MOISE tests new ways to 
"listen" 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 (ROV). 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 l'Univers.

Using MBARI’s 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, Ventana’s 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 one—which is stationed at the Farallon Islands northwest of San Francisco—are 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, and—with the broadband seismometer—detect 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 seismometer’s 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 Earth’s 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 noise—produced mostly by waves and currents—is 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 Coordinator