Can Earthquakes be Predicted?

Predicting the next major earthquake may depend more on finding quiet ground than on listening for rumbles and shakes.

New research shows that periods of "seismic quiescence"---times when there are far fewer small earthquakes in an area than normal---sometimes precede the world's strongest and most damaging earthquakes.

Geophysical Institute Professor Max Wyss, who holds the Wadati Chair in Seismology, is developing new methods to test the hypothesis that major earthquakes often occur after the ground has been unusually quiet for months or years.

Much of Earth shakes with a constant number of small earthquakes created by the continual movement of plates around the globe. The theory of plate tectonics suggests that these semi-rigid plates move independently on a viscous underlayer in Earth's mantle. When the plates grind against each other, the contact can cause earthquakes or volcanic eruptions.

"When there is a sudden decrease of earthquakes in an area, it suggests something unusual is happening," Wyss said.

Wyss hopes that identifying periods of seismic quiescence in areas that normally shake with small tremor will someday help scientists forecast moderate and large earthquakes around the world. Periods of quiet activity already have been observed before the occurrence of large earthquakes in California, Hawaii, Turkey, Utah, Japan, Italy, Armenia, and the Pacific.

In a recent study, Wyss and Seismologist Walter Arabasz, director of the University of Utah Seismograph Station, searched for periods of seismic quiescence before seven large earthquakes recorded in Utah from 1974-'96. Quiet periods preceded at least three of those earthquakes.

To find out why large earthquakes don't always follow seismic quiescence, Wyss researched earthquakes in Japan, where much has been published in support of the quiescence hypothesis. There, Wyss found that seismically quiet periods precede significant earthquakes only when sufficient underground stress has accumulated.

Wyss studied three areas around Tokyo that have been experiencing seismic quiescence for three years. Each of the areas has experienced large earthquakes in the past.

Within the areas, Wyss mapped and tested asperities, or hard spots, which can supply the source of energy for an earthquake. He assigned each asperity a "b-value" as a measure of the accumulation of underground stress. The measuring of b-values can help seismologists rule out other reasons for a decrease in the rate of small earthquakes caused by physical factors, such as ground water movement.

High b-values mean it is unlikely the ground contains enough stress or energy to produce an earthquake. Low b-values mean the ground has great potential for producing an earthquake.

All three of the areas of seismic quiescence near Tokyo contained asperities with high b-values. Consequently, Wyss predicts there is less than a 20 percent chance of major earthquakes occurring in those areas any time soon. Wyss calls areas containing seismic quiescence that do not produce major earthquakes, "false alarms."

"False alarms do not disprove the hypotheses. They just mean the hypothesis doesn't work in this case," Wyss said. "Scientists who predict the weather experience false alarms quite frequently."

Wyss has made progress in confirming his hypothesis, but he is careful to say it is not ready yet to be used as a prediction tool. He advocates using multiple methods of prediction because earthquakes follow a variety of patterns.