Experts say recent quakes are wake-up call to county
By Robert Krier
UNION-TRIBUNE STAFF WRITER
September 11, 2007
The good news: The series of seven quakes that has rattled the region during the past week and a half probably doesn't mean The Big One is more likely to hit soon. The bad news: The Big One is probably not any less likely, either.
“If you had asked me two weeks ago, I would have said a 6.0 earthquake could hit today, or it might hit in 100 years,” said Patrick Abbott, a geologist with San Diego State University. “My answer today would be the same. This sheds no light. We just don't have short-term ability to predict.”
Abbott and other local seismologists hope the quakes, which have shaken residents but done no damage, serve as a wake-up call. The county is near major fault lines that could someday cause severe damage, and laced with lesser fault lines.
Five quakes, ranging in magnitude from 3.2 to 4.0, hit beneath the ocean floor off the county's coast between Aug. 30 and Sunday.
Two other quakes, a 4.7 on Sept. 2 and a 3.4 on Sept. 5, struck the Lake Elsinore area, about 70 miles north of San Diego. The larger Elsinore quake was felt by some residents in San Diego.
Quake clusters are unusual but not unheard of in San Diego. Abbott said a similar cluster occurred around San Diego Bay in 1985 and 1986. The largest quake of that series was a 4.7 five miles southeast of downtown on Oct. 29, 1986.
Graham Kent, a seismologist at the Scripps Institution of Oceanography, said there is less than a 1 percent chance the cluster of quakes could be foreshocks, precursors of a larger quake.
“Even if something is a foreshock, that doesn't mean the next one will be much bigger,” Kent said. “Typically, what you see is the first one is bigger, then the magnitude goes down.”
The offshore quakes occurred either along unnamed faults or what seismologists call “splays” – or branches – of the Coronado Banks fault off the coast. The underwater area is not well understood and mapped by geologists, Kent said.
“Out there, it's really beat up,” Kent said. “It's very complicated. There are dozens of faults.”
Abbott said the Elsinore and offshore quakes most likely are not directly related. He said usually it takes a much bigger quake to trigger movement on a neighboring fault.
The strongest quake ever in San Diego was in 1862. It was estimated to be 6.0. It damaged adobe buildings in Old Town and the old lighthouse at Point Loma. The largest quake since then was a 5.3 quake off the coast of Oceanside on July 13, 1986.
Two local faults are capable of producing quakes as large as magnitude 7.0, Kent said. The Rose Canyon fault, which runs through the middle of San Diego, is roughly parallel to the Coronado Banks fault offshore.
But Kent said the San Jacinto fault, a few miles west of the Salton Sea in Imperial County, and San Andreas fault just to the east of the sea, pose a greater risk. Those faults could cause quakes much greater than 7.0. They are close enough to San Diego to cause widespread damage and are overdue for a big quake. The southern stretch of the San Andreas has not had a major quake in nearly 300 years, and seismologists believe pressure is building.
The Marina District in San Francisco, which was devastated in the 1988 Loma Prieta quake, is about the same distance from the quake's Santa Cruz epicenter as San Diego is from the San Jacinto Fault, Kent said. Homes built on fill in San Francisco's Marina District crumbled during the Loma Prieta quake, and similar damage could occur to structures in San Diego if they are built on sandy bay soils.
Kent said the danger of local quakes is greater than most people realize, and many residents have not prepared their homes or stored emergency supplies.
“I don't think the average person in San Diego County is anywhere near as prepared as they should be,” Kent said. “People should go through their houses and figure out what will get thrown off the walls.”
The state's Earthquake Authority provides residential earthquake insurance and encourages Californians to reduce their risk, said Tim Richardson, acting chief executive officer. Only about 13 percent of homeowners in California have quake insurance.Richardson said he does not expect the recent round of tem blors to spur a buying spree. That takes death and damage.
“Anything short of that doesn't seem to get people to do anything,” he said.
Union Tribune
