Geophysicists map youngest section of San Andreas Fault

12/12/95

CONTACT: Stanford University News Service (650) 723-2558
COMMENT: Bruce Beaudoin, Geophysics, (415) 723-4417
Simon Klemperer, Geophysics, (415) 723-8214

Geophysicists map youngest section of San Andreas Fault

STANFORD -- Stanford geophysicists have nearly completed the first three-dimensional picture of the northern end of the San Andreas fault.

Their map is the first to show where the fault lies as it curves out under the ocean from Point Arena to Mendocino - its exact location was uncertain until now. And they are the first to see this youngest section of the San Andreas in three dimensions, as a steep, curving slash between two continental plates, plunging 10 to 12 miles down, apparently to the base of the earth's brittle crust.

The picture is still preliminary, according to Stanford postdoctoral research associate Bruce Beaudoin who spoke during presentations at a briefing Dec. 14 and at a session Dec. 15 at the American Geophysical Union meeting in San Francisco. With fellow postdoc John Hole and geophysics Professor Simon Klemperer, Beaudoin is winnowing through one million traces of seismic data recorded in a 1994 experiment along the Mendocino coast. When all that data are analyzed, the team expects to have a three-dimensional picture of the birth of the San Andreas fault.

They and their colleagues also expect to have a new understanding of how earthquake and volcanic zones along the entire West Coast are linked by three continental plates that jostle and grind against one another at a junction near Mendocino. Their findings will help assess hazards in Northern California, Oregon and Washington State.

The Mendocino Triple Junction Project, funded by the Continental Dynamics program of the National Science Foundation, with support from the Deep Continental Studies program of the United States Geological Survey, is a multi-year experiment to create a three-dimensional model of the region. It is led by principal investigators from five research institutions, including Stanford.

In 1993 and 1994, the researchers sent sound waves deep into the earth's crust from ship and shore stations, then recorded the echoes as the sounds bounced off different layers of rock. Now they are using these data to create seismic images of the faultlines and subduction zones formed at the meeting place of the three massive crustal plates.

Part of the Stanford team's role in this project is to take those images to three dimensions. Traditional seismic images are two dimensional: they show a cross-section of layers of rock like a slice through a layer cake. Using computer codes that Hole developed as a graduate student at the University of British Columbia, Beaudoin and Hole are working to combine many such slices to create a model of the whole cake. "The best analogy for the difference may be that it is like looking at many x-rays, versus one three-dimensional CAT scan," Beaudoin said.

Birthplace of a fault zone

The Mendocino Triple Junction, located in the vicinity of Cape Mendocino near the town of Petrolia, is the place where three of the earth's tectonic plates meet. The Pacific Plate to the south and the Gorda plate to the north are oceanic plates. To the east of both of them is the North American continental plate.

The boundaries between these plates are faults. Two are strike-slip faults, one running from east to west at the border of the Pacific and Gorda plates (the Mendocino fracture zone), the other running north to south where the Pacific meets the North American plate (the San Andreas fault). The third boundary is a thrust fault: the Gorda plate thrusts inland under the North American plate, forming the Cascadia subduction zone, the source of earthquake and volcanic hazards in the Pacific Northwest.

For the past 29 million years, the two ocean plates have been slowly moving northward along the edge of the continental plate, grinding out earthquakes; triggering tsunami waves; squeezing hot magma from beneath the crust to feed hot springs, geysers and volcanoes; and forming mountains and valleys and suboceanic canyons.

The Pacific plate grinds against the continent, slipping along the San Andreas faultline and sometimes releasing its energy in a violent shake. The Gorda plate moves northward but also thrusts inland so far that it triggers earthquakes 40 to 50 miles deep under the continent, as far east as Redding, Calif.

At the boundary between the two oceanic plates, the continental margin undergoes a transition. No longer overriding the Gorda Plate, it clashes against the edge of the Pacific Plate. That transition is the birthplace of the San Andreas Fault.

Key to the faultline's past

When the Stanford team's three-dimensional profile is completed, it should give a clearer picture of the transition zone between the three continental plates. It should show what happens to the crust as it changes from subduction zone to strike/slip fault - in effect, how a fault is born.

"This transition is fascinating, said Klemperer, "because the same process happened in the San Francisco Bay Area six million years ago. The present, at Cape Mendocino, is the key to the past at San Francisco."

In addition, the seismic profile may:

Raise new questions about what happens to a fault after it is first formed. Beaudoin's preliminary data show that the newly formed Mendocino segment of the San Andreas angles almost straight down through the crust, all the way to soft, hot mantle beneath the crust.

If this profile is confirmed, it will mean that the new fault looks much different than the segment of the San Andreas under San Francisco Bay, which was formed when the triple junction passed that region six million years ago. Another seismic profile completed last year shows that south of San Francisco, the San Andreas stops part-way down in the crust. At that point, a structure that looks like a horizontal fault angles off sharply to the east deep beneath the bay. If both pictures are accurate, some yet-to-be-determined forces have changed the configuration of Bay Area faults after they were formed.

Help settle an ongoing scientific discussion about what happens to the North American continental crust south of the triple junction, where it no longer is supported by the underthrusting slab of the Gorda Plate. Some scientists think that broken rock from the Gorda slab keeps the continent afloat. Others think that the continental crust in the region just south of Mendocino may be floating on soft, partly melted rock, warmed by the hot mantle below. This warm rock from what geophysicists call the "asthenosphere" would help explain the geysers and hot springs that are common in Northern California from Calistoga to Clear Lake.

Location of three continental plates and the faultlines that form their borders.

Three-dimensional schematic of southern segment of San Andreas Fault near Cape Mendocino. Future data analysis should show whether the fault continues straight down to the base of the crust, and should show the northern end of the fault as it is formed at the meeting of the Mendocino Triple Junction.

-jb-

REPORTERS NOTE: Principal investigators in the Mendocino Working Group are Anne Meltzer of Lehigh University; Anne Trehu of Oregon State University; Alan Levander of Rice University; Simon Klemperer and Bruce Beaudoin of Stanford University; and Jim Luetgert, Walter Mooney and Sam Clarke of the U.S. Geological Survey.

For details and updates about the study, visit the Mendocino World-Wide Web home page, http://quakes.oce.orst.edu/mendocino

For color copies of the figures in Beaudoin's AGU paper, see his Web page at http://pangea.stanford.edu/~bruce/abst/mendo3d_agu95.html

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