UCR earthquake physicist Abhijit Ghosh discusses the “Big One”

 

 Courtesy of Youtube
Courtesy of Youtube

Abhijit Ghosh is an assistant professor in UCR’s Department of Earth Sciences with a research focus in seismology and earthquake physics. He holds two master’s of science degrees to his name in geology and earth and atmospheric sciences, respectively and earned his doctorate in geophysics from the University of Washington in 2011. Ghosh’s list of awards and honors is a lengthy one, beginning with a book grant awarded in 2003 by the Geological Mining and Metallurgical Society of India and ending with his postdoctoral fellowship awarded by the U.S. National Science Foundation in 2012.

What can be gathered here is simple: Ghosh knows a heck of a lot more about earthquakes than we do. Considering this, and Governor Jerry Brown’s issuance of an official earthquake warning in late September, we sat down with Ghosh for a lengthy conversation on earthquake prediction, dispelling earthquake myths and California’s looming behemoth known as the “Big One.” Read the discussion in full below. The interview has been lightly edited for clarity.
Myles Andrews-Duve: So, where should we start?

Abhijit Ghosh: Well, first (you) must know we can not predict earthquakes. So, that’s the first thing. What we try to do is to forecast earthquakes, but we have to be very cautious when forecasting earthquakes more than, let’s say, weather forecasting, because of the socioeconomic impact.

Myles: Could you elaborate?

Abhijit: Well, what I mean by that is … let’s say there is a forecast of 80 percent chance of rain today, okay? And let’s say you ignore it, then you forget your umbrella. So what’s the worst that can happen?

Myles: You get drenched.

Abhijit: Right. But if we forecast an earthquake and it does happen, but nobody really cared, thousands of people can die, multiple cities can be destroyed … so if you think about the effects and socioeconomic impact of the results, it’s hugely different than other forecasting. Unless, of course, the margin of error is really, really tiny, we can not really predict or forecast earthquakes.

Myles: Hm, yeah. Giving it more thought, it seems highly illogical to ever suggest we could predict one.

Abhijit: Right. And, to be clear, there are multiple things involved in the effort to predict them. It’s about where, when and how, but it also involves how big it is. Because let’s say, hypothetically, I predict a magnitude 2.0 earthquake happening somewhere at some time, you probably wouldn’t care. If I predict a magnitude 8.0 earthquake, that is important. You need to know and you will care. So, size is very important and that’s why the “Big One” attracts more attention.

Myles: So, if we can not predict an earthquake, what makes the “Big One” in California imminent?

Abhijit: So the San Andreas Fault (SAF), in total it runs from the Salton Sea to Northern California, it’s 1,300 kilometers (800 miles) long along California. So, it’s a massive fault. To give you context, in 2004 there was a magnitude 9.2 earthquake in Sumatra. Again, magnitude 9.2. It created a worldwide tsunami and killed an estimated 200,000 people worldwide. That earthquake ruptured a fault that is similar to the length of the SAF. If the entire fault ruptured at the same time, it can create a very, very large earthquake.

Myles: That’s … scary.

Abhijit: It is very scary. But, there is good news.

Myles: Please share!

Abhijit: The good news is, we think that the SAF is segmented and, very broadly speaking, there are two main segments: Northern SAF and Southern SAF. What do I mean by segments? I mean that these two parts of the fault rupture separately, not together.

Myles: And are these of equal length?

Abhijit: More or less. Though they are separated by smaller segments that may or may not produce damaging earthquakes. So, the northern segment last ruptured in the 1906 San Francisco earthquake and the southern segment — that is, “our” SAF, which is essentially 13 miles from UCR’s campus — last ruptured in 1857. Based on paleoseismic study (study of earthquakes that happened hundreds of years ago), we think more or less, that the recurrence interval is about 150 years or so — meaning, roughly every 150 years on average, this fault will produce that magnitude of an earthquake. And that magnitude here is, about 7.8. We think that’s the largest earthquake we can have in the Southern SAF. Again, there are some uncertainties, so it is not fixed, but that is most likely the largest magnitude earthquake the fault could have.

Myles: So, doing the math …

Abhijit: Well, if we follow the simple logic, we take the year 1857 with a recurrence interval of 150 years, add 1857 plus 150 and …

Myles: We’re at 2007.

Abhijit: 2007, right. But, wait, this has already passed. So what does that mean? It’s overdue. And as I said, there is a margin of error, it’s not always 150, it’s an average value. Sometimes it may come a little earlier, sometimes it may come a little later, but the point is, the fault is ready to rupture to produce a large, damaging earthquake. When you hear the “Big One,” that is what they are referring to.

