Data on ‘Ground Failure’ Sought : Parkfield Analyzed, Poked as Scientists Await Earthquake

Scientists have long been frustrated in their efforts to study one of the leading causes of damage from an earthquake because they always arrive on the scene too late.

During a major quake, some soils assume the physical characteristics of liquid in a process called liquefaction. That leads to what scientists call “ground failure,” where the ground becomes so unstable that structures built upon it collapse.

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, but that may soon change.

Quake Expected at Parkfield

A team from Brigham Young University has set up an array of instruments in central California near Parkfield, where a quake is expected to hit within the next two or three years, to study liquefaction as it occurs.

That should give them hard information on which to base their conclusions, according to the scientists; until now they have had to rely on conjecture and after-the-fact analysis.

Seismologists believe that 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. This expectation is based on the historical record.

A quake of 5.5 to 6 has struck that area once about every 22 years, most recently in 1966. That record has led scientists to set up a wide range of instruments at Parkfield to determine exactly what happens just before an earthquake hits as part of an ongoing effort to learn how to predict them.

Thus the little farming community has become one of the most intensely studied areas in the world.

But until now, most of that research has been concentrated on documenting changes in land contours and stresses in nearby rock formations, because any change could illustrate the dynamics that immediately precede an earthquake.

Likely Liquefaction Site

Now the Brigham Young team, working with scientists from the U.S. Geological Survey, has installed 12 instruments at various depths below a ranch seven miles from Parkfield where earlier studies have indicated that liquefaction is most likely to take place.

“What we will have will be a case history,” said T. Leslie Youd, professor of civil engineering at Brigham Young.

That case history should provide the raw data necessary to create more accurate models for predicting the susceptibility of any area to liquefaction, he said in a telephone interview.

Youd said several recent earthquakes have demonstrated the urgency in generating a better understanding of liquefaction.

“In the 1964 Alaska quake about 60% of the damage was caused by ground failure,” he said. Some of that failure was in the form of landslides, but “the majority was from liquefaction,” he added. He said liquefaction was blamed for the failure of more than 200 bridges.

A similar record was compiled during the 1971 San Fernando quake, including the near failure of a major dam, Youd said.

For years scientists have been able to make what Youd called “crude” predictions of how the ground would perform by drilling for core samples that told them the nature of the soils.

“That will give us a crude yes-no answer, but if you have a very important site, you may want to know what magnitude of earthquake will cause liquefaction,” he said.

That led the Electric Power Research Institute to fund the study because of problems with siting such critical facilities as nuclear power plants.

Typical in Sandy Soils

Generally, liquefaction occurs in loosely compacted, sandy soils found throughout much of California, including wide areas of the Los Angeles Basin. During an earthquake, shaking causes the lower soil to compact, thus 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 at depths ranging up to 100 feet in the Cholame Valley near Parkfield, east of Paso Robles. Five of the instruments will measure ground movement during the quake. Seven of the instruments will measure changes in water pressure.

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

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

The results from Youd’s project will be part of what scientists hope will be a rich legacy left behind when the shaking stops at Parkfield, a sleepy little community that is being poked, prodded and scrutinized by seismologists, geologists and engineers from around the world.