California Geological Survey

Rose Canyon faults

The California Geological Survey released new earthquake fault zone maps in late September – and San Diego-area homeowners may be surprised to learn their property sits atop a fault.

A press release from the agency notes that the maps “detail where local governments must require site-specific geologic and engineering studies for proposed developments to ensure this hazard is identified and avoided. Generally, new construction for human occupancy must be set back 50 feet from the active surface trace of a fault.”

Thomas Rockwell: "Really, it’s going to be Redlands which will be right next San Francisco."

CNN

The maps are part of the Alquist-Priolo Act, which was passed into law following the magnitude 6.6 San Fernando Earthquake in 1971. A lot of buildings were damaged during this event.

Thomas Rockwell is a San Diego State University professor of geology and noted expert when it comes to the local Rose Canyon Fault. He provided his thoughts on the new maps.

“I think they have done an excellent job identifying where there are obvious traces of the fault,” he explained. “It used to be that we didn’t have enough information to map the entire onshore portion, but we know the fault comes onshore in La Jolla, and it comes up over through Soledad, then down Rose Creek and then down through the Morena district right through Old Town. Then it starts to splay out through the downtown area and offshore into the bay.

"It just wasn’t completely zoned before for whatever reason. It wasn’t as well identified in terms of where the traces are, but going back to the 1927-era photos we can see scarps [features on the surface of the earth that look like a step caused by slip on the fault] so we can see where the fault is. They have done a very good job identifying the likely locations of the main strands of the fault.”

Rockwell continued, “I know some people may not like the fact that the fault goes close to their house or through it, but they can talk to the Geological Survey about that.”

So, what happens if you discover your house sits atop or near one of these new fault traces?

“What it affects is what you can do [with the structure] in the future,” Rockwell said, explaining the issue of new construction on a fault line. “If it’s a vacant lot, of course, you won’t be able to build a house directly on top of the fault — obviously. If the house already exists, then you’re fine as long as it doesn’t burn down or something. As long as the house is there, it’s grandfathered in.” In other words, you don’t have to move or rebuild – unless so inclined.

But even if a house is grandfathered in, what are the actual odds that it will have to ride out a strong quake? Rockwell made it clear that he would be making no predictions regarding when a Rose Canyon quake could occur and then detailed why attempting to predict earthquakes is a futile gesture.

“Look, that fault could produce a [magnitude] 6 at any time, but it could be 100 years, it could be 300 or 400 years from now,” he said. “The one I’m really worried about would be the 7 or the 6.9, but that’s probably a few hundred years off. The problem is that from all the work that we have done on looking at recurrence of large earthquakes on faults, they don’t go off like clockwork. If I say the average return time for a 6.9 on the Rose Canyon is 700 years, that’s plus or minus 300 or 400 years. They’re not nice and regular like we wish that they were. It’s been 200 to 300 years since the last large earthquake on the Rose Canyon, so the likelihood of it happening tomorrow is low, but could it happen? Yeah. That’s why we want to be prepared for it.”

During our discussion, Rockwell also mentioned that the Rose Canyon Fault’s estimated accumulated, annual slip is about 2-3mm per year. This is quite low when compared to the San Andreas Fault’s rates, which range from about 20mm per year in the Salton Trough to as high as 35mm per year in Central California. That translates to 2-3 meters of displacement every 1,000 years for the Rose Canyon Fault, and 3.5 meters of displacement every 100 years for the San Andreas Fault. The latter’s more aggressive movement has led to the theory that Los Angeles will be adjacent to San Francisco in 20 million years.

“We’ve joked about that for decades,” Rockwell said. “They’re going to have to argue about who is a suburb of who. Really, it’s going to be Redlands which will be right next San Francisco. So, L.A. will be as far from San Francisco as it is from San Bernardino now. At the rate it’s going, it’s gonna take a long time. I don’t think I’ll be around for that.”

If you are curious as to whether your property exists within an earthquake hazard zone, the EQ Zapp app will let you know. Simply enter your address to learn your property’s proximity to an earthquake hazard zone and whether the Geological Survey has evaluated the hazards in your area.