LA developers in quake prone-areas must now look for faults first

In quake-prone parts of Los Angeles, the new rule is dig before you build.

That's according to a new measure announced Friday that says developers looking to build near certain faults must first do a geological survey to make sure any new structure won't sit directly on top of the rift.

Three zones fall into these new "Preliminary Fault Rupture Study Areas," including a stretch of Santa Monica Boulevard near Brentwood, part of Glassell Park near the 2 Freeway and a portion of San Pedro and Palos Verdes.

(The Santa Monica Preliminary Fault Study Zone is shown in green. Image via L.A. Dept. of Building and Safety.)

Developers looking to build a new structure in those spots must first prove they won’t be doing so on top of a dangerous fault, according to Ashley Atkinson with Mayor Eric Garcetti’s Office. 

"Essentially they need to dig down... far enough to determine that there is no active fault running under the location of the building," she explained.

An active fault is one that’s moved sometime in the last 11,000 years.

Building directly on active faults is very dangerous; when they move they can split the earth and tear apart any structures above.

That happened during the 1971 Sylmar earthquake, prompting legislators to pass a law known as the Alquist-Priolo Act the following year. It mandates that no new building can sit on an active fault and any new building near such a fault must be set back by at least 50 feet.

(The Palos Verdes Preliminary Fault Study Zone is shown in yellow. Image via L.A. Dept. of Building and Safety.)

For a fault to be subject to these rules, the California Geological Survey must first make a detailed map indicating, to the best of its knowledge, where the fault lies.

However, the state doesn't have the funds it would need to map every potentially dangerous fault, leaving a number of well-known faults unmapped, including several in Los Angeles.

So the city is taking the lead, creating its own Alquist-Priolo-like rules for active faults in the area.

"It's a very positive action on their part," said Brian Olson, an engineering geologist with CGS.

Los Angeles is not the first local government to make such a move. Both San Bernardino and Riverside Counties, as well as the city of Pasadena have similar safety measures in place, Olson said.

The L.A. areas in question include stretches that have seen a lot of development in recent years.

The new rules will likely slow down some future projects since digging to look for a fault — or "trenching" as it's known — is time-consuming and can cost tens of thousands of dollars.

However, some developers might be able to provide enough evidence by drilling for core samples, Atkinson explained, which is a less involved process.

"That would really be on a case by case basis," she added.

Prior to the new measure, the Department of Building and Safety decided to make developers carry out these geological surveys on an individual basis. Now, it will be required for all new structures in these fault zone that are designed for human occupancy.

The California Geological Survey is currently working on maps for both the Santa Monica and the Hollywood-Raymond fault zones. However, it currently has no plans to map the Palos Verdes area.

The city of L.A. says its Preliminary Fault Rupture Study Areas are only temporary and will be replaced by the state maps once they're released.

In the meantime, city officials say they will share any data they gather about these natural hazards with CGS.

Preliminary Fault Rupture Study Areas