Southern California's 5.2 Earthquake: Geophysicist Explains the Aftershocks

Courtesy of Temblor
Friday’s earthquake was centered near the desert town of Borrego Springs, population 2,989. It was a magnitude 5.2 temblor, but it spawned more than 600 aftershocks along a vast network of interconnected faults, leading experts to question when, and how big, the next quake will be.

Former U.S Geological Survey earthquake geophysicist Ross Stein discusses the quake that rippled across southern California in the early morning hours of Friday. No casualties were reported, but the quake could portend bigger, more ominous shaking to come, says Stein.

Friday’s 5.2 earthquake along the San Jacinto fault network, part of the San Andreas system, rippled across southern California communities in the early morning hours, causing no casualties, but raising questions about what the quake could portend.

Ross Stein is an earthquake geophysicist who spent 34 years with the U.S Geological Survey. He left in 2014 to start Temblor.net, a startup that aims to help people assess their own risk from earthquakes, and take steps to mitigate that risk. He is also a consulting professor at Stanford, with an upcoming course this fall. He helps scientists and P.h.D students learn how to better convey the results of their work to the public.

He spoke to The Hollywood Reporter on Friday morning. Excerpts:

What’s the big picture here?

Faults are like dummies and dogs, they don’t know their names. For all practical purposes they’re the same. In this part of California, about half of the quakes were on the San Jacinto fault. The San Andreas fault is relatively smooth. The San Jacinto fault is kind of gnarly and grungy, and maybe it has a harder time knocking off really large earthquakes. In the historical record, which is maybe 150 years long, there hasn’t been anything 7 or larger on the San Jacinto fault. There was a 6.8 in 1890 just southeast of where this one occurred by about 10 miles. And there have been a lot in the vicinity in the low 5s. The real question is: Could the San Jacinto fault knock off a really large one? A 7? A 7.5? It’s anyone’s guess. Many would say that fault is not continuous enough to get a real freight train of an earthquake going. But we’ve been fooled before. The 1992 Lander’s earthquake was a 7.3, and it struck along faults that didn’t look like they were capable of linking up — but they did. That’s humbled us. What’s interesting here is that it produced a huge spread of aftershocks. Some are on the main strand, but most aren’t. This could be the harbinger of something larger. It’s always possible. There is plenty of real estate available, and we haven’t had anything of real size in last 150 years.

What’s the significance of this quake?

With any earthquake, regardless of size or location, there’s always a possibility of something larger. Something between 2-10 percent of all quakes turn out to be foreshocks of larger quakes to come. So there’s a small probability that this is a foreshock of a 6. Once you have a 5, the possibility of a bigger earthquake increases over what it was before. But this quake is peculiar in some ways. It had lots of aftershocks that weren’t on the fault, but on bookshelf faults and secondary features, and this suggests the entire region is close to failure.

Close to failure? That sounds ominous.

Imagine you put some Home Depot non-skid on your desk, and then you stuck a brick on it. Then you attached a bungee cord to the brick and started pulling the bungee cord sideways, the brick is going to sit there until the bungee has enough strain that the brick suddenly slides. Then you start pulling again until the cord is taught again. The brick isn’t going to move until it gets close to failure. We have this situation with all these shocks, and it suggests that a lot of the bungee cords are ready to go. Faults have to be close to failure. If not, we wouldn’t have had the aftershocks.

Is this unusual, the rate of the aftershocks?

The rate may be typical, but they’re very widespread, and that’s interesting. Magnitude 5s are typical in this area. There have been four others since 1980 of same size and in the same general location. That’s not unusual. It makes sense. It’s an active area. What’s unusual is that the bookshelf faults that connect the strands create opportunities. And there’s a lot of run-out to the northwest, with a significant portion that hasn’t erupted in 150 years. If that were to happen, it’s not unexpected and it wouldn’t be a surprise.

But the aftershocks were surprising here?

There were an awful lot of them — at least 600 aftershocks spread over an area of about 10 miles wide. The rupture area for a magnitude 5 should be 2x2 miles. So that’s odd. They were widely distributed and some or most of them are on these weird, secondary bookshelf faults.

So they’re connected?

These parallel faults, the Clark, the Coyote and others are linked by the bookshelf faults, and all have been excited by this earthquake. That could lead to rupture — or, this could be the end of the story, which is what happened the last four times. Another way to look at it: Since 1980, which is when the good records start, we’ve had one of these a decade and none have triggered something bigger. So there’s a three in four chance that this won’t trigger something bigger. What we don’t know is if this is scattershot and broad, or similar to previous events. We don’t know that. A good gambler would say that there’s at least three of four chances that this is the end of the story. But there’s a one chance in four that another shoe will drop.

So you’re a gambler, where are you putting your money?

I’d say the area to the NW looks more likely. My chance of getting that right is about 51 percent. It’s so hard to call any of these things.

So NW? What’s up there?

Not much. This is the desert area. There’s Anza, then idlewild, then Hemet, then San Jacinto, from which the fault takes its name. They’re all relatively remote places until you get to the agricultural area of San Jacinto.

Any threat to L.A.?

No, I wouldn’t think so. It’s a long way from L.A. Riverside, San Bernardino and Redlands are communities astride the San Jacinto fault. If a large earthquake occurred, then those would be strongly shaken.

What should people be doing now?  

Our app, Temblor, allows anyone to go to any location to tell you what your hazard is. It lets you see the faults and shows how you can reduce your own risk. These things are real, they do happen. Most of us are in fairly deep denial. Temblor uses the best available data. We’re really trying to help people understand what their risk is and how to reduce it.

You always hear that you can’t predict when the big one is coming — but can you predict when the big one is coming?

A: No. it’s such a humbling science to be in. The San Jacinto fault is slipping at 10 mm a year; that’s a third to half an inch a year. With a large quake, in three seconds it speeds up to 5,000 miles an hour. As far as we can tell, there’s very little that happens before that acceleration. If there’s a zone of acceleration, it’s eight miles down. It’s an extremely difficult target. And no little quake when it starts to rupture knows it’s marked for future greatness. Most 8s start out as 4s. Big ones don’t start out any differently. There’s always a chance a little one will cascade into something bigger. Most don’t.

What’s required for one to become bigger?

All the so-called bricks I mentioned earlier along the fault have to be just about ready to go. Then as the rupture starts moving down the fault, additional stress has to be enough to keep triggering other parts of the fault. Most ruptures are stop-and-go, and the shakiness occurs when it speeds up or slows down. What we don’t know about nature of the rupture front is just staggering. It’s an incredible opportunity for young people to jump into field and figure it out. 

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