Risk of a big San Andreas Fault earthquake should be top of mind, this NAU researcher says

The San Andreas Fault has long been a sort of bogeyman in Southern California life. Worries that eventually “the big one” will hit are always around, but not always top of mind.

Bridget Smith-Konter says we should probably make it top of mind.

Smith-Konter is a Northern Arizona University researcher who studies the San Andreas fault. And her newest research is stark.

The fault sits on the state line between Arizona and California, and her research suggests it’s currently experiencing its highest levels of accumulated tectonic stress in more than 1,000 years. She spoke more about it with The Show.

Full conversation

BRIDGET SMITH-KONTER: These tectonic plates, they keep moving regardless of whether faults are slipping on them, and the Pacific Plate and the North American Plate push against each other at a rate of about 45 millimeters per year. So that’s about two inches a year. And all this motion has to go somewhere.

Now, the fault itself, though, is a place where stickiness happens. The faults are locked in place, and friction builds along these sections and accumulates this thing called tectonic stress in the surrounding rock. And that stress builds over time and eventually gets high enough to where the conditions are set for the fault to not be able to hold on to it anymore, and it breaks in an earthquake.

And so, in Southern California, we’re looking at really a place for two different faults that kind of come together that make up the San Andreas. The San Andreas has like a little sister called the San Jacinto. And this is a little branch in Southern California that we’re sort of concerned about its interaction with the San Andreas and the fact that these two haven’t produced an earthquake in Southern California near Los Angeles in almost 170 years.

And that’s a pretty long quiet period for earthquakes.

LAUREN GILGER: Right. So it’s sort of like a building up. Like you’re watching, and your research is focused on finding out when and if we might be due for one.

BRIDGET SMITH-KONTER: That’s right.

LAUREN GILGER: And bad news, right? It seems as if that pressure is building. Is that what you’re finding here?

BRIDGET SMITH-KONTER: Yeah, so this is all based on what we call a physics-based computer model. So we designed this model almost 20 years ago to study the San Andreas. But in this case, we’re looking at a thousand years back in time of fault motion due to earthquake history.

So we have all this computer code, and we need data to put into it. So we fed data from past earthquakes into this model based on reconstructed geologic evidence — stuff like radiocarbon dating of displaced sediments in the crust, tree ring anomalies caused by past earthquakes.

And we used all this to track how tectonic stresses build up and are released on the fault over time. And what we found is that the stress levels in this area of the San Jacinto and San Andreas junction, they’re really at a thousand-year high, where current stresses are really exceeding any level that we’ve seen throughout our simulation.

And our simulation covers the entire San Andreas plate boundary from Southern California to Northern California.

LAUREN GILGER: So things are building up. Tell us about this specific place in your research that you’re looking at — which is the Cajon Pass — and its potential importance. This you describe as a conditional earthquake gate?

BRIDGET SMITH-KONTER: Yeah, so it’s kind of a new word in earthquake science. It’s basically thought of as like a barrier to earthquake rupture. So at this point, we were asking the question, does this thing act as a gate where it can either stop a large earthquake rupture that’s already happening in its tracks, or does this region allow earthquake ruptures to continue moving forward and cross the fault and onto another one, creating a much larger, more destructive event?

And so, this model really suggests that whether the gate opens or closes, whether this Cajon Pass acts as a green light or a red light to ruptures, that really depends on how similarly stressed these two fault junctions are at the time of rupture. So it’s really the critical factor is not just how much stress has been built up on any one fault, but really how closely aligned these stress levels on these two nearby faults are to each other.

Unfortunately, we’re in the current situation where it looks like both faults are highly stressed, and that kind of historically points to a joint rupture, which could be a bigger type of earthquake.

Bridget Smith-Konter

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Handout

LAUREN GILGER: So that’s the kicker, right? Like this is the gate, it’s the red light or the green light, and it looks like it could be a green light.

BRIDGET SMITH-KONTER: It’s a green light, yeah.

LAUREN GILGER: What would that mean? I mean, I don’t want to paint like an action film picture here, but is that what we’re looking at?

BRIDGET SMITH-KONTER: So, big breath here.

LAUREN GILGER: Right.

BRIDGET SMITH-KONTER: OK, so what we can tell from this model is that the system is critically loaded. It is at the highest point that we can tell from the earthquake record it’s been in a thousand years in Southern California.

This doesn’t mean that an earthquake will happen tomorrow, but what it does mean is that the system is loaded enough that it is significant to pay attention to and really ask yourself the question of not really if, but when. And we’re not at the point where we can really predict exactly when and where an earthquake will occur. Unfortunately, that’s just not part of our earthquake science portfolio that we’re able to really give numbers on.

But we can estimate probabilities of significant earthquakes in Southern California, and we can plan and prepare ahead for what that looks like for the people of Southern California.

So I can just throw a statistic out there: The probability of a significant earthquake in this region in the coming decades is relatively high. The U.S. Geological Survey puts the chance of a magnitude 6.7 or larger on the Southern San Andreas at roughly 59% over the next 30 years.

So if you’re going to Disneyland, Arizonans, in the next 30 years, just keep that in mind as you’re picking your hotel and figuring out where you’re making your pit stops. But it’s nothing new. We’ve known that the San Andreas is a potential very big seismic risk all along.

LAUREN GILGER: Right, so I mean this study, this research of yours that just came out is getting a lot of attention, particularly in Southern California, which makes a lot of sense. I wonder if you can just speak to the importance of doing this kind of research. I mean, there’s nothing you can do to prevent this from happening necessarily, but I’m certain there’s a lot you can do in terms of preparation.

BRIDGET SMITH-KONTER: I hate to use the term “the big one,” but we’ve all grown up hearing it.

LAUREN GILGER: Right. Yes.

BRIDGET SMITH-KONTER: And these results really reinforce what earthquake scientists have long been saying, that Southern Californians face a significant and growing seismic risk. And so now is the time to prepare.

And so for city managers and emergency responders, really the implication of this work is that for planning scenarios, they should really be thinking about including cases where we have these joint ruptures because that impacts more people.

And they should consider this not just as a remote worst-case scenario, but as a realistic possibility given these current tectonic stress conditions.

For people like you and I, for the public, this means knowing how to prepare for and respond during an earthquake. Have an updated earthquake kit. You should have a family and friends communication plan, and you should really familiarize yourself with evacuation routes. And we were all taught as kids to stop, drop and roll during a fire.

LAUREN GILGER: Right.

BRIDGET SMITH-KONTER: But the earthquake community has gotten together and tried to message a different phrase for earthquake response, and that is “drop, cover and hold on.”

So drop to the ground, make yourself as small and compact as possible, cover your head and neck, and hold on to something strong and sturdy or try to get under a table or hold on to a chair or something, and look out for falling debris. So don’t run out of a building, stay put and make yourself as small as possible so that you can minimize the impacts of shaking.

LAUREN GILGER: So if we’re looking at, “the big one,”— right, Bridget? Would this be something that impacts us here in Arizona?

BRIDGET SMITH-KONTER: We could easily feel shaking here in Arizona. Certainly our friends in Yuma will feel the impacts. But it’s really those 20 million people in Southern California right around Los Angeles and San Diego that would feel the largest brunt of this hypothetical but certainly reasonable large earthquake that could occur.

LAUREN GILGER: Yeah, OK. We’ll leave it there. Bridget Smith-Konter, professor at NAU School of Earth and Sustainability, part of their Radian Center for Remote Sensing there at NAU. Bridget, thank you for coming on, thank you for all of the details here. This is, you know, scary but important stuff.

BRIDGET SMITH-KONTER: Thank you, Lauren. It’s been a pleasure.