Morris Tribune
June 10, 1993

'Periodicity' aids predicting . . .

Predicting quakes

Cotter said geologists try to predict quakes based on a number of criteria. In California, where there are large and frequent earthquakes, small adjustments in the earth are monitored. Geologists can also monitor increased intensity of those adjustments, or "foreshocks."

There is little monitoring in this area, though, he said.

Some faults seem to move on a periodic cyclic basis. Cotter said "periodicity" is a viable method of predicting quakes. His estimate that an earthquake along this zone will occur every 15 to 20 years is based on that periodicity.

A fault, he said, is a break or facture in the rocks -- a structural unsoundness. If there is a failure as the rocks move, that failure will be where there is a break. An earthquake is the energy released when rocks move on each side of the fault, he said.

Cotter said a magnitude 4.1 earthquake is not dangerous.

Measuring quakes

Measurement of the earthquake last Friday came from the U.S. Geological Survey in Colorado.

Cotter said that when the earth's energy is released in a quake, it's like ringing a bell. The bell is struck in one place, but the vibrations are felt all over.

It's the same with the earth. Vibrations from an earthquake are felt worldwide.

He said an earthquake has three types of waves that move at different speeds -- like the classic eighth grade algebra problem of two trains moving at different speeds.

Geologists from a network of sites use the distance in time between when the primary and secondary waves are recorded on a seismograph to determine the intensity of an earthquake and where the epicenter is located.

He said the network that determined the magnitude and location of Friday's earthquake was probably in Colorado, in the Twin Cities and maybe Grand Forks, ND.