New perspectives on
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 everyones
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 arent even aware ofinformation that makes it imperative for
the regional population to stay prepared for major earthquakes that could happen at any
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
of MBARI; seismologist Gerry Simila of California State University, Northridge; and
McNally, of the Department of Earth Sciences and Institute of Tectonics,
The research involved several components. First, MBARIs 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 earthquakesinformation crucial for defining the
zones of activity and the causes of the earthquakes. Using novel instrumentation and
methods developed at MBARI, Stakess 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 episodesincluding 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 havent 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.
Similas 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 were 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.
"Whats 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 smallerperhaps 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."