More faults found off West Coast

Five times more mapped faults than previously imaged by scientists apparently cross the fractured Gorda Plate about 125 miles off the Northern California and southern Oregon coasts, according to an Oregon State University, Corvallis, study. More than 340 strike-slip and spreading-center related normal faults or fault segments were found in the region, says Jason Chaytor, a master's candidate in the College of Oceanic and Atmospheric Sciences.

Researchers once had plotted more than 60 faults spilling out across the Gorda Plate from the Gorda Ridge spreading zone, he points out. However, analysis of data collected in 1997 imaged more suspected faults. Using multibeam bathymetry, sidescan sonar, and seismic reflection data that provide soundings of water depth and echo strength, seafloor imaging pro grams can interpret depths and echo strengths and allow scientists to create a snapshot of the seafloor. "Long linear features strongly suggest the faults are more pervasive throughout the Gorda Plate than originally thought," Chaytor indicates. "A lot of the faults are actually created at the spreading ridge--the Gorda Ridge spreading zone. This isn't unusual. It's quite common to see this many faults in these areas."

The Pacific Coast from northern British Columbia to Punta Gorda, near Cape Mendocino in California's Humboldt County, is the region known to geologists as the Cascadia Subduction Zone. There, two large slabs of the Earth's crust called the Juan de Fuca and the Gorda plates have been diving ponderously down beneath the North American continental plate for millions of years. The Gorda Plate, about 5,000,000 years old at the point where it enters the subduction zone, is a relatively young, hot, thin plate in geological terms. Chaytor explains. Many researchers believe the Juan de Fuca-Gorda North American plate system is a likely source for a much anticipated mammoth earthquake that could threaten Portland, Seattle, and parts of Northern California.

The Pacific Plate, south of the Gorda Plate, creeps northward about one inch a year along the California coast, compressing the Gorda Plate and triggering frequent small to moderately sized earthquakes within the plate. The geological record shows that deep faults inside the diving plates have generated tremendous quakes with magnitudes of 8.0 to 9.0 every 500 years. "We will be trying to figure out if more activity on the plates may actually lessen the strain in the subduction and mean less chance of a larger earthquake," Chaytor notes.

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