Rose Canyon Fault Designated Active Hazard : Quakes: The decision could hurt property values in the exclusive neighborhoods from La Jolla to downtown San Diego.

In a move that could rattle the property values of some San Diego homes, state geologists say they intend to upgrade the Rose Canyon fault to an active earthquake hazard that could slip and destroy homes in some of the heavily populated areas around Mt. Soledad park and east of Mission Bay.

The state’s Division of Mines and Geology has already declared “special study zones” in rural areas of San Diego County, but its current proposal would be the first to officially raise the specter of a potential earthquake along the fault that runs under the high-priced neighborhoods of La Jolla, Pacific Beach, Clairemont, as well as downtown San Diego.

“The big concern is that the fault may produce an earthquake in the city of San Diego,” said senior state geologist Earl Hart, who has overseen the state’s efforts to map active fault lines for 17 years. “And the questions are, ‘How big?’ ‘What’s the probability?’ and ‘When?’ ”

San Diego State University geology professor Thomas Rockwell determined that the Rose Canyon fault was active after he took underground samples last summer at a utility construction site along Interstate 5.

He predicted that the earthquake there would happen within the next 500 years and register from 6.25 to 6.75 on the Richter scale--the size of temblors that rocked the San Fernando Valley in 1971 and Coalinga in 1983, but considerably smaller than the 7.1 temblor that shook the Bay Area last October.

“It would feel like someone was pulling the rug out from under your feet,” said Rockwell. “A lot of chimneys would go down, and a lot of glass would break. There would be some structural damage.”

In the past, state geologists have always considered the fault potentially active, but lacked the proof to warrant the official designation as a truly active earthquake site.

Now, spurred by Rockwell’s research and prediction, state geologists have drawn up maps proposing the special study zone--an official designation that warns of possible ground movement during an earthquake--along 2 to 3 miles of the Rose Canyon fault as it runs under the La Jolla Cove, through Mt. Soledad, and across Interstate 5 into western Clairemont.

The maps also indicate the possibility of the state imposing the earthquake zoning on two shorter, quarter-mile legs of the Rose Canyon fissure that have been discovered near the new San Diego Police Department headquarters downtown, along 12th Avenue between C and E streets.

Under the state’s plan, the special studies zone could cut a swath ranging from 200 to 500 feet on either side of where the fault is determined to touch the Earth’s surface. Hart said all the initial work on deciding where to put the zones won’t be finished until next March, with a final designation possible by next summer.

The proposal was disclosed to city officials in a March 21 letter, which asks for more information about the fault line between Clairemont and downtown.

Hart said this week that his staff is working carefully on the Rose Canyon proposal because it is a “very sensitive” move that could hurt the values of buildings that lie within the special studies zone.

Part of the reason, he said, is a state law that forces homeowners and landlords to disclose the existence of an active fault to prospective buyers.

In addition, any residential or commercial property owner within the zone who attempts development or extensive renovation would be required to conduct a geological inspection to make sure any new construction is set back at least 50 feet from the fault line, he said.

“Obviously, if the fault goes through your property it is going to lower the value of your property,” Hart said. “So, we don’t want to overzone and . . . I don’t want people to be unnecessarily alarmed.

“We’re aware that it is an exclusive area, and they have a lot of homes, and we want to make sure the fault is identifiable before we go in and zone it.”

Marjorie McLaughlin, president of the San Diego Assn. of Realtors, agreed that the state’s actions would hurt some property values on the hillside homes, which county tax roles show have fetched between $400,000 and $940,000 in recent transactions. But McLaughlin, who once lived in a house on the fault in Clairemont, said she would expect the effect to wear off in about a year.

“Some buyers won’t buy because of it, I can tell you that right now, and especially if they are from another area and they won’t know the meaning of it,” said McLaughlin, an agent with Century 21. “Other buyers, it’s not going to affect them. They’re used to living in California.

“In the beginning, there are some people who think they should get the property for substantially lower because of that fact,” she said. “After that, it will become business as usual.”

McLaughlin also said that San Diego real estate agents have normally disclosed the presence of the Rose Canyon fault to prospective buyers even when the fissure was not thought to be active.

Although geologists have long been suspicious of the fault, they have failed in the past to find any evidence that it was an active threat to San Diego, a city that until now has been regarded as one of the safer spots in Southern California for temblors. It has always been overshadowed by fissures that were demonstrably active, such as the infamous San Andreas fault, which runs through San Francisco Bay and east of Los Angeles.

State geologists have imposed the special study zones throughout the state extensively since they were given that power under a 1973 law. So far, the state has declared special study zones over active earthquake faults running through 33 counties, including along the San Jacinto and Elsinore faults that run through rural portions of northern San Diego County.

But the state abruptly turned its attention to Rose Canyon, within the San Diego city limits, after SDSU geologist Rockwell conducted trenching and unearthed the first evidence last summer of recent geological activity on San Diego Gas & Electric property on Santa Fe Avenue.

The discovery caused state geologists to reconsider the fact that small parts of the fault were documented in 1984 during the excavation for the new downtown police station.

“Most of the Rose Canyon fault is developed on, and that’s part of the problem,” Rockwell said Wednesday. “Much of the canyon fault zone was developed on by the ‘30s and ‘40s, and it is a very difficult fault to study because so much of it is under asphalt and buildings.”

Rockwell said Wednesday that carbon-dating tests on samples taken from seven spots along the fault in La Jolla and near Mission Bay show that there has definitely been activity in the recent geological past--about 11,000 years in seismological terms. He said the fault has shown repeated movements within the last 7,000 years, and a surface rupture as recently as the last 1,000 years.

Although Rockwell’s research has prompted new interest in the Rose Canyon fault, state geologists still consider it to be less a threat than the San Andreas. Rockwell said the more notorious fault is considered 20 times stronger and more likely to yield an earthquake than Rose Canyon in San Diego.

“It’s an important fault, but I’m not sure I would call it a major fault,” Hart said of the Rose Canyon fissure.