UCSD Seismologists: Recent Quakes Unlikely to Trigger `Big One’
KPBS Midday Edition Segments / July 8, 2019
Speaker 1: 00:00 Many of us living in southern California haven't thought much about earthquakes recently. That's because it's been years since the last big one here, but that changed at the end of last week when rear back to back. Major earthquake struck near the town of Ridgecrest in the Mojave Desert 225 miles north of San Diego. Joining me to discuss what this might mean is Frank Vernon, he's a research geophysicist at UC San Diego's Scripps Institution of Oceanography. Frank Vernon, welcome to midday edition. Thank you for having me. Well, let's start with the magnitude of these earthquakes. The first was a magnitude 6.4 last Thursday morning, followed by a magnitude 7.1 on Friday evening. How unusual is it to have big quakes like these back to back? It's unreasonably unusual. It's about one chance in 20 as the usual percentages that you have an earthquake and then followed by a bigger earthquake. So this is about 5% chance on that just happened this time.
Speaker 1: 00:55 Now what's known about the nature of these two big quakes, a about the faults where they occurred? Well, these are actually occurring on false that we're not a well known beforehand. And so we're learning a lot about the faulting structure in this particular region right now based on the seismicity and where these aftershocks are occurring. And they were, uh, were they parallel or were they a perpendicular or the first earthquake? Looks like it was on a north east trending, a left lateral fault, which teed into what became the right lateral main 7.1 shock. And let's talk about, uh, we talked about the magnitude of those. How much bigger is a 7.1 and a 6.4 or five? I mean, it doesn't sound like much, it's just a few numbers in between, but the magnitude is quite different, right? Well remember that magnitude is on a logarithmic scale.
Speaker 1: 01:43 So if you actually translate that into energy, it's about 11 times more energetic than the previous, than from the 6.4. Okay. A much bigger one. We all talk and worry about the big one that would occur along the Senate. Dreyfus fault. Why is that fault? So worrisome? And these two quakes we're talking about, could they trigger something along the San Andreas? So what people can generally considered to be the big one that will we expect to happen at some point is on the San Andreas fault rupturing between Bombay beach and going up past um, Palmdale and now up into uh, along those Andres for a couple hundred kilometers. So, which would be our place in our a replicant of these 19 or 1857, a 7.9399999999999995 to honers quite for instance, this earthquake care is fairly far away. It's maybe 60, 70, 80 miles away from the San Andreas fault. And if you look at that in historical times, and in 1992, we had the landers earthquake, which was much closer, same size as this one effectively.
Speaker 1: 02:46 And in 1999, we had Hector mind, which was also much closer San Andres fault. Neither one of them did anything that actually were causative of any other earthquake followup on the San Andrea. So this earthquake being much further away, it would be as less likely that something is going to happen. Significant damage has been reported in Ridgecrest and the small town of Trona nearby have 7.1 quake were to hit in the middle of San Diego or the Los Angeles area. Well, there be a lot more damage. I mean, we can look at um, what happened in the North Ridge in San Fernando Worth Casey and Allie and the uh, and back in the last century now and then, uh, and thankfully we haven't had anything that big in San Diego and recorded history. All right. And these two earthquakes, especially the bigger one on Friday evening fell for hundreds of miles. Explain how some people fires ways San Diego to Ana did the South Las Vegas to the east, we'd Phoenix, uh, up, uh, farther north.
Speaker 1: 03:42 A lot of people can feel quakes way far away and then others in the same area, same neighborhood almost don't feel anything. It's going to depend on what you're doing. It's going to depend on where you're situated, what the geology is, can dependent on how type of structure in and how high up in the structure you are. If you're in a tall building, you're more likely to feel it because of the shaking, the long period shaking from the surface waves. If you're near a sedimentary, really soft geology, you will be more likely to feel it. If you're up running around or driving, you won't feel it. So, okay. Just depending on real specific to where you are. All right. And in the aftermath of these earthquakes last week, a, what are you and other, uh, seismologists learning? What sorts of things are you looking for to help understand, maybe even predict a big earthquake?
Speaker 1: 04:27 Well, I don't see a whole lot in here that's going to lead me to a prediction. I see a lot that we're going to learn about the structure of the fault, how it evolves as a function of time between the two events. Why the seismicity evolved in a way did the evolution of the rupture pattern and the directivity of it. These are the types of basic physics problems that we're trying to address right now. And Are we a, in this day and age, have a lot more, uh, sensors and a lot more, uh, equipment to be able to gauge these quakes than in decades past? Absolutely. We've, uh, thanks to the, uh, USG gs and their, and their development towards an earthquake early warning system, they put a lot more centers out, a lot more. A data's coming in real time and that'll give us a, a real treasure trove of information that we can mind going forward.
Speaker 1: 05:12 And can we expect more earthquake activity in the Ridgecrest area following these two big events? Well, there's certainly going to be a good long aftershock sequence going on after that, but I mean, the expectation is that we'll be tapering off as a function of time over the next several months. I mean, but if you remember the 2010 earthquake at Kuka file, my or we also had aftershocks going on for six months or a year later that were actually noticeable. Okay. And that was the Easter Sunday [inaudible] people will remember from back then. All right. I had been speaking with Frank Vernon. He's a research geophysicist with Scripps Institution of Oceanography. Thanks, Doctor Vernon. Thank you.
Last week's back-to-back quakes are not likely to trigger the "Big One" in Southern California, according to University of California San Diego scientists.