data over the same period from land-based monitoring stations. Such information from the
previously unmonitored seafloor will allow scientists to more accurately pinpoint geologic
events on the offshore faults that slice across Monterey Bay and comprise the westernmost
branches of the San Andreas System. It may also assist researchers in interpreting the
apparent northward shift in the frequency of earthquakes offshore of Central California
over the past few decades and unraveling the geologic history associated with the movement
of the triple plate junction that currently lies off Northern California.
Several large, distant earthquakes were recorded on the MOISE instruments, as well as
several moderate regional ones. "This was truly an experiment that has local,
regional, and global implications," Stakes noted. "The suite of instruments we
selected gave us the ability to effectively monitor local earthquakes, answer questions
about regional plate processes, andwith the broadband seismometerdetect
earthquakes on the far side of the globe whose seismic waves give us information about the
internal structure of the planet as they pass through Earth."
Because of the broadband seismometers sensitivity, other instruments measuring
seawater pressure and local currents were moored on the seafloor nearby to measure tidal
effects, which were subtracted from the gross seismic signals to obtain more accurate
information. A magnetometer was also installed at the MOISE site to measure local changes
in Earths magnetic field in correlation with seismic signals. Some studies have
suggested that electromagnetic signals occur in association with seismic activity, perhaps
due to the movement of water through underlying rocks.
Securing the seismic instruments at the seafloor so as to minimize background
noiseproduced mostly by waves and currentsis perhaps the biggest challenge in
setting up such equipment. In the effort to firmly entrench the MOISE seismometer in the
ocean floor, ROV operators excavated a hole in the seafloor sediments, sunk a caisson into
the cavity, and inserted the seismometer so that it was buried halfway up its length. In
previous experiments MBARI researchers have used Ventana to place seismic instruments into
small-diameter coreholes drilled in the granite walls of Monterey Canyon. Comparisons of
the usefulness of the data derived from the new, ROV-deployed instrument systems with
conventional ocean-bottom seismometers will assist seismologists and engineers in
perfecting seismic monitoring technology.
Contact: Debbie Meyer, Communications