Myles: So in this time of a nearly 10-year delay, what is happening to the fault?

Abhijit: It is building “moment.” Or, storing more energy waiting to be released. But, there are other ways for that energy to be released. Smaller earthquakes that happen all the time (and the rate of which fluctuates from time to time), they also release some of this stored energy. There are creep movements, meaning slow movement of the fault, that also may release some of the energy. So it’s not like all of the energy that’s been stored over the last century and a half is going to be released during the “Big One.” There are constant, almost quasi-continuous releases of energy.

Myles: So, was the recent 2.8 earthquake in Loma Linda an example of what you’re referring to?

Abhijit: Yes, absolutely. Tiny earthquakes release some of the energy, right.

Myles: Any chance we can get the “Big One” in small doses, then?

Abhijit: (laughs) Well, the problem is, one magnitude increase translates to about 30 times increase in moment. For example, let’s say you go from a magnitude 2.0 to magnitude 3.0, the moment released by the magnitude 3.0 is 30 times greater than the 2.0. So, following the same logic, if you compare a magnitude 5.0 event to a near-8.0 event such as the “Big One,” the 8.0 would release 1,000 times more energy. So it’s like having 1,000 magnitude 5.0 earthquakes at once.

Myles: Also very scary. But it’s often suggested California has infrastructure in place to better sustain such a potentially devastating earthquake, is this something you believe rings true?

Abhijit: So, there are building codes. And what scientists can do is estimate the maximum magnitude and predict, with some uncertainty, what the ground movement will be. Here (pointing to his desktop) is a computer simulation of a magnitude 7.8 earthquake along the southern SAF. The red means very high intensity of ground shaking, really high as in everything will be destroyed … So, builders know what they need to do if they want to build a building near here and want to protect their building from a potential earthquake. That’s the process engineers follow to make building codes. This does not mean, that if you follow the building codes, it will not be damaged at all but it can significantly reduce the fatalities. So, more modern buildings should have updated building codes and would be much more resilient than older buildings.

[embedyt] http://www.youtube.com/watch?v=blTx92TuWHA[/embedyt]

Myles: So, as a state, we aren’t collectively prepared?

Abhijit: There is no short answer for whether we are prepared. For example, looking at the simulation, there are strong channelized ground motions in the Los Angeles (LA) Basin because LA sits under thick sediments, which amplify the magnitude of seismic waves. This means the amplitude of ground motion will be high and it will ring for a longer period of time. But, again, the good news here is we can predict this. We know what the ground movement will be so we can prepare for it.

Myles: But even with all this information, it remains impossible to gather an idea of when the next devastating earthquake will hit.

Abhijit: Well, we cannot predict earthquakes, sure. But we can give you a quantitative estimate of the likelihood of having an earthquake of a certain magnitude in a certain time period and a certain place. This gives you some idea of the hazard. To give an example, in the Southern California region, the likelihood of having a magnitude 7.0 earthquake in the next 30 years is 75 percent. Over that same span, the likelihood of having a 6.7 earthquake (as was the 1994 Northridge earthquake), is 93 percent, which is pretty, pretty high. So, when we say the likelihood is “high” this is what that means.

Myles: Was it data such as this that offered reason for Governor Brown’s warning issued in September?

Abhijit: Well, his warning was prompted by a swarm of around 200 earthquakes under the Salton Sea on September 26. But those earthquakes were not on the SAF, they were actually a little beyond the tip of the fault. Often, other ruptures can affect faults nearby.

Myles: How should we react to such a warning being issued?

Abhijit: Take it seriously. Make sure you are prepared. There are three steps in this preparation. One, know what to do when an earthquake occurs: Drop, cover and hold on. Two, have an emergency kit prepared. This includes flashlights, water, a radio to communicate and energy bars. Third, make sure you have prepared your family and friends. Make sure they know what to do in the instance of an earthquake and that their homes are secured.

Myles: So essentially, everything they told us in elementary school was true.

Abhijit: Yes, it is all very true and also very important.

Myles: Well, if that’s the case, weird weather must also mean an earthquake is coming too!

Abhijit: There have been studies looking to connect weather and earthquakes and other potential predictors, but there is no connection, no.

Myles: I guess my dog can’t predict them either then.

Abhijit: (laughs) No, sorry.

Ghosh, in partnership with UCR Campus Emergency Manager Lisa Martin, is coordinating UCR’s Great California ShakeOut drill held at the HUB upper plaza on Thursday, October 20 at 10:20 a.m. The drill will also feature a booth offering a demonstration on how earthquake faults operate and numerous interactive displays.

 

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