Quake Maps Could Plot Data Block by Block : Seismology: Computer studies will show likely paths and effects of temblors throughout L.A. Basin. They could lead to suggested new building codes for each neighborhood.
A $2.15-million study grant from the National Science Foundation is likely to lead to recommendations for new earthquake-resistant construction codes for the Los Angeles Basin that would vary every few blocks, based on computer simulations of how particular neighborhoods would shake in future quakes.
The three-dimensional simulations will be developed over the next four years by civil engineers and computer scientists at Pittsburgh’s Carnegie Mellon University using information about Los Angeles area earthquake faults provided by the Southern California Earthquake Center at USC.
Jacobo Bielak, a professor of engineering at Carnegie Mellon and the principal investigator for the project, said the simulations will indicate the source, the “propagation path” and “local site conditions” for many potential earthquakes.
The source is the place where the quake originates. The propagation path is the direction of earthquake waves. And the site conditions are the geologic makeup--such as the stability of underlying rock or sediment--that result in specific types of shaking, Bielak said.
Still, it will be up to local and state officials, not the computer scientists, to decide on any alterations to building codes, Bielak said.
“Our principal objective is to delineate the mechanisms of shaking that take place, so the authorities have the information they need to take action,” he said.
“We believe we will be able to make substantial recommendations to incorporate into building codes and practice.”
Certain visible faults, such as the Newport-Inglewood and the Sierra Madre, have long been well known in Los Angeles and Orange counties. But the 1987 Whittier Narrows earthquake pointed up the existence of deeply buried thrust faults, often not visible on the surface, that have not been completely mapped by scientists.
To the extent they are known, these are essential to the computer modeling, which will require billions of equations to adequately represent quake behavior in such a large area.
“Certain areas within the Los Angeles Basin are more prone to experience ground motion than others, just like happened in Mexico City in the 1985 earthquake, although the variations in L.A. are going to be a lot smaller,” said Bielak, who was born in Mexico City.
Portions of that city’s downtown sustained major damage, despite the fact that the source of the shaking was 200 miles away. Ancient lake beds under the city allowed the distant earthquake to resonate, causing buildings to sink and crumble.
Resonance--meaning the echoing of earthquake waves--is also a factor in parts of the Los Angeles Basin, and the computer simulations will incorporate that factor, Bielak said.
Already, he said, a scientist at UC Santa Barbara, Ralph Archuletta, has developed a map of earthquake amplifications in the basin from 1933 to 1992. He has rated them on a scale of severity from 1 to 6 for such temblors as Long Beach, Tehachapi, Borrego Mountain, Sylmar-San Fernando, Whittier Narrows and Landers.
“The work we do will examine what might happen in the future,” Bielak said. “It will take a while. We need to develop appropriate mathematical models based in part on the properties of the soil in the various neighborhoods.”
Bielak said his team has little knowledge of the political problems that have often surfaced in implementing laws, such as the Alquist-Priolo Act, that mandate restrictions on building in areas particularly prone to quake damage.
Under the Alquist-Priolo Act, local authorities are supposed to sharply restrict construction within 50 feet of a fault. But the Landers quake demonstrated that there had been little adherence to the act in the quake-damaged zone north of Yucca Valley, and that the faults involved often were half-mile-wide zones, possibly making the 50-foot requirement obsolete.