Good News About Quakes For Southern California

December 10, 1997

The Los Angeles basin's sediments seem to moderate the type of ground motion that threatens single-story and low- rise buildings in a severe earthquake, a new study of data from the 1994 magnitude-6.7 Northridge quake has revealed.

University of Southern California seismologist Edward H. "Ned" Field, Ph.D., calls his team's research, which will be published in the Nov. 11 issue of the journal Nature, "good news" for Southern California.

Geologically, the Los Angeles basin is a valley filled with debris (sediments) that eroded from neighboring mountains over hundreds of thousands of years, Dr. Field notes.

For more than a century, scientists have known that such sediments usually amplify ground motion in earthquakes. But seismologists and engineers disagree as to whether the degree of amplification will change as the level of shaking increases.

Do all sediment-filled valleys shake like a bowlful of jelly in larger earthquakes, as they do during smaller quakes' Or do some behave like a bowlful of sand in which seismic energy is "absorbed" as the grains rub together and effectively reduce ground motion? (The jelly-like reaction of soils is called "elastic" behavior, while the latter is dubbed "nonelastic" or "nonlinear" behavior.)

Based on laboratory studies of sediments, engineers have argued for the bowl-of-sand theory and have designed structures on the assumption that amplification factors go down as the level of shaking increases -- that is, the shaking effects of a stronger earthquake aren't boosted as much as those of a smaller quake.

On the other hand, seismologists have traditionally argued for the bowl-of-jelly model. They have seen little evidence that sediment amplification is reduced, especially when the soil is of the stiff, dry variety found in the Los Angeles basin. They have therefore been concerned that some engineering designs may fail to account for the degree of seismic hazard that sediments actually pose.

The new study provides the first evidence based on large-scale measurements that the answer is closer to the engineering view than seismologists had thought. Sediment amplification in the Los Angeles basin would be significantly reduced during large earthquakes by the nonelasticity of the local soil, the researchers found.

Although the paper takes a step toward settling the debate, the researchers believe more work is needed to assess whether current engineering practices adequately reflect the degree of seismic hazard posed by the local sediments.

"This result will help us refine methodologies to assess seismic hazard," says Field, a research assistant professor of earth sciences in the USC College of Letters Arts and Sciences.

Collaborating with Field on the paper, titled "Nonlinear Ground-Motion Amplification by Sediments during the 1994 Northridge Earthquake," were researchers Paul Johnson of the Los Alamos National Laboratory; Igor A. Beresnev of Carlton University, Ottawa, Canada; and Yuehua Zeng of the University of Nevada, Reno.

The research was supported by the Los Alamos National Laboratory. The National Science Foundation provided additional funding through the USC-headquartered Southern California Earthquake Center.


EDITOR: Dr. Field will be attending the meeting of the American Geophysical Union in San Francisco during the week of publication. Call him at the Westin St. Francis Hotel at (415) 397-7000.

University of Southern California

Related Earthquakes Articles from Brightsurf:

AI detects hidden earthquakes
Tiny movements in Earth's outermost layer may provide a Rosetta Stone for deciphering the physics and warning signs of big quakes.

Undersea earthquakes shake up climate science
Sound generated by seismic events on the seabed can be used to determine the temperature of Earth's warming oceans.

New discovery could highlight areas where earthquakes are less likely to occur
Scientists from Cardiff University have discovered specific conditions that occur along the ocean floor where two tectonic plates are more likely to slowly creep past one another as opposed to drastically slipping and creating catastrophic earthquakes.

Does accelerated subduction precede great earthquakes?
A strange reversal of ground motion preceded two of the largest earthquakes in history.

Scientists get first look at cause of 'slow motion' earthquakes
An international team of scientists has for the first time identified the conditions deep below the Earth's surface that lead to the triggering of so-called 'slow motion' earthquakes.

Separations between earthquakes reveal clear patterns
So far, few studies have explored how the similarity between inter-earthquake times and distances is related to their separation from initial events.

How earthquakes deform gravity
Researchers at the German Research Centre for Geosciences GFZ in Potsdam have developed an algorithm that for the first time can describe a gravitational signal caused by earthquakes with high accuracy.

Bridge protection in catastrophic earthquakes
Bridges are the most vulnerable parts of a transport network when earthquakes occur, obstructing emergency response, search and rescue missions and aid delivery, increasing potential fatalities.

Earthquakes, chickens, and bugs, oh my!
Computer scientists at the University of California, Riverside have developed two algorithms that will improve earthquake monitoring and help farmers protect their crops from dangerous insects, or monitor the health of chickens and other animals.

Can a UNICORN outrun earthquakes?
A University of Tokyo Team transformed its UNICORN computing code into an AI-like algorithm to more quickly simulate tectonic plate deformation due to a phenomenon called a ''fault slip,'' a sudden shift that occurs at the plate boundary.

Read More: Earthquakes News and Earthquakes Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to