Experts Study Rupture Zone for Clues

LANDERS, Calif. — Navigating four-wheel-drive vehicles across desolate parts of the desert floor, scores of geologists, seismologists and private consultants on Wednesday combed over the surface rupture zone of last weekend’s magnitude 7.4 earthquake looking for clues.

It is here along the Landers rupture--a dramatic 44-mile long swath of fissures, depressions, scarps and generally crumbled earth up to 100 feet wide--that efforts to learn about Sunday’s quake and later aftershocks are focused.

Though careful to avoid property where the owners are sensitive about interlopers, the scientists are working with some urgency. Thunderstorms are common in the High Desert in August, and the cloudbursts, high winds and flash floods might obliterate many of the features that are of most interest.

So, on an unusually cool morning for July, four researchers from the Bay Area stood in a draw here and examined the seismic evidence.

“This little hill could be a nice master’s project, it’s complex enough,” said Pat Williams, a staff scientist with the Lawrence Berkeley Laboratory at UC Berkeley. “We’re making a major effort to record what’s here before it might be damaged by wind and rain.”

Nearby, William Lettis--a consultant performing studies for three federal agencies and Pacific Gas & Electric Co.--carefully examined a series of chaotic fissures. The rumpled ground was a sign that running a high-pressure natural gas line across the earthquake fault here would require a careful strategy.

“The best thing to do would be to put it in the air over the fault,” he said. “But that’s not feasible. Shooters would make it a target. . . . A proper angle, to ensure the pipe will be extended, not compressed, by shaking would be important.”

The state Division of Mines and Geology has opened a scientific clearinghouse in nearby Yucca Valley in an attempt to coordinate the work of experts from all over California--and from as far as Columbia University in New York--who have come to study the rupture zone.

At a Tuesday night meeting, more than 30 geologists shared information and received suggestions about where to work in an attempt to avoid duplication.

Already, study of the rupture has led some experts to conclude that the state’s 20-year-old Alquist-Priolo Act, which regulates construction in quake fault zones, needs to be strengthened to better protect dwellings close to faults.

Some property owners who thought their dwellings were well clear of faults have found this week that the fissures came right through their homes.

Under the guidelines of the Alquist-Priolo Act, local authorities are supposed to sharply restrict construction within 50 feet of a fault. Sometimes, however, developers have been allowed to build homes right up to the edge of a rupture.

What has alarmed researchers inspