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Danger posed by earthquake fault will lead to some tighter building restrictions, including in La Jolla

The state is creating new earthquake fault zones in San Diego.

The state is creating new regulatory zones to protect people against the Rose Canyon fault.

It will soon become harder to develop many properties in San Diego, including La Jolla, due to growing evidence that the Rose Canyon earthquake fault, which runs beneath the city, is larger and more active than scientists once thought.

About 7,000 parcels in and around La Jolla, Old Town, San Diego International Airport and downtown San Diego will be placed in new regulatory fault zones.

The California Geological Survey is creating the fault zones where developers of residential, commercial and public buildings may be required to show that their projects do not sit on top of active faults or are located a safe distance from such systems.

The zones, which are expected to be adopted this summer, are based in the 1972 Alquist-Priolo Act, which is meant to minimize the sort of death and destruction that can occur when an earthquake ruptures Earth’s surface.

The law generally does not pertain to minor structures such as retaining walls and swimming pools. But CGS says the regulations will apply to major projects that are being considered but have yet to be permitted.

The CGS wants to reduce the chances that people will be hurt or killed by the Rose Canyon fault, which comes ashore in La Jolla, cuts through the city and goes back offshore along the Silver Strand.

The fault is not as dangerous as the San Andreas system to the east. But Rose Canyon produced a magnitude 6.0 quake in San Diego in 1862 that caused damage locally and generated shaking as far away as Los Angeles.

The strike-slip system is capable of producing a 6.9 quake that could damage 100,000 residences in greater San Diego and displace 36,000 households, according to a 2020 study by the San Diego chapter of the Earthquake Engineering Research Institute. In a strike-slip system, one side of a fault moves horizontally in relation to the other when a quake occurs.

“A 6.0 quake could occur at any time, and a much bigger one — something in the 6.8 to 6.9 range — appears to occur every 700 to 800 years,” said Tom Rockwell, a San Diego State University seismologist who has been studying the Rose Canyon system for decades.

“The last really big one happened between 1700 and 1750, but we shouldn’t relax. Rose Canyon is an active system that could produce something big during our lifetimes.”

The entire fault was once thought to be inactive. But scientists have learned otherwise, partly from conducting geological studies tied to the development of the city’s trolley system, renovation and expansion at the airport, and reviews of Seaport Village, which is targeted for major redevelopment.

The new regulatory fault zones being created in San Diego arise from a catastrophe that occurred 50 years ago — the 6.6 San Fernando Valley earthquake in Los Angeles County, which killed 64 people, heavily damaged freeways and inflicted at least $500 million in losses.

The earthquake made it clear that more needed to be done to avoid constructing buildings — particularly those that house people — on top of active faults.

The tragedy led to passage of the Alquist-Priolo Act, which identifies areas where active faults are known or thought to exist.

The state created one of the zones in La Jolla in 1991 and one in Point Loma in 2003. The city of San Diego created its own fault zone in the downtown area about 20 years ago.

The laws have not hobbled growth.

Since 2000, the population of San Diego has grown by about 175,000, to roughly 1.4 million. During that period, 60 buildings over eight stories tall were built downtown and seven more are under construction, according to the city’s Development Services Department.

To a degree, such growth has been a boon to science, leading to geological studies throughout the city.

“We’ve learned that the Rose Canyon fault is more spread out, more distributed, than we knew and some of its strands are active,” Rockwell said.

Scientists cannot accurately forecast when and where a fault will break, how long the rupture will be and whether it will trigger other faults.

But that doesn’t stop them from producing estimates. The San Diego chapter of EERI ran computer programs that simulated a 6.9 quake on the Rose Canyon system. The algorithms led scientists to believe that such a quake could knock out gas and water service between La Jolla and the Silver Strand for months, shut down the San Diego-Coronado Bridge and cause important municipal buildings to collapse. In some places, the surface would shift six to seven feet and parts of Mission Bay would sink about a foot, the study said.

“It’s been a real wake-up call for stakeholders,” Jorge Meneses, president of EERI-San Diego, said last year. “But they have time to make San Diego more resilient to the kind of damages that could occur.” ◆