San Francisco, Calif. -- The rewritten and revised geological history of coastal California, developed through recent seismic studies, has important implications for earthquakes north of San Francisco and along the San Andreas fault, according to a Penn State geophysicist.
"The model of the Mendocino Triple Junction -- the place where three tectonic plates in the Earth's crust come together -- as a slab window, is too simple to accurately portray events at the northern end of the San Andreas fault," says Dr. Kevin P. Furlong, professor of geosciences.
The Mendocino Triple Junction is where the North American, Pacific and Gorda plates come together. The North American plate is moving south while the Pacific plate is moving north. These two plates slip by each other with resultant earthquakes on the San Andreas fault and subsidiary faults. While the Gorda plate is also moving north, it is, at the same time, moving east, underneath the North American plate. This subduction of one plate beneath another creates the Cascade mountains, a volcanic range known for Mt. St. Helens and Mt. Rainier.
"At the point where the three plates come together, a complicated process that we dubbed the Mendocino crustal conveyer takes place," Furlong told attendees today (Dec. 15) at the fall meeting of the American Geophysical Union in San Francisco.
Recent seismic studies indicated that the crust at the Mendocino Triple Junction was nearly twice as thick as that north and south of the area.
"We needed to determine if this thickening was caused by the mechanism at the triple junction or if it was simply an inherited attribute of the geology," says Furlong.
Working with Susan Schwartz, University of California, Santa Cruz; Rob Govers, University of Utrecht, the Netherlands; and Chris Guzofski a Penn State graduate student in geosciences, Furlong modeled the triple junction to explain what happens in that area of Northern California.
The researchers found that as the North American plate moves south and the Gorda plate moves under it, a gap forms at the intersection that allows hot magma from beneath to well up. As this magma hits the relatively cooler edge of the North American plate, it cools and becomes viscous, sticking the North American and Pacific plates together in that area.
"As the Pacific plate moves north, it drags the North American plate with it," says Furlong. "However, the North American plate was moving south and so the crust bunches up on itself and thickens."
The material for this thickening comes from south of the junction, an area that had previously thickened as the triple junction moved past. The thickening and thinning of the crust alter the underlying rock characteristics, which influences how the area responds during an earthquake.
"In the area where the plates stick together, the plates are essentially moving in the same direction," says Furlong. "This explains why the area about 150 miles north of San Francisco has only minor earthquakes but never anything like the earthquakes that occur farther south on the San Andreas. If the two plates are not slipping past each other, there is no mechanism for a major earthquake."
As the Mendocino Triple Junction traveled north, it left what could be thought of as a giant worm trail. Near the junction, the thickened crust forms the first hump of the worm followed by a valley and a second smaller hump. South of these humps is the area where the worm passed, leaving the underlying rock forever altered by thickening and thinning.
"The rocks around the San Andreas fault began as sedimentary rock that came off the Gorda plate, but the process of thickening and thinning very rapidly turns the sedimentary to metamorphic crystalline rock," says Furlong. "Rather than taking tens of millions of years, the process takes only a few million years."
Furlong's model matched the geology of the area and also answers some longstanding questions. Researchers have looked for the northern end of the San Andreas fault, but have never pinpointed it. According to Furlong, that may be because the thickening and thinning process obscure the end and the fault is only clearly seen in areas where the triple junction has already passed.
The model also explains the puzzling river geometry in Northern California. North and south of the area immediately affected by the triple junction, the rivers flow north and south respectively, but between the two raised areas, rivers tend to meander and turn back on themselves, complicating flood prediction and protection. This indicates an area where rivers once flowed north, but in a short geological time, will flow south.
The trail left in the geology of coastal California is not only important for the earthquake structure of the area, but can also lead to the location of other places where similar triple junctions occurred. These disturbed areas of rock beneath the surface have a characteristic behavior pattern that influences activity along the slip fault line. The Penn State scientist suggests that the thickening and thinning of the Mendocino crustal conveyor have been going on for 20 million years and effect a broad strip of land down to central California.
EDITORS: Dr. Furlong is at (814) 863-0567 or at firstname.lastname@example.org by email.