Nevada seismology researchers develop model that estimates impact of large earthquake in Los Angeles

Stiffer building codes in the Los Angeles basin may come in the near future as a result of a new study completed by University of Nevada, Reno seismologists of an anticipated large thrust-fault earthquake.

“Our study in California’s Kern County is a good indicator of what could happen in Los Angeles because the geology of the areas is so similar,” said James Brune, seismology pioneer and University Foundation Professor. The conditions, he said, would indicate “high motion” in Los Angeles.

His team’s study in Kern County is a scenario for a large earthquake in the Los Angeles basin because the downtown portion of the city is on the hanging-wall of the Puente Hills thrust fault. The study involved looking at what levels of peak ground acceleration were necessary to topple a balanced rock or a rigid transformer. Their conclusions reinforce that the hanging-wall side of large thrust-fault earthquakes experience more extreme motion and, therefore, more damage than the footwall side.

A thrust-fault earthquake occurs when the land on one side of a fault (the “hanging-wall”) gets driven up and over land on the other side (the “foot-wall”), he explained. This is different from a normal fault earthquake where land on one side of the fault gets pulled down and away from the land on the other side.

Brune and his team of University professors, Abdolrasool Anooshehpoor, Baoping Shi and Yuehua Zeng, have further proved that the geometry of thrust faults can dictate the range of an earthquake’s damage in their study, “Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the Ms=7.7 Kern County Earthquake: an Analog for Disastrous Shaking from a Major Thrust Fault in the Los Angeles Basin.” This article will soon be published in the Bulletin of the Seismological Society of America. “This is the first extensive data set from a major thrust-fault earthquake in the United States,” said Anooshehpoor. “The only other data on these faults come from a 1999 quake in Taiwan.”

Brune and his team have supplemented data from Taiwan with recent research which shows that precariously balanced rocks and overturned transformers in the vicinity of the White Wolf fault, south of Bakersfield, Calif., provide an understanding of thrust-fault earthquake ground motion that has been otherwise unavailable. The team also complemented their rock research by studying large foam-rubber models of faults and computer simulations of quakes.

According to Brune, precariously balanced rocks evolve naturally unless shaken down by earthquakes. As a result, these rocks are effective earthquake seismoscopes. Zones of precarious rocks are direct evidence that no strong ground accelerations have occurred for thousands of years, and as a result they potentially provide important information about seismic risk. “The brilliance of this balanced-rock research is that it gives us a way to test ground-motion probability which can dictate building codes,” said John Anderson, University professor of geophysics and director of the Nevada Seismological Laboratory.

The next United States Geological Survey hazard methodologies which are based on ground-motion probability are slated for release in 2007. These methodologies are used by engineers to frame building codes. Many anticipate that Brune’s study will be used to update these codes because the research provides new quantitative constraints on seismic hazard.