How Much Can Seismic History Tell Us About the Next “Big One”?

Egill Hauksson, research professor of geophysics, has been at Caltech researching geophysics and seismology since 1989. In this excerpt from his Caltech Heritage Project (CHP) oral history, Hauksson explains how to think on earthquake time.

David Zierler, director of the Caltech Heritage Project: In looking for patterns in the earthquake catalog going back 90 years, what does that tell us about the possibility that there is something cyclical about earthquakes?

Egill Hauksson: Unfortunately, the historical record is not quite long enough, because the expected average return time of major earthquakes on the southern San Andreas is 150 to 300 years. Here, we have 90 years of data, but ideally we would like to have 900 years so we would be able to analyze numerous cycles to understand the details of the phenomena many times. Just like the meteorologist who predicts the weather every day, he gets a new chance at it and actually sees how predictions have gotten a lot better over time than what they used to be, and much more detailed and much more accurate.

Having said that, there are people who work in the field of earthquake geology who like to dig trenches across the major faults. By studying the walls of a trench, they can find cracks that come up from depth, and some of them don't go very far up, and others go all the way up to the surface. Some of those cracks they identify as being caused by an older earthquake, and that allows them to get information maybe back 2,000 or 3,000 years. They date those cracks in the trench wall by dating charcoal with carbon-14 methods, and that gives an idea of what has happened over the last 2,000 or 3,000 years. That's where the numbers 150 to 300 years comes from, but it still is not a detailed enough view to exactly say what's going to happen next.

Zierler: To square the circle, the idea that earthquakes are fundamentally unpredictable because they're chaotic, how might that work with a broader effort to determine cyclicity within a 1,000-year timescale? In other words, how can earthquakes potentially be cyclic to some degree but also are unknown how they start because of their chaotic nature?

Hauksson: It's not only unknown how they start. I can tell you that there's probably going to be 50 magnitude 1 or greater earthquakes in Southern California today, so we know 50 of them will start. Then, the question is, on any given day, which one of those 50 is going to become a big earthquake. That means it has to start near a big fault that accommodates the rupture. Then we have to figure out, where is this thing going to stop? That determines the length of the fault, the rupture, and the magnitude of the earthquake. So, there are all these aspects. If it starts in the south, say, down by Palm Springs, and ruptures north to Gorman versus rupturing the other way, it may pick different strands of the same fault, and that may result in different behavior. The [United States’] attempts at predicting earthquakes failed in the early 1980s, and, since then, the focus of the community has been more to accept the fact that we are going to have earthquakes and, rather, predict the potentially damaging effects of earthquakes.

A lot of the data that we collect are measurements of ground shaking. How hard does the ground shake in different types of earthquakes? That makes it possible to make maps to say, "Every 30 years or so, you will have these type of ground motions happening." That allows an engineer to design a building or a structure to withstand those ground motions. Also, we know which faults are likely to have earthquakes that may rupture the surface, so we tell people, "Don't build a building across this fault, because it's going to be two buildings at some point in the future." Those, in my mind, are very important things. Similarly, understanding liquefaction, where the ground may shake so hard that it becomes flowing mud, so any building built on that will topple over.

See Hauksson’s full interview here.

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