Big San Diego Quake Called Unlikely but Possible

The obvious question: Can it happen here? After closely monitoring the death and damage that struck the Bay Area, San Diego experts who study earthquakes, fault lines and seismic movements said Wednesday that it is unlikely this area would experience the magnitude of destruction that is unfolding in Northern California.

But they also cautioned that San Diego’s most crucial fault line--the Rose Canyon network that runs under Mission Bay and La Jolla and then ventures out under the sea--is still capable of wrenching the ground to a level that could buckle highways, tear down brick buildings and flatten some homes.

The problem, though, is that authorities have not acquired enough scientific knowledge about the Rose Canyon Fault, not nearly as much as officials know about the deadly San Andreas Fault where the Bay Area earthquake erupted late Tuesday afternoon.

They know the Rose Canyon Fault will slip; they don’t know when or how fierce will be the destruction it causes.

“We can say it’s clearly an active fault,” said Steve Day, a seismologist at San Diego State University. “It has had offsets in the past 10,000 years, roughly, but we don’t have enough data to pin down how often.”

But, he said, “the fact that it has had past offsets suggests it probably is capable of producing earthquakes in the range of 6 and 7” on the Richter scale.

Tuesday’s quake in the Bay Area was measured at a magnitude of 6.9 and is believed to have caused at least 273 deaths and more than $1 billion in damage.

Day said the Rose Canyon Fault runs along shore from Mission Bay to La Jolla for about 25 kilometers. It then proceeds north under the ocean and is believed to eventually resurface underground in the Los Angeles area.

“But that’s not known with certainty,” he said. “That’s just an estimate.”

He said officials theorize that the fault is accumulating a potential for slipping about 1 to 2 millimeters a year. “That means that, for example, after 500 years it would have accumulated one meter of potential slippage, and that’s about enough to create a large earthquake.”

But, even with those odds, it still remains unclear when the Rose Canyon might next make its fury known.

Daniel Eberle, director of the San Diego County Office of Civil Defense and Disaster Preparedness, said he believes the Rose Canyon Fault has the capacity to cause an earthquake of a magnitude of 6.5 or 7.2. But he, too, cautioned that his projection is only speculation.

“The problem with the Rose Canyon Fault is it goes underwater, and then on land that is built over,” he said. “So you can’t get good information.

“Plus, there’s lot of strands. The Rose Canyon is not one line. It’s a whole series of strands. That’s why it’s really called the Rose Canyon Fault System.”

Both Eberle and Day noted that there is a debate over whether the entire San Diego County area should be upgraded from a Zone 3 earthquake safety designation to a Zone 4. The higher zone designation places greater building code restrictions to better protect lives and property in the event of a major quake.

The officials noted that the populated parts of San Diego County, which would be affected by a Rose Canyon temblor, are under a Zone 3 classification, while the relatively open mountain and desert areas in East County are under a Zone 4. He said the East County portion carries the higher designation because it is nearer the southern San Andreas Fault that stretches through Riverside and Imperial counties.

In addition, the county is crisscrossed by other faults--such as the San Diego Bay Fault and the Old Town Fault in San Diego. Although an earthquake along one of those minor faults would cause damage to buildings on or near its pathway, damage would not be as severe elsewhere in the county, said Bruce Westermo, an SDSU earthquake engineer.

However, earthquake experts “can’t be confident because we don’t know all of the faults,” he said.

“If there is (a major fault), it’s done a pretty good job of hiding,” Westermo said. “It’s possible it’s there, but we don’t have decent historical records” of earthquakes in San Diego.