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Saturday, Jul 21 2012 10:00 PM

EARTHQUAKES OF 1952: A conversation with a Caltech seismologist

By THE BAKERSFIELD CALIFORNIAN

Kate Hutton, staff seismologist at the California Institute of Technology in Pasadena, has been studying earthquakes and the science behind them for more than three decades.

Seismologists, she said, can help estimate earthquake risk in various locations, increase our understanding of the physics of earthquakes, and estimate the severity of shaking in different seismic scenarios. Seismologists may also assist in emergency planning, zoning and the design of buildings and other structures that better withstand the shaking that comes with powerful earthquakes.

TBC: Depending on the source, I'm seeing the magnitude of the Tehachapi quake described as 7.3, 7.5, 7.6 and 7.7. Is there a definitive number?

Hutton: This question is complicated. First, there are several different ways of calculating an earthquake magnitude. By present standards, the best method, especially for the larger quakes, is "moment magnitude," and the best moment magnitude value for this quake is 7.5. (Note: In simple terms, moment magnitude is a complex mathematical calculation used to estimate the total energy released in an earthquake. It is now the preferred magnitude scale in use by seismologists.)

Another issue is that the instrumentation available in 1952 was definitely not as good as what we have now, so even considering only moment magnitude, there is some uncertainty.

Still, I think almost all seismologists would agree that 7.5 is a very good estimate for the Kern County earthquake.

TBC: Here's a layman's question: Can you talk about why the much larger first quake did relatively little damage to Bakersfield, while the much less powerful 5.5 magnitude aftershock on Aug. 22 wreaked so much destruction?

Hutton: The 5.5 earthquake on Aug. 22 was much closer to Bakersfield, hence the shaking was stronger there, even though the earthquake had a smaller magnitude.

There is often confusion over the difference between magnitude, which is intended to characterize the true size of the earthquake -- how much rock moved how far, say -- and intensity, which is a measure of the effects of the shaking and perceptions. Ideally, an earthquake has only one magnitude and a whole map full of intensities.

TBC: In 1952, Bakersfield's population was 46,725, with about 129,000 residents in the greater metro area. Today, close to 355,000 live within the city limits, while more than a half-million people reside in greater Bakersfield. Are you able to speak to what we might expect if we had a similar string of earthquakes on the White Wolf Fault? What about the Garlock Fault?

Hutton: There are competing factors here. On one hand, the building codes are more stringent now than they were in 1952. We don't build unreinforced masonry anymore at all. This will reduce deaths, injuries and property damage in future large earthquakes, no matter which fault they happen to be on.

On the other side, however, population has increased and is now more widely distributed. So there is more exposure to the risk.

TBC: Bakersfield exists atop a web of seismic faults. Do those faults interact? Could a quake on one fault trigger a quake on another?

Hutton: The occurrence of any earthquake releases strain from the crust, but it also rearranges strain to some extent. In some areas there may actually be an increase. Depending on the strain level that is already present, the increment might or might not trigger another earthquake, immediately or at some later time.

As you might guess, small quakes make small changes and larger quakes make larger changes in the strain field. So it is much more common for large quakes to trigger additional earthquakes, which are called aftershocks. Most aftershocks occur along the same fault where the mainshock occurred, because that is where the largest changes in strain occur.

Occasionally, other nearby faults are affected. A good example is the Big Bear aftershock (magnitude 6.3) of the 1992 Landers earthquake (magnitude 7.3), which was on a different fault.

TBC: Considering the wealth of research that has been done on the physics and history of earthquakes in California, it there any way scientists can estimate when or where the next large quake will occur?

Hutton: This is a good question. After several decades of research, we've gotten better at recording, analyzing and predicting what ground motion will be like for specific earthquake scenarios. We understand why earthquakes happen where they do, which we really did not before plate tectonics became well-established in the 1970s.

We also know a lot more about historic and prehistoric seismic activity, which we can use to predict the future statistically. We are also close to an "early warning" system, whereby users are notified that a large quake has started before the shaking arrives at their location. What we are still missing, however, is a way to know when a particular earthquake scenario is going to happen. This is a very hard problem. Earthquakes apparently occur suddenly, with little or no warning.

Even if we could make these types of predictions, they would not be as accurate as society would require, at least in the beginning. People would still have to be prepared all the time.

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