Why Southern California Is Ripe For A Large Earthquake

Speaker 1: 00:00 So this Easter Sunday marks nine years since an earthquake was felt in southern California, in Mexicali. There was a lot of damage after that. 7.2 earthquake happened almost a decade ago. But as we close in on a decade, a new study shows that we are in an earthquake drought and here to talk about what an earthquake drought is. Paleo seismologist, Dr. Thomas Rockwell. Dr Rockwell, welcome. Thank you. Glad to be here. So researchers from the US Geological Survey say there had been no major ground rupturing earthquakes along these three key faults over the past 100 years. Why is that significant? Speaker 2: 00:35 So almost all of the motion between the Pacific and North American place, which is almost 50 millimeters a year, occurs on the fault of the San Andres system. And principally in southern California, that's the San Andres, the Senate sent on Elsner faults and northern California, the San Andrei as the Calaveras, the conquered and the Tacoma, the Rodgers Creek, there's a whole system of faults up there as well. And in the past hundred years, none of these faults have ruptured and in a large earthquake, whereas in the previous thousand years, there had not been a period of a hundred years when one of these faults has not ruptured. Speaker 1: 01:10 Well. So given the passage of time then, is it fair and accurate to say that we're overdue for a large earthquake? Speaker 2: 01:16 We don't like to say overdue, but to show you how bad it is or why we're so concerned. I'm sure you've heard people talk about the San Andreas is overdue or due for an earthquake. So from dating pastor earthquakes from the geology that we do, uh, we know that the average recurrence interval, the average time between large earthquakes on the southern most San Andres is about 180 years. It's now been 300 years since the last large earthquake. So this has led to the idea that maybe we're overdue a, but there's caveats to that. Uh, in any case, it's very clear that the southern San Andres is certainly ripe for a large earthquake. And the question is why hasn't it happened? And then why hasn't a large earthquake occurred on the Santa Santa's? We've had moderate earthquakes. Some people may remember earthquakes on the southern Santa Santa's fault in 1968, uh, there was an earthquake on the imperial fought in 1979 so there have been some earthquakes, but they really, they released so little energy. It's the big ones that really, uh, really released most of the energy. And those are the ones who are worried about. Speaker 1: 02:24 So which fault line should be of the greatest concern for those living in San Diego? Speaker 2: 02:29 Well, for San Diego is the Rose Canyon fault. No question. But from our recent studies, and we just had a paper come out in the last few weeks, uh, the Rose Canyon fault only produces these relatively large earthquakes in the high magnitude six is about every six to 800 years. And the last one was just pre mission. So in the last 300 years, so it's one that we're, that's not one that we say is overdue by any account. On the other hand, a strong earthquake on one of the offshore faults or the Elsinore was certainly pretty strong shaking and San Diego, uh, the one that we're really worried about is the San Andreas and I'm not so worried in San Diego for the San Andres fault, that one, if it ruptures from south to north modeling, studies have shown it does a tremendous amount of damage in Los Angeles. So that's the main concern because that's where the main population center is. Hmm. Speaker 3: 03:21 [inaudible] Speaker 1: 03:32 you know, I, I remember the, the bay area earthquake, seeing that on television. I was living back home in St Louis, Missouri. And shortly after that happened we had someone come through who predicted that there would be a major earthquake in Missouri and there were all of these measures taken to secure school buildings and school desks and things like that. Is there really a way to predict when an earthquake will happen? Speaker 2: 03:57 No, we cannot predict earthquakes. And in terms of prediction, we're talking short term prediction. What we do is we study the past history of earthquakes on a fault. And then based on the statistics, we forecast the probability or the likelihood of an earthquake occurring and say a 30 or 50 year window that's very different than these, I don't want to necessarily call them quacks, but uh, uh, people who make these short term predictions based on uh, the behavior of their cat or because they had a bad dream, you know, Jean Dixon, the whole California is going to slide into the ocean. No, he can't do that. Uh, continental crust is lighter than oceanic crust. It is impossible to do that. So, you know, there's a lot of these predictions that happened because somebody has a nightmare or something and they think there's something special maybe in terms of psychic ability or whatever, I don't know. But I would not put much faith or any faith in those types of predictions. However, if you can base it on science, if you can base it on what we know has happened in the past, we can forecast a likelihood that it will happen within a window in the future. A 30 year window is too small of a target to capture a broad statistical distribution. But we can make those and we have made those. And those are all, uh, uh, published in an accessible to the public. Speaker 1: 05:24 So better to put your faith and preparedness then predictions. Speaker 2: 05:28 Absolutely. Everyone should be prepared for an earthquake. Even the Rose Canyon fault, we say, you know, just had a big earthquake a few hundred years ago, uh, but it could still have a magnitude six at any time. And we had one in San Diego historically, uh, in 1862. And we didn't have a newspaper at the time. We didn't have KPBS, but it was called the day of terror in the Los Angeles Star. So it was about a magnitude six and it ruptured in the town area. We know that from the work we've done in old town, we found the actual, the displacement from that earthquake at the surface and at fault historical deposits. So a six in any time was still do a lot of damage in San Diego. Speaker 1: 06:11 I've been speaking with Dr. Thomas Rockwell, a Paleo seismologist at Sdsu. Dr Rockwell. Thank you very much. My pleasure. Speaker 3: 06:23 Okay.