Stress on Southern San Andreas Hits 1000-Year High. Scientists Say LA’s Bill Is Coming Due

The Big One may finally be around the corner. Why scientists say the time is now to pay attention. 

I know, I know. We’ve been hearing it for as long as any of us have been living in SoCal. Whether it be your entire life or you just moved here: The Big One is coming. Thirty years out. It’s always in the next thirty years. Funny (or maybe not so) how that timeline never shifts.

This time might be different.

New research published last week in the Journal of Geophysical Research says Southern California’s earthquake faults are under their highest stress level in 1,000 years. Not in recent memory. Not since Northridge. A thousand years and not in the past 30 years.

A geophysicist team ran a physics-based computer model that simulated earthquake activity along the San Andreas and San Jacinto faults going all the way back to 1100 AD. They fed it radiocarbon dating, tree-ring data, historical ground rupture records. When they got the results, it was disturbing — and for those of us living nearby, let’s just say it wasn’t great news.

Here’s where it gets technical and we may lose you, but stay with us. Turns out stress on the San Jacinto-Bernardino section has hit 3.6 megapascals — the highest number recorded anywhere in the entire 1,000-year simulation.

The southern portion of the San Andreas fault is sitting at 2.8. The worry isn’t just that both numbers are high. It’s that they’re high at the same time. These numbers are pretty meaningless unless you have a point of reference. One megapascal equals roughly 145 pounds of force pressing on a single square inch of rock. So 3.6 megapascals is about 520 pounds per square inch bearing down on a fault that hasn’t ruptured in over 150 years. To help you visualize IRL: your car’s tire runs at about 0.2 megapascals. The rock beneath Cajon Pass right now is carrying 18 times that with nowhere to go. This inevitable rupture could be The Big One.

The Gate to Hell
If you’ve ever driven to Vegas then you’ve driven over the San Andreas fault. Most don’t know or notice — it’s where the 15 freeway cuts through the mountains. It’s also where the San Andreas and San Jacinto faults converge. It’s this convergence the study says is way more dangerous than anyone realized.

Burkhard’s team calls it an “earthquake gate.” 1857, Fort Tejon, magnitude 7.9 — that one stopped at Cajon Pass. Stayed on the San Andreas. The 1812 Wrightwood quake didn’t stop. It crossed through and blew out both fault lines at the same time. What’s the difference? Stress conditions at the junction.

Right now both faults are running hot simultaneously. That’s the scenario the model flags as the one where a rupture crosses. “Not only is it concerning that the stresses are reaching historic highs,” Burkhard said, “but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously — and that is a scenario with much larger consequences for the region.”

Last major rupture in the wider LA region was 1857 — 169 years ago.

Burkhard’s team was careful. No prediction. No date. What the model does show is that both fault systems are simultaneously approaching the stress threshold the researchers associate with joint ruptures. That hasn’t happened in 1,000 years of simulated history. It’s happening now. A joint rupture of the San Andreas Fault and the San Jacinto Fault that crosses the Cajon Pass would be a much more severe event than one limited to a single fault. Not news to anyone, the damage would be catastrophic and widespread. The affected region includes some of the most densely populated, infrastructure-critical corridors in the U.S., including the greater LA area, San Bernardino, Riverside and the Coachella Valley. 

A Look Back

SoCal’s earthquake history has definitely been active, as the timeline below shows. But when you look closely, these quakes have all been everywhere but on the San Andreas:

1933 — Long Beach (M 6.4). 115 dead. School buildings collapsed. Led to the Field Act — seismically safe school construction, California law.

1971 — Sylmar (M 6.6). 65 killed. VA hospital partially collapsed. Freeway overpasses failed.

1987 — Whittier Narrows (M 5.9). 8 dead. A blind thrust fault nobody could see coming, sitting right under the LA Basin.

1994 — Northridge (M 6.7). 57 killed. 7,000-plus injured. Up to $20 billion in damage. Freeway collapses live on TV. Still the costliest quake in California history.

What’s New Since Northridge

One thing we didn’t have in 1994: a warning system. ShakeAlert went live across California a few years back. It monitors seismic stations statewide, detects the first waves off a quake, and fires an alert to your phone before the bad shaking arrives. We’re talking five to eight seconds in most of LA. That’s not a lot. But it’s enough to get under a table, pull over, or get away from a window. The alert comes through as a wireless emergency notification — same loud tone as an AMBER alert. It’s already on your phone. Just don’t turn it off.

What To Do

So now’s the time in the story where you go — that’s nice but what the hell do you want me to do about it?

You’ve probably already noticed seismic retrofitting happening across the city with older apartment buildings. That’s part of the soft-story retrofit program at work, and it’s not optional.

West Hollywood passed an ordinance back in 2017, making seismic retrofits mandatory for wood-frame buildings built before 1978 where the ground floor has parking or open-wall construction — the classic tuck-under configuration that is likely to collapse in a major quake. Turns out there are more than 700 buildings in WeHo that fall under the ordinance. The City has been phasing compliance deadlines by building size and risk level. If you’re a renter in one of those buildings, your landlord is required to file a Tenant Habitability Plan before construction begins. And here’s something renters should know: the City Council voted unanimously in 2018 that landlords bear the full cost. No surcharge gets passed to tenants. If a landlord claims financial hardship, they can petition for a rent increase — but that’s not automatic and it’s not guaranteed. 

Los Angeles passed its own soft-story ordinance in 2015. Santa Monica followed in 2016. The regional push has been underway for a decade. If you own a wood-frame home built before 1978, the state’s Earthquake Brace + Bolt program offers grants up to $3,000 to help cover the cost of bolting the frame to the foundation. Check current enrollment at earthquakebracebolt.com.

On the insurance front, most know a standard homeowner’s or renter’s policy doesn’t cover earthquake damage. Both owners and renters need a separate policy. The California Earthquake Authority is the main provider. Retrofitting first can lower the premium.

The latest science doesn’t give us a date. Ironically, sometime in the next 30 years feels more real than ever before. What it does tell us is the bill has been running for 169 years and soon it will come due. Are you ready?

Editor’s Note: A previous version of this story incorrectly stated that West Hollywood landlords could pass retrofit costs to tenants. The City Council voted unanimously in 2018 that building owners bear the full cost.