Contact: Robert Irion (408/459-2495)

AFTERSHOCKS EXPOSE PLATE MOTIONS AND A WEAK FAULT AT THE MENDOCINO TRIPLE JUNCTION OFF NORTHERN CALIFORNIA

NOTE: This release is embargoed until 1:30 p.m. Tuesday, December 6, 1994, when session U22C begins at the American Geophysical Union meeting in San Francisco. Susan Schwartz is the fifth speaker in the session, in room 134 of the Moscone Center. Schwartz also will present her research in poster session S51A (#10) on Friday morning, December 9.

SAN FRANCISCO, CA--Aftershocks of three large earthquakes at Cape Mendocino, California, in April 1992 have shed new light on the interplay among three tectonic plates at the Mendocino Triple Junction, one of the country's most complex seismic settings.

A detailed study of the aftershocks suggests that the Gorda plate, a wedge of Pacific Ocean seafloor, is plunging in fits and starts under the North American and Pacific plates in the process called subduction. Other scientists have believed that the Gorda plate subducts only beneath the North American plate.

In addition, the study indicates that the fault between the Gorda and North American plates is weak: It broke even though the stress in the direction of the earthquake rupture was relatively low before the main quake.

Study author Susan Schwartz, director of the W. M. Keck Seismological Laboratory at the University of California, Santa Cruz, compares the fault to the Santa Cruz Mountains segment of the San Andreas Fault, site of the Loma Prieta earthquake of 1989. "In both cases, the regional stress field was not favorably oriented to make the faults slip," she says. "Only a little shear stress was enough to make them move. However, both events showed that even with a weak fault, you can still have large earthquakes."

The Mendocino Triple Junction marks the southern limit of the Cascadia Subduction Zone, which threatens the Pacific Northwest with potential quakes of magnitude 8 or larger. Two other faults converge on the subduction zone at the triple junction: the offshore Mendocino Fault and the northern end of the San Andreas Fault (see attached Figure 1). Near the junction, all three faults largely vanish, leaving seismologists in the past to scratch their heads about how the tectonic plates interact there.

That all changed on April 25, 1992, when a magnitude 7.1 earthquake struck near the triple junction at Petrolia, about 30 miles south of Eureka. Two magnitude 6.6 aftershocks caused further damage the next day. Within a day, Schwartz and a team from Lawrence Livermore National Laboratory and the University of Oregon began to deploy portable seismometers. The team placed eleven sensitive instruments in a 50-mile zone around Petrolia. The instruments measured ground motions in such detail that Schwartz could calculate both the locations of the aftershocks and their focal mechanisms--the exact ways in which they ruptured the earth. For her study, Schwartz used about 30 of the biggest aftershocks (greater than magnitude 3), plus records from 30 other quakes of previous years.

The data painted a clear picture of the stresses in the region of the triple junction, which are dominated by the motions of the Gorda plate, Schwartz found. During the main quake, the plate slid at a shallow angle--nearly horizontally--under the North American plate. However, most of the aftershocks sliced through the earth on vertical or steeply dipping faults on the boundary between the Gorda and Pacific plates. "In general, most aftershocks occur on the same plane, and with the same sense of motion, as the main shock," says Schwartz. "Here, none of the aftershocks did so."

Schwartz used these results to make a model of the plate interactions (see attached Figure 2). Far west of the triple junction, the young and hot Gorda plate--born at the undersea Gorda Rise-- butts up against the old and rigid Pacific plate, which prevents the Gorda plate from subducting. This creates earthquakes along the Mendocino Fault and within the Gorda plate itself as the plate is squeezed. Toward the triple junction, where the Pacific plate ends, the Gorda plate dives under both the North American plate and a corner of the Pacific plate. Schwartz notes that seismic profiles of the region reveal a cold slab dipping both to the east and the south, which supports the model.

The Cascadia Subduction Zone runs almost parallel to the direction of the regional stress at the triple junction, Schwartz found. Only a small component of the stress points in the direction of the Gorda plate's subduction under the North American plate. Nevertheless, it was enough to trigger the magnitude 7.1 quake. To Schwartz, this indicates that the fault is weak, perhaps because fluids lubricate it far underground. As an analogy, consider sliding one brick past another by pushing them together, with just a pinch of side-to-side force. Bricks with smooth surfaces or with a thin layer of water between them are much more likely to slide than rough and dry ones.

Schwartz cautions that her conclusions are unique to the triple junction. "This is just the southern tip of the Cascadia Subduction Zone," she says. "The presence of a third fault makes the situation much more complicated than in the rest of the zone. This simply is a model that may help us better understand the plate geometry at this one location."

Schwartz's study was made possible by the Rapid Array Mobilization Program of the Incorporated Research Institutions for Seismology (IRIS).

Editor's note: You may reach Susan Schwartz after the AGU meeting at (408) 459-3133 or at susan@earthsci.ucsc.edu.

This release is also available on UC NewsWire, the University of California's electronic news service. To access by modem, dial (209) 244-6971.

Figure 1: General tectonic setting at the Mendocino Triple Junction (*) and the Pacific Northwest. The magnitude 7.1 Cape Mendocino earthquake of April 25, 1992, struck onshore near the triple junction; two magnitude 6.6 aftershocks occurred offshore the next day.

Figure 2: Model of plate interactions at the Mendocino Triple Junction by Susan Schwartz, seismologist at UC Santa Cruz. West of the triple junction, the old and rigid Pacific plate prevents the young and hot Gorda plate from subducting. Stresses are relieved by earthquakes along the Mendocino Fault and within the Gorda plate. At the triple junction (*), the Gorda plate subducts beneath both the North American plate (overlying, not shown) and a corner of the Pacific plate. The Cascadia Subduction Zone appears to be weak in this region; it broke even though the regional stress field (roughly north-south) is nearly perpendicular to the direction of the main quake's rupture (roughly east-west).

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