OREGON STATE UNIVERSITY

Scientists discover hundreds of faults off West Coast

05/14/2002

CORVALLIS - 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.

Preliminary data presented today (Tuesday, May 14) at Oregon State University, reveals more than 340 strike-slip and spreading-center related normal faults or fault segments in the region, said Jason Chaytor, an OSU master's candidate in the College of Oceanic and Atmospheric Sciences.

Chaytor presented the data at the 98th annual meeting of the Cordilleran Section of the Geological Society of America. "Where Plates Collide" is the theme of the conference, which is hosted by the OSU Department of Geosciences.

Researchers had once plotted more than 60 faults spilling out across the Gorda Plate from the Gorda Ridge spreading zone, Chaytor said. But analysis of data collected by NOAA in 1997 imaged more suspected faults, he said. During the past several months, Chaytor, Chris Goldfinger, an OSU associate professor of oceanic and atmospheric sciences and Robert Dziak, an OSU assistant professor of oceanic and atmospheric sciences and research scientist with the National Oceanic and Atmospheric Administration, have studied data collected during mapping of the region.

Using multibeam bathymetry, sidescan sonar and seismic reflection data that provide soundings of water depth and echo strength, seafloor imaging programs 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 said. "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, Chaytor said. 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, now about five million years old at the point where it enters the subduction zone, is a relatively young, hot and thin plate in geologic terms, Chaytor said.

Many researchers believe the Juan de Fuca-Gorda-North America 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 three centimeters 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 said.

One curiosity of the region is the peculiar form the faults take on the coastal side of the Gorda Spreading Ridge, he said. Faults that fan out from the ridge on the seaward side exhibit fairly typical and uniform lines. Faults marching shoreward, however are skewed into sharp deviations that resemble question marks, Chaytor said.

Researchers are unsure if there is any significance to the shape of the faults in the region.

"As the northern portion of the zone is spreading faster than the southern part and as the volume is being forced into the continental margin, pre-existing faults are being reactivated," Chaytor said.

"Right now we are just looking at the fundamental science of it all. What it will eventually mean in terms of seismic hazards remains to be seen."