PARKFIELD, Calif. (AP) _ Brigham Young University researchers are waiting for the next big earthquake here to analyze how a big shake sometimes makes the earth take on the characteristics of a fluid.

Ground failure, or liquefaction, is the term for the phenomenon which is one of the leading causes of quake damage where the Earth becomes so unstable that structures collapse.

But since no one has been able to predict exactly when and where a large earthquake will strike, scientists have never been able to study liquefaction during an actual quake.

The Brigham Young team, working with scientists from the U.S. Geological Survey, has installed 12 instruments at various depths outside this small farming town 170 miles northwest of Los Angeles.

Based on the historical record, seismologists believe a quake of at least 5.5 on the Richter scale is due to strike the Parkfield area along a section of the San Andreas fault within the next two or three years.

A quake of 5.5 to 6 has struck that area once about every 22 years, most recently in 1966.

''What we will have will be a case history,'' said T. Leslie Youd, professor of civil engineering at Brigham Young.

The case history should provide data needed to create more accurate models for predicting how vulnerable an area is to liquefaction, he said.

Until now, most research in the Parkfield area focused on documenting changes in land contours and stresses in rock formations, because any change could illustrate the dynamics that immediately precede an earthquake.

Youd said several recent earthquakes have demonstrated the urgency in generating a better understanding of liquefaction. ''In the 1964 Alaska quake about 60 percent of the damage was caused by ground failure,'' he said.

Liquefaction occurs in sandy soils found throughout much of California, including wide areas of the Los Angeles Basin. During an earthquake, shaking causes lower soil to compact, pushing the water table up quickly.

The water permits the soil to shake so violently that it loses its strength and is unable to support heavy structures.

Youd's instruments have been buried as deep as 100 feet in the Cholame Valley. Five instruments will measure ground movement during the quake. Seven instruments will measure changes in water pressure.

Readings from the instruments are being continually recorded so that when a quake hits, the data should show how much the water pressure changed due to compaction of soil and the relationship between liquefaction and the rate of ground acceleration.

Youd said the data should be valuable for engineers who design and build structures in areas where liquefaction may occur.