Loma Prieta: 20 Years Later, Bay Area Safer

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Detailed Description

Host Leslie Gordon interviews three USGS earthquake scientists, David Oppenheimer, Carol Prentice, and Tom Holzer about the scientific advances made in the last 20 years since the Loma Prieta earthquake struck northern California. Oppenheimer, Holzer and Prentice discuss the work they did immediately after the quake in 1989, and how 20 years of scientific and technological advances have changed their work, and made the San Francisco Bay Area safer and better prepared for the next big earthquake.


Date Taken:

Location Taken: US


Leslie Gordon: My name is Leslie Gordon, and today we're going to be talking about the 20th anniversary of the Loma Prieta earthquake in Northern California on October 17th, 1989. It was the middle of the World Series between the San Francisco Giants and the Oakland A's when the earthquake hit.

And today, I have three US geoscientists with me: Carol Prentice, a geologist, David Oppenheimer, a seismologist, and Tom Holzer, a geologist. And what we're going to be talking about is where we have come in the last 20 years, what kind of scientific and technical advancements we've made, and where it holds us for the future.

What I'd like to know... Let's start out and think about in 1989. Where were you and what work were you involved with immediately after the Loma Prieta quake. Carol?

Carol Prentice: Well, immediately after the earthquake, all of the geologists congregated and set out together to try to figure out a plan, because the first thing you want to do as a geologist after an earthquake like that is to get out in the field as quickly as possible. And given the location and the magnitude of this event, we really expected to see surface rupture on the San Andreas fault, perhaps a meter, a meter-and-a-half of displacement.


So, we got our maps out, broke up into groups, kind of assigned where we're going to go. Because it was late in the day when the earthquake happened, we actually decided it would make most sense to go out early the next morning. And that's what we did.

Leslie Gordon: David, you run the monitoring system for northern and central California. What was the Loma Prieta earthquake like in your view?

David Oppenheimer: Well, at the time the earthquake occurred, it was almost chaos for us because the facilities that we had were not hardened to record an earthquake. Luckily, there was sufficient resiliency in the system that we actually did record the earthquake, but in 1989, computer networking was primitive, to say the least.

The power that drove all the computers went down, and we did have some auxiliary power that kept critical recording systems operating, but in the first few hours after the earthquake, we were just scrambling around trying to get all the computers back up and plugged into diesel-generating supply power and meanwhile watching television, seeing all the destruction.


And when we finally got up the displays and looked at the automatic locations, which we did have in 1989, we were kind of confused because all the locations were in the Santa Cruz mountains and all the damage was in the cypress structure in Oakland and the arena district, and it for a moment caused us to question whether we had terrible processing issues.

But, you know, fast forward to today, we're much more hardened. We have redundant systems. And the systems that we have hopefully will serve the public better the next time. In one particular area, we have the internet. We didn't have the internet in 1989 and if people can remember back to that point, what would happen is local radio stations like KCBS would call up on the phone and they would say, "Where's the earthquake and what's the magnitude?" And you would talk to a reporter live on radio. Today we have the internet. We can pump that information out. Everybody can have it so this rapid situational awareness.


Leslie Gordon: Tom Holzer, what was your first reaction to the quake and how did it affect your work?

Tom Holzer: Well, I was in a similar situation as Carol was. We had to find portable seismic instruments and just make sure everybody was organized. I am also sympathetic to the problem that Dave had in figuring out where the earthquake was. So I remember one of the first things we did was walk over to the old-fashioned smoke drum recorders, try to estimate where the earthquake was the old-fashioned way. And some of our rather famous seismologists at the time were measuring travel times and making estimates of locations.

We sort of had an idea it was south of us even though, like Dave said, all the reports were coming from the north. But that turned out to be an important lesson because what it meant was, areas that are heavily damaged are the quiet ones after an earthquake and you don't necessarily hear from them. So just like Katrina, when  things get quiet, you should get suspicious.


Leslie Gordon: And, in fact, we found out that some of the most heavily damaged areas were in the town of Watsonville and Santa Cruz. And what would happen today would--we didn't find out about that until maybe days later after the--in 1989. How are our systems today that we would know about those quiet areas?

Tom Holzer: Well, that's the great thing about ShakeMap. All the recordings get assembled into this map that portrays the regional picture of shaking. So we'll have a pretty good idea of the aerial extent of the strong shaking from ShakeMap.

Leslie Gordon: So, ShakeMap is a computer-produced map that shows how strong the ground shook in different areas?


Tom Holzer: Yeah. And it's generated within minutes of the earthquake.

Leslie Gordon: Within minutes? So...

Tom Holzer: Thanks to all that hardening that Dave did, we'd have a snapshot of the earthquake within minutes.

Leslie Gordon: What are some of the other advances, or what have we learned by studying the Loma Prieta earthquake and where have we come in 20 years?

David Oppenheimer: When the earthquake occurred, a lot of buildings were damaged. And there was a group of seismologists that were concerned about engineering issues. But I think the bulk of the field at that time was more interested in earthquake forecasting and prediction. And we were roundly criticized by the engineering community for failure to transfer information from our research into the application side. And I think, over the last 20 years, there's been substantial improvement in that area.

Leslie Gordon: And these are building codes you're talking about?

David Oppenheimer: These are building codes. And the engineering seismologists are working with the geologists to do the forecasting for the likelihood of earthquakes and seismologists do the forecasting of the ground motions from earthquakes. We're taking the on-scale recordings that we now get from modern instrumentation to guide code changes.


So what's happened in the last 20 years is this complete integration of information from people walking out on the surface looking at faults and trying to figure how often that fault breaks, to the seismologists that are trying to predict the ground motion, to the actual seismograms themselves that the engineers are using in their buildings. All of that is being used to make buildings safer. That really was not happening very effectively in 1989.

Leslie Gordon: Carol, you mentioned how the geologists got together and made a plan to go out in the field and see what happened, you expected to find large ruptures in the earth's surface. Tell us what you found.

Carol Prentice: Well, we did find large ruptures in the earth's surface, but they weren't actually any of them. Certainly none of the big ones. None of them were along the San Andreas fault. None of them were along any tectonic fault. They were all produced by strong shaking, gravity failure rather than actual tectonic displacement.


So that was actually a big surprise to us. An earthquake of that magnitude in this part of the world, I think everybody at the time would've expected surface rupture on the causative fault to show up, and we looked and looked and looked and did not find it.

Leslie Gordon: How is your geology fieldwork? How is it done any differently today than how you went out to look for the fault and surface displacement in 1989? Do you have tools at your disposal that you didn't have then?

Carol Prentice: The tool that we have today that has impacted field research the most is something called LiDAR data. There's an instrument that flies in the airplane, sends out a laser pulse, you can get many returns from each pulse. The last return that you get is actually the ground surface no matter what vegetation you have in between the plane and the ground surface.


So what that allows us to do in northern California is literally to see through the forest cover. And being able to do that and have a digital elevation model of the ground surface underneath that forest really makes finding and mapping active faults much, much easier, much more precise, and also allows us to more easily find areas where we can do more detailed studies.

On the other hand, some things are very much the same. We still are very interested in finding locations where we can dig trenches across the fault. All of that technology is--that's not exactly the same. We have some new tools like digital photography. But we still are looking for places where we can go out with a backhoe and literally dig a trench across the fault to tell us when and how fast faults are moving.

Leslie Gordon: And the information we learn about how and where the ground shakes to varying degrees, this information is also used by the insurance industry to set insurance rates. Is that correct? 


Tom Holzer: Well, it's interesting. Back then, people were really guessing when it came to setting premiums for earthquake interest. They really didn't have a clue how to set the rates because we didn't have any history of earthquakes. There was no sort of damage statistics they could use.

And one thing that began to develop after Loma Prieta, and really came to a head after the North Ridge earthquake, were that people started actually modeling the ground motion, and we could do it better because of Loma Prieta, and we would model how structures responded. And so you could put an economic value on that response when they failed, when they were damaged.

And so you could calculate a loss, and so did they. Our earthquake premiums are actually based on fairly sophisticated modeling of how an earthquake--well, it starts with when an earthquake will happen, on through to how the ground shakes, to how the structures will respond. 


Carol Prentice: From a geological point of view, we've really learned quite a bit about our faults in the San Francisco Bay area in the last 20 years. The field of paleoseismology, the sort of geologic study of active faults, was really in its infancy in 1989, just getting going in the Bay Area. We had very little information about how often earthquakes have happened in the past on any of our faults, or even how fast they've been moving in geologic time. And we just have a better picture. We still have a long way to go, however, before we really can say that we understand fully our faults in the Bay Area.

Tom Holzer: Another aspect, as we commemorate the 20th anniversary of Loma Prieta, was, in some respects Loma Prieta was like a warning shot. Because since the 1906 earthquake, we hadn't had any really large earthquakes in the Bay Area and we had had a lot of people move into the Bay Area and become a major urban area. 


And so Loma Prieta highlighted the vulnerability of a lot of our things to two earthquakes. And it's prompted, during the last 20 years, a lot of efforts by major utilities to retrofit structures, to build things better, so that we're much more attentive to the earthquake hazard than we were 20 years ago.

Carol Prentice: People need to understand that it's important to prepare for large earthquakes if you live in the Bay Area. Something much larger than the 1989 earthquake is likely to happen in the future.

Leslie Gordon: Well, I'd like to thank my guests Carol Prentice, David Oppenheimer, and Tom Holzer. Thank you for taking the time to chat with me today. My name is Leslie Gordon. And CoreCast is a product of the U.S. Geological Survey, U.S. Department of the Interior.