Career Spotlight: Q & A with Geophysicist Mary Lou Zoback

Mary Lou Zoback is a seismologist and Consulting Professor in the Environmental Earth System Science Department at Stanford University. From 2006-20011 she was Vice President for Earthquake Risk Applications with Risk Management Solutions, a private catastrophe modeling firm serving the insurance industry.

Zoback previously was a senior research scientist at the USGS in Menlo Park, CA and served as Chief Scientist of the Western Earthquake Hazards team.

Dr. Zoback is a member of the U. S. National Academy of Sciences, past President of the Geological Society of America, and past chair of the Advisory Committee for San Francisco’s Community Action Plan for Seismic Safety (CAPSS) program. She is currently a member of the National Academy of Sciences’ Disaster Roundtable.

Q: What is your area of expertise?

MARY LOU ZOBACK: I am a geophysicist and study the forces in the earth’s crust that cause earthquakes. I got interested in geophysics when I was studying in college. It was the early 1970s and the theory of plate tectonics had just burst on the scene. Before plate tectonics, some scientists had proposed the theory of continental drift, recognizing that present-day continents could be “fit together” as pieces of pre-existing larger land masses, or "supercontinents". However, this theory did not explain how the oceans formed or how the continents broke up and moved apart. Plate tectonics, on the other hand, was a wonderful unifying theory based on geophysical evidence including the location of earthquakes, the bathymetry (depth and structure) of the ocean floor, and variations in the earth’s magnetic field. Plate tectonics had new ocean crust forming at underwater ridges and moving continents apart while at the same time old ocean crust was being consumed at “trenches” where this crust plunged deep into the earth’s mantle, generating big earthquakes and melting to form volcanoes. The Pacific “ring of fire” of volcanoes and earthquakes rimming the margin of the Pacific all of a sudden made sense. It was an exciting time as scientists all over the world were rapidly finding additional evidence supporting the new theory.

One of those supporting observations is right here in California. For years geologists had recognized that there were very different rocks on either side of the San Andreas Fault, and had determined that fault was strike-slip, or that the two blocks on either side of the fault slid horizontally with respect to one another. In particular, they had mapped some very distinctive volcanic rocks along the fault, but separated about 200 miles. How can that be? Because if the fault had actually slipped horizontally 200 miles, where were the ends of the fault and what was happening there?

Geologists knew that this fault ended in the Pacific Ocean to the north and into the Gulf of California to the south. In plate tectonic theory the San Andreas is known as a transform fault, part of the plate boundary of the North American plate. Over geologic time plate boundaries form and reform, and at the ends of transform faults the motion transforms to a different kind of plate boundary, usually associated with a major bend in the fault system.

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 the physical properties, like the gravity and magnetic fields, of the Earth’s interior. And I think I was really drawn to the field because I saw a chance to use a more physical and mechanical approach to understanding the earth. It’s more of an engineering approach. We now pretty much understand the earthquakes along plate boundaries like the San Andreas. As the plates move past one another, the stresses in the earth’s crust result in earthquakes, but we still don't understand so well what happens in the middle of plates and why we can potentially have really large earthquakes there.

Q: What is magnitude?

MARY LOU ZOBACK: Earthquake magnitude is a number that describes the relative size of an earthquake. I think one of the biggest confusions in the public’s mind is understanding what earthquake magnitude is and why seismologists give different magnitudes for the same earthquake. Often the media refers to a Richter magnitude, and I think there’s a sense among the public 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 maximum motion of seismic waves as recorded on early seismographs. And that worked fine for small earthquakes in California. But those early instruments couldn’t record really large earthquakes, their recording pens literally pegged out. When we started getting modern digital instruments, all of a sudden we had enough bandwidth to record really large ground motions as well as the small motions with great accuracy. Seismologist then found that the Richter magnitude and other magnitudes related just to maximum motions of different parts of the seismic wave did not adequately measure the size of very large earthquakes.

Seismologist needed a better measure of magnitude. Today most seismologists and geologists have agreed to use a moment magnitude scale, which is based on the “seismic moment”. The seismic moment of an earthquake is related to the amount of energy released. It can be determined directly by the area (length X width) of the fault that broke, and the amount of displacement on it. It’s also measured by looking at changes in the frequency content of the seismogram. So there’s two different ways of measuring it, and we get pretty consistent measurements.

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

MARY LOU ZOBACK: A whole California-wide earthquake forecast came out in 2008, and the Hayward Fault was identified as the second most dangerous fault in the state of California. They assigned about a 30% likelihood of a magnitude 6.7 or larger earthquake in the next 30 years. The highest likelihood was in Southern California, along the Southern San Andreas, where they estimated about a 60% chance of such an earthquake. On that stretch of the southern San Andreas Fault, we’ve got geologic evidence that on average, there’s been a big earthquake every 100 years over the last several thousand years. And we know from historic records that there hasn’t been one there 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 mapped the Hayward Fault further north, trending more northwesterly. And they seem to come close together, within a few miles, but never really completely join.

However, some recent studies by scientists at the USGS looked at some very precisely located small earthquakes, and 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: What we’ve also learned from recent earthquakes is that the actual rupture on the fault plane -- that is, the area of the fault that is slipping rapidly during an earthquake -- that rupture doesn’t happen all at once. It progresses along the fault. And what we’ve learned recently is that in the direction that the rupture progresses, 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 could happen then is that seismic energy could get trapped in those valleys and just bounce back and forth, so it really extends the length of time of shaking and it actually amplifies the seismic waves, causing much greater damage.

This phenomenon, which we call directivity, also indicates that the 1906 earthquake, as bad as it was, was probably a best-case scenario for an earthquake of that size in the Bay Area. The reason is that 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 3-D computer modeling to reproduce the 1906 shaking pattern and other potential future earthquakes. The modelers found that if that same earthquake began near Cape Mendocino on the San Andreas and ruptured southward, we’d have tremendous impact in the Bay Area, as all that seismic energy got focused and pushed into the Bay Area.

Q: Why is there expected such a major offset?

MARY LOU ZOBACK: The size of a potential earthquake depends on the length of fault that ruptures. The longer the fault, the larger the earthquake and the greater the potential offset. In 1906, the San Andreas ruptured 300 miles with a maximum horizontal offset of about 28 feet; that was a huge earthquake. The Loma Prieta earthquake only ruptured 25 miles with a maximum offset of less than seven feet. The Hayward fault will be something in between, with an offset of six to perhaps ten feet. The fact that we have a very long fault and we have a lot of evidence that that fault moved in the past in something like a magnitude 7.0 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 fault 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 2,000 years (if we dig any deeper in the soil we hit the water table). By dating the different layers of soil that are offset, geologists have 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 there is a high likelihood of a repeat of that event close to the 140^th anniversary of that big quake.

Q: Why is the Hayward Fault so dangerous?

MARY LOU ZOBACK: Two reasons: 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. Many of these older buildings are what we call “soft stories,” where the ground floor is an open space, typically a garage. When the whole ground floor is open and you get very strong horizontal shaking in an earthquake, the building starts moving backwards and forwards, and often 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 also had open garage space on the ground floor, similar to the older buildings. There are also old concrete frame buildings built largely in the 1960s. These have insufficient steel reinforcing in the concrete frames and many may be collapse threats. Now, not all of them will collapse, but many of them will be damaged severely and unusable.

In the next Hayward Fault earthquake there’s going to be an awful lot of damage and destruction. Major roadways will be shut down, the soft water-saturated sandy deposits along the margin of the Bay are likely to liquefy, ripping apart the roads. These areas are also where we have airports as well as approaches to bridges. I think we can we assume transportation networks will have major disruptions and some destroyed. Water pipelines crossing the Hayward Fault may rupture and will likely be shut down for some time. There will also be fires because underground 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 Internet, at least for a while, most likely.

Transportation systems in addition to the roads will be disrupted. The BART train crosses the Hayward Fault twice, and actually a number of the stations are right along the fault. [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, they felt it would crack possibly allowing water to flood into the Tube, which would not be good if people were in 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 here. We have lots of pipelines that are running underground, both gas and water. We have a bay, surrounded by wet, sandy soil, and when subjected to very strong shaking, that sandy soil liquefies, literally turns to something like quicksand and that flows and literally rips pipes apart. So you have gas pipe leaks -- you have a trigger for a fire to start. You have water pipe leaks, so it really hampers your ability to fight the fire. Another significant factor is that much of our buildings both residential and commercial are wood frame, which burns easily.

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, in 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 distressing 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 take steps to get prepared, 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 not that people say, “It’s too awful to think about, I can’t deal with it,” but rather I hope that most of us are thinking about what we can do to control our own situation. Take charge of your safety, and be prepared. You don’t want to lose your home as a result of a natural disaster, because then you’ve 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 for your family to reunite.

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. It may be more like camping out, you may not have water and you may not have electricity. But they can drop off water and Porta-Johns at the end of your block. If your home survived the earthquake, you’re generally better off sheltering in place than wandering around, trying to find a shelter and being separated from all your possessions.