New Method Of Dating Past Earthquakes, Assessing Future Ones Discovered

Jay S. Noller, assistant professor of geology, made the discoveries while studying the Hebgen Lake fault in Montana. His research is featured in the Nov. 6 issue of the journal Science.

Noller and his research partner Marek Zreda of the University of Arizona discovered while studying bedrock formations that the chemical makeup of the bedrock had changed after it was affected by an earthquake. By taking a closer look at the bedrock, they were able to determine how long ago the quake had occurred, how often quakes occur in that area, and the probability of another quake happening.

Earthquake dating and predicting is currently done by studying events that occurred either before or after an earthquake - for example, by looking at rivers that crossed over a fault line. Noller's method looks at what occurred at the very moment an earthquake hit.

"That's what's revolutionary about this," Noller said. "This opens up a new range of potential study sites. Bedrock exposures last tens of thousands of years, but river deposits don't last as long - they get buried or eroded away, so they last only a few thousand years. The longer the track record you have of an area, the better idea you have of how things work. The track record lets us estimate the likelihood of future earthquakes. "

Most of the faults that run through the United States are in bedrock, such as the New Madrid fault in West Tennessee and faults near New York City. Because there are more bedrock faults and fewer river deposits than in the West, it has been very difficult to assess past, and hence future, seismic action in that area.

"There are some bedrock faults in New Jersey and New York that have bothered geologists for a long time now. They've looked at them and said, 'We don't know how to date when the last time this fault moved,' because there are no river deposits there."

Noller's research is funded by the U. S. Nuclear Regulatory Commission, with the express purpose of better assessing the activity of faults around the nation's nuclear facilities, which are primarily located in the eastern United States - which is where most bedrock faults are located. Noller's research now provides a tool with which to find answers to how seismically safe the nuclear facilities are, as well as how safe the major metropolitan areas in the East are.

This is how Noller's research works: after a large-magnitude earthquake occurs, the earth's crust is shifted, exposing parts of the earth that had previously never seen daylight. These rocks contain elements such as potassium and calcium, which become chlorine after exposure to cosmic rays. In particular, Noller looks for the isotope 36Cl, which although found in nature, occurs in very small amounts in most rocks. But because of the bedrock's exposure to cosmic rays, 36Cl is found in high numbers in seismically altered bedrock.

After thousands of years and many quakes, more and more of the earth is exposed, creating bands of different colors that Noller describes as looking like "a big piece of bacon," the bands near the top being the oldest. Noller takes samples of the rock and counts the number of atoms of 36Cl found - the longer a piece of rock has been exposed to cosmic rays, the more 36Cl is present.

"You can see exactly how much of the earth moved during these earthquakes," Noller said. "The exposure of rock is directly proportional to the size of the earthquake. It's very clear and plain to see - it's elegant and simple and it tells us the exact time it happened."

The Hebgen Lake fault was chosen as Noller's base of research because it was the site of a catastrophic 7.5 magnitude earthquake in 1959. During that quake, the ground was raised 21 feet - "the largest amount of displacement ever recorded anywhere on the globe."

Noller also discovered while investigating the Hebgen Lake fault that when the 1959 quake occurred, it had an effect on the geysers in nearby areas. Old Faithful in Yellowstone National Park slowed down, new geysers were formed and old ones stopped. "It's interesting to see how it affected the natural history of that area," he said. "This record of earthquakes should shed light on the history of changes in the geyser fields."