Explaining Earthquakes

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Q: What is your area of expertise?

MARY LOU ZOBACK: I got interested in geophysics and studying earthquakes in the early ‘70s when I was studying in college. The theory of plate tectonics was brand new at the time. Although the basic tenets had been spelled out, people were discovering new things all the time. That was a really exciting time, because it all of a sudden it integrated several centuries of geologic observation, some of which had no good explanation. But it provided this unifying theory that an awful lot of things could be fit into.

And one of those observations is right here in California. Geologists had mapped very distinctive volcanic rocks on both sides of the San Andreas fault, which they knew about. Then they were able to age date them, and they said, “Well, these have to be the same rocks. But they’re 200 miles apart. How can that be? Because if the fault moved 200 miles, what happens at the ends?”

We had no solution for what happens at the ends of these faults. Because we know they didn't go all the way around the world, so it wasn’t like a shift around the world. But plate tectonics showed that the San Andreas part was part of a plate boundary. And yes, thousands of kilometers of motion have occurred, but these plate boundaries form and reform, and the motion transforms to a different kind of motion as you go around a bend in a plate boundary.

Q: What is a geophysicist?

MARY LOU ZOBACK: Geophysics really merges the study of geology -- the study of the Earth’s surface, the rocks, the processes that created it -- with the physical understanding of the forces acting on it, as well as things like the gravity and magnetics of the Earth’s interior. And I think I was really drawn to the field because I saw a chance to use the more physical approach and mechanical, too. It’s more of an engineering approach. Also, we pretty much understand the earthquakes along plate boundaries like the San Andreas. As the plates move past one another, that’s accommodated by earthquakes, but we don't understand so well what happens in the middle of plates and why we can have potentially really large earthquakes there.

Q: What is magnitude?

MARY LOU ZOBACK: I think one of the biggest confusions in the public’s mind is this whole question of earthquake magnitude and why does it change. I think a lot of the reporting has talked about a Richter magnitude, and I think there’s a sense that there’s some “scale” around, [that there’s] a Richter “ruler” that people use.

In fact, the Richter magnitude was based on actually measuring the amplitude of seismic waves as measured on a seismograph. And that worked fine for small earthquakes in California. But those instruments couldn’t record really large earthquakes. When we started getting modern digital instruments, all of a sudden, we had enough bandwidth to record really large ground motions as well as recording small motions with a lot of fidelity.

But we needed a better, more robust measure of magnitude. And that’s now most seismologists and geologists agree to use something we call a moment magnitude. All that means is that the magnitude of the earthquake is related to the amount of energy released, as measured by the size of the fault that broke, and the amount of displacement on it. The nice thing about that is that magnitude can be estimated from direct observations, and it’s also measured by looking at variations in the frequency content of the seismogram. So there’s two different ways of measuring it, and we can get pretty consistent measurements.

Q: What is the forecast for the Hayward fault event?

MARY LOU ZOBACK: A whole California-wide forecast came out, and the Hayward fault was the second most dangerous fault in the state of California. And I think they assigned about a 30% likelihood of a major event in the next 30 years. The highest likelihood was in Southern California, along the Southern San Andreas, where I think they estimated about 60% chance of a major earthquake. On that stretch of the fault, we’ve got geologic evidence that on average, there’s been a big earthquake every 100 years. And we know there hasn’t been one there from historic records for 300 years.

Q: What is the length between the Hayward and the Calaveras fault?

MARY LOU ZOBACK: The Bay Area has a whole series of sub-parallel faults. And traditionally, we had mapped the Calaveras fault to the south, and then we had the Hayward fault further north. And they seem to come close together, but never really completely join.

However, some recent studies by scientists at the USGS looked at what we call micro-earthquakes, or very small earthquakes, found a really surprising result. Although at the surface, you can’t see any connection between the two faults, at depth these small earthquakes indicate the faults are actually connected. Now that’s really important because typically the size of an earthquake depends on the length of the fault that ruptures. And the maximum earthquake we could have if the entire Hayward fault ruptured is about magnitude 7.1. But if you add an earthquake that would continue on to the Calaveras fault, you could get a substantially large earthquake, maybe about a 7.4, which would have devastating impact across the region.

Q: What is the range of damage?

MARY LOU ZOBACK: There’d be a lot more damage than to the south. And what we’ve also learned from recent earthquakes is that the actual rupture on the fault plane -- that is, during the earthquake when the fault is slipping rapidly -- that rupture doesn’t happen all at once. It progresses along the fault. And what we’ve learned recently is that the direction that the rupture progresses, the so-called propagation direction, you actually focus seismic energy in that direction.

That’s important because if the earthquake began to the north, say near Point Pinole on the Hayward fault and ruptured south, it would be pushing energy into San Jose and into the valleys that underlie San Jose. What happens is that energy gets trapped in those valleys and just bounces back and forth, so it really extends the length of time of shaking and it actually amplifies the seismic waves.

This phenomenon, which we call directivity, also indicates that the 1906 earthquake, as bad as it was, was probably a best-case scenario for the Bay Area. The 1906 earthquake began offshore from San Francisco, and it ruptured in two directions at once. It ruptured to the northwest and to the southeast. Both of those ruptures carried energy out of the Bay Area away from the main population center. We’ve been doing modeling 3-D computer modeling and if that same earthquake began all the way to the north, we’d have tremendous impact in the Bay Area, as all that energy got pushed into the Bay Area.

Q: Why is there anticipated such a major offset?

MARY LOU ZOBACK: The size of a potential earthquake depends on the length of fault that ruptures. Now in 1906, the San Andreas ruptured 300 miles; that was a huge earthquake. The Loma Prieta earthquake only ruptured 25 miles. The Hayward fault will be something in between. The fact that we have a very long fault and we have a lot of evidence that that fault moved in the past was something like a magnitude 7 event, and that’s based on the fact that we can actually see evidence of past surface ruptures.

Q: What are the lengths of time between ruptures?

MARY LOU ZOBACK: By literally digging trenches across faults, we get the past history of movement by looking at offset soil layers. We’ve been fortunate that for the Hayward fault that we have a very good record that goes back about 2000 years. And by dating the different layers of soil that are offset, we’ve been able to come up with an average -- what we call recurrence or repeat time -- for that earthquake. It’s roughly about 140 to 150 years. The last big event was in 1868, so we can do the math and find out that this shear is coming close to the anniversary of when we might expect that earthquake again.

Q: Why is the Hayward fault so dangerous?

MARY LOU ZOBACK: It has potential for very large earthquakes, and it runs right through a very built-up area. Unfortunately, in the Bay Area, we’ve got an awful lot of older residential buildings. We typically find these in the downtown areas of San Francisco, Oakland or Berkeley. They could either be what we call “soft stories”, where the ground floor is a garage. When the whole ground floor is open and you get very strong shaking, that could hold a building upright. But when the building starts moving backwards and forwards, that soft first story collapses.

Most of the people that were killed in Los Angeles in 1994 were killed in relatively new apartment buildings that had open garages. The older buildings have a similar style, and then there’s also old concrete frame buildings built largely to 1960’s. All these are collapse threats. Now, not all of them will collapse, but many of them will be damaged severely.

There’s going to be an awful lot of damage and destruction. Roadways will be shut down, the soft water-saturated sandy deposits along the margin of the Bay where we have airports, approaches to bridges, they’re likely to liquefy, ripping apart the roads. So I think we can we assume transportation networks will be completely disrupted and destroyed. Water will likely be shut down. There will be fires because gas lines will rupture. There won’t be enough water to fight the fires. Power will be down. Communications will be down. There won’t be the internet, at least for a while, most likely.

One of the other major risks for the Bay Area with a big earthquake on the Hayward Fault will be the transportation systems. Everything from the freeways that cross the fault will be disrupted. The BART train crosses the Hayward fault twice, and actually a lot of the stations are right along there. [The Transbay Tube] runs underneath the Bay; BART went to the public a couple of years ago to ask for a bond measure for a retrofit, admitting that they feel it’s likely the Tube would fail. By failing it would mean it would crack and water would go into the Tube, which would not be good if people were there. The Hayward fault also runs right across the opening of the Caldecott Tunnel in Oakland. All these things are going to be offset five, seven, eight feet laterally, so they’re not going to be functioning.

Q: What is the risk of post-earthquake fires?

MARY LOU ZOBACK: In 1906, most of the damage was caused by fires. But fire following earthquakes is always a huge threat, and that’s because particularly here in the Bay Area, we’ve got lots of pipelines that are running underground, both gas and water. We’ve got a bay, we’ve got wet, sandy soil, and when subjected to very strong shaking, that sandy soil liquefies, literally turns to something like quicksand and that flows and it rips pipes apart. So you have gas pipe leaks, so you have a trigger. You’ve got water pipe leaks, so it really hampers your ability to fight the fire.

Q: How many people would be exposed to an event?

MARY LOU ZOBACK: If we actually look at what we call the footprint of this earthquake -- the area that would be strongly impacted -- that includes roughly 5 million people in the counties around the Bay Area. And if you look at the total value of the buildings, their contents, we’re talking about something like $1.4 trillion -- a huge number. So the losses are relative to that value.

Q: Why do you live here?

MARY LOU ZOBACK: I think all of us get this question: “Why do you live here knowing all of this?” Well, we’re all making decisions every day about what risk we’re willing to accept. And it’s far riskier for us to drive on Highway 101 on a daily basis than to worry about an earthquake, because the likelihood of being involved in an accident is much higher. I think the difference, though, is that we all think we can manage our own personal risk, how we drive, and that sort of thing. We can’t control when the next earthquake will occur or how large it’ll be. And I think the frightening thing is that when we start thinking about the ramifications and the true impacts, it’s so frightening that most of us are in denial, which means that we don't bother to get earthquake insurance, and we are sort of betting that it’s not going happen to us.

Q: What advice would you give to Bay Area residents?

MARY LOU ZOBACK: I hope one outcome of the publicity about the earthquake threat in the Bay Area is that rather than people saying, “It’s too awful to think about, I can’t deal with it,” I hope that most of us are thinking about what we can do to control our own situation. Be prepared, and you know, if you lose your home as a result of a natural disaster, you’ve sort of lost everything.

There’s an awful lot that we can all do to make our situations better. Have the necessary supplies. Expect that you are going to be responsible for your family and your neighbors. Have a plan. Recently there’s been a lot of talk among seismologists and engineers that probably the best goal for the Bay area would be to have most people be able to shelter in place. That means that rather than go to shelters where we’ve seen horrific situations, stay in your home. You may not have water, you may not have electricity, they may drop off Port-a-Johns at the end of the block, but if your home survived the earthquake, you’re generally better off sheltering in place than wandering around in all the consequential things that could happen related to that.

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