PREDICTING EARTHQUAKES

August 18, 2004
		In recent years, scientists have been moving ever closer to one day being able to predict when and where a major earthquake could occur. A leading scientist in earthquake prediction answers your questions about the latest developments in the field.

Online NewsHour Special Report: Predicting Earthquakes Forum Introduction What is the difference between a "return interval" and a "recurrence interval"? Seismologists study earthquakes, but who studies how to mobilize society for disaster? Why is it that the New Madrid Fault is very rarely mentioned in quake discussions? If people could turn on their TVs to find out an earthquake was coming, wouldn't there be widespread panic? Are we going to have a 19-year quake cycle here in California? What are the chances of a major earthquake through the mid-U.S.A., where none has occurred since the early 1800s?	Larry Hart of Culver City, Calif., asks: Might we have a 19-year quake cycle here in California? From the large event in 1857 till Long Beach 1933 is 76-years which is devisible by 19. Nineteen more years till 1952 near BakersField. Nineteen more years till Silmar in 1971. While I have not really studied Northern California, you have a Freemont quake in 1868, 38 years later is 1906, again a multiple of 19. I realize with this cycle, in the south Landers should have happened in 1990. I look forward to reading any comments -- and thanks for listening. Dr. William Ellsworth responds: It's tempting to find patterns in earthquake catalogs, and many have tried but largely without success. If we analyze the intervals between large earthquakes in California, we find that they occur at random. In other words, the chance of another event immediately following the last one are just the same as at any other time and there is no cycle. Technically, we say that the intervals between earthquakes follow an exponential probability distribution and form a Poisson process. It gets more interesting as we focus on smaller and smaller pieces of real estate. If we look at the intervals between ruptures at a particular spot on a fault, they do indeed occur with some degree of regularity. Very small earthquakes on some California faults are even more remarkably regular. Work is underway this summer to find out what makes one of these little recurrent earthquake sources tick in the EarthScope Program's ambitious effort to drill into the San Andreas fault. You can learn more about this program at www.earthscope.org.

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