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New perspectives on 
Monterey Bay seismicity

December 7, 1998

SAN FRANCISCO, CA —Scientists measuring seismic activity in Monterey Bay and reanalyzing seismic events from the past 72 years have uncovered evidence that all recent major earthquake activity in the bay occurs along the San Gregorio and Monterey Bay faults, which fracture the seafloor off Central California. The detailed investigations, led by geologist Debra Stakes of the Monterey Bay Aquarium Research Institute (MBARI), have enabled the researchers to hone in on the mechanisms by which the earthquakes occur and their relationship to local geological structures. For residents of the Central California coast the findings give new emphasis to a longstanding warning.

"The 1989 Loma Prieta earthquake on the San Andreas is still on everyone’s mind," said seismologist and collaborator on the studies, Karen McNally. "But now, thanks to new technology, we have information about activity on these other faults that many people aren’t even aware of—information that makes it imperative for the regional population to stay prepared for major earthquakes that could happen at any time."

The studies were conducted as part of the MBARI Margin Seismology System Deployment Project. Collaborators with Stakes included postdoctoral researcher Michael Begnaud and University of California, Santa Cruz (UCSC), masters degree candidate Vicky Gallardo, also of MBARI; seismologist Gerry Simila of California State University, Northridge; and McNally, of the Department of Earth Sciences and Institute of Tectonics, UCSC.

The research involved several components. First, MBARI’s R/V Point Lobos and remotely operated vehicle (ROV) Ventana were employed to set up seafloor stations in Monterey Bay for recording seismic waves from events during 1997 and 1998. Historically the absence of seafloor stations has hampered efforts to pinpoint the locations and depths of offshore earthquakes—information crucial for defining the zones of activity and the causes of the earthquakes. Using novel instrumentation and methods developed at MBARI, Stakes’s team set up an array spanning five key locations on the ocean bottom. To minimize disturbance to the instruments and optimize the quality of the data recorded, the instruments were secured either in small-diameter coreholes drilled in the hard rock, or mounted to low-profile bases and placed on the sediments.

"This array of the most sophisticated instruments available provided information in unprecedented detail," Stakes said. "With stations spread around the bay we were able to precisely locate not only the larger events but also the much more frequent microseismic episodes—including many that were not even recorded by the permanent U.S. Geological Survey seismic network. We learn a great deal about earthquake mechanisms from these small events." A number of seismic instruments were also loaned to the University of California, Santa Cruz, by the Incorporated Research Institutes for Seismology, a consortium that supports the global seismic network. To complement the seafloor stations, McNally selected nine coastal sites for setting up these state-of-the-art seismic instruments during 1998.

Armed with reliable data on a small number of events, researcher Michael Begnaud began to compile a model of the structure of the subseafloor rock layers based on recorded travel speeds for the seismic waves. "We needed to determine a new velocity model for earthquake location on the San Gregorio and Monterey Bay fault zones. In the past, earthquakes in this area have been located with land-based models, which are not appropriate or accurate for offshore events, and so the locations haven’t been reliable. And if you have inaccurate locations, you get inaccurate source mechanisms," he explained.

Begnaud was able to ground the model on discoveries about Monterey Canyon geology that Stakes and other MBARI scientists have made over the past several years from studies on rock samples collected with the ROV Ventana. As additional seismic data became available from the seafloor and coastal stations during 1997 and 1998, Begnaud integrated it into the model, continually refining it.

The final step of the studies expanded the findings to a historic perspective. Gerry Simila used the results of the new velocity model, in combination with data on master (most accurately located) events captured by the offshore and onshore arrays and past seismic data from the University of California, Berkeley, and California Institute of Technology, to "relocate" the largest earthquakes in the bay since 1926. Simila’s analysis showed that all the episodes examined (ranging in magnitude from 4-6.1) occurred along either the San Gregorio or Monterey Bay fault zones.

"We suspected a high degree of uncertainty in the past estimates of event locations, so we’re not surprised to see that this analysis has shifted some locations many kilometers," Simila commented. In the case of two closely spaced, major earthquakes in 1926, the relocations moved the events from an area of no known faults to one in each of the known fault zones.

"What’s really intriguing is the revelation of recent activity, including some of the largest events, within the Monterey Bay Fault Zone, which crosscuts the Monterey Peninsula. Previously only the San Gregorio was thought to be active," Stakes noted. "This analysis also shows that although much activity occurred on the northern San Gregorio west of Santa Cruz, there were no historic events along the southern section in Monterey Bay. One could conclude that the southern half of the San Gregorio fault has been ‘locked’ in recent time, and stress on it may be building."

"This was the situation on the San Andreas system, where there were no earthquakes for the 50 years preceding the 1906 event," McNally added. "However, a future event expected on the San Gregorio would be smaller—perhaps 7.3 in magnitude." The California Division of Geology and Mines recently upgraded the San Gregorio to a "Class A" fault, indicating its potential for a major earthquake.

The results of the studies also have shed light on the nature of the earthquake mechanisms for each of the fault zones. "From initial runs of the new velocity model, it appears that motion along the Monterey Bay faults occurs as strike-slip movement on a vertical fault plane," McNally explained. "In contrast, activity on the northern San Gregorio appears to be a combination of thrust faulting and strike-slip movement; that is, the fault plane dips to the east and thrusts under the continental margin." Knowing the types of motions occurring along the faults will help the scientists to understand the magnitudes of the forces at work beneath the seafloor and to reconstruct past geological history.

"MBARI undertook this effort to develop seismic instrumentation for extended seafloor deployment and chose sites in Monterey Bay because of the pressing need for advancing our understanding of seismic-tectonic activity there," Stakes said. "We hope the success of this collaboration demonstrates the value of seafloor instrumentation and serves to encourage continuing development of this much-needed technology, both offshore of California and, ultimately, to more remote locations."

Contact: Debbie Meyer, Communications Coordinator