Ocean Fault Raises Estimate of Quake Risk

A UCLA scientist has found evidence of a previously unknown undersea fault off the Southern California coast that he says raises the estimate of overall quake risk from a known fault in the Los Angeles area by at least 50%.

The undersea fault is causing the islands off Southern California to slip northwestward at about one-half inch a year, possibly building up seismic strain for a major earthquake that could affect cities from Ventura to San Diego counties, the scientist reported here Wednesday.

The fault, which appears to run 20 to 50 miles offshore and parallel to the San Andreas Fault, has been slipping without the characteristic small earthquakes to relieve crustal strain. This makes an earthquake of about magnitude 7 a strong possibility, seismologist David D. Jackson of UCLA said at a meeting of the American Geophysical Union.

Before this new evidence, the U.S. Geological Survey estimated the risk of a quake of magnitude 7 or higher in Southern California at 60% over the next 30 years, or 10% within the next five years.

The findings, which have yet to be confirmed officially by the U.S. Geological Survey, would boost those estimates to 90% and 15% respectively, according to Jackson.

Jackson and his colleagues discovered evidence of the fault’s existence after analyzing data from an array of ground instruments used in conjunction with space satellites. The research group measured motion along the offshore fault at about one-half inch a year Jackson said.

“It’s at least as fast as some sections of the San Andreas Fault,” said Jackson. “If you were to have an earthquake the size of the Loma Prieta quake, which is not unthinkable at all, it could cause widespread damage in Los Angeles or San Diego and in many of the towns in between.”

That earthquake on Oct. 17, 1989, near San Francisco measured magnitude 7.1.

“I think for a long time we’ve thought there was probably motion offshore in that area (off Southern California), and this new evidence looks very clear to me,” said James Savage, a crustal deformation specialist with the USGS in Menlo Park.

Savage said he believes the deformation Jackson’s research group found is sufficient to prove the existence of the fault, but more work needs to be done to precisely localize it and characterize it.

Earth scientists envision the Earth as a molten core with vast plates of crust floating on top, moving slowly past or under each other. California is located at the margin where the Pacific Plate is grinding northwestward past the continental North American Plate. This creates massive strain that results in cracks, or faults, along which the strain is relieved through earthquakes.

Along the San Andreas Fault that bisects Southern California, this motion is from about 1 1/2 inches a year in the north to about half an inch in the south.

The newly detected offshore fault differs from the San Andreas and most other faults in that it lacks a distinct pattern of small earthquakes. Jackson suspects that strain is building up for a larger event.

“The kinds of faults we know about are those which are creeping and have lots of small earthquakes, and those which are being displaced but not creeping and have big earthquakes,” Jackson said. “Then there’s a third kind that just isn’t slipping very fast and doesn’t have earthquakes or has very few. We think ‘the offshore fault’ is the second kind.”

The fault itself has not yet been observed, but Jackson said its western side represents the edge of the Pacific Plate. It is causing movement from San Miguel Island in the north to Santa Catalina Island and San Clemente Island off the coasts of Orange and San Diego counties.

The crustal monitoring on which Jackson reported was done with the nation’s first regional continuously monitoring network that uses Department of Defense satellites to do civilian research. This satellite system is known as the Global Positioning System, and is being used in Saudi Arabia to allow U.S. troops there to know where they are in the featureless desert.

In California, the research consortium involves Scripps Institution of Oceanography at UC San Diego, the Jet Propulsion Laboratory, Massachusetts Institute of Technology and UCLA.

Every minute, its ground stations throughout Southern California send signals to the satellites that, when they return, can be used to tell the precise locations to within an inch.

Portable receivers, hooked into the system, then can be placed temporarily on remote survey sites to measure how much the ground there has moved over the previous year.

The studies on which Jackson reported began in 1986.

POSSIBLE QUAKE FAULT

Scientists have found deformation in islands off the Southern California coast that indicates they are moving northwestward relative to the mainland at 10 millimeters a year. This suggests the existence of a fault between the islands and the coast, from 20 to 50 miles offshore. It will take extensive work to find out specifics about the fault, but depending on its location it would affect seismic risk from Ventura to San Diego, scientists say.

Source: David Johnson, UCLA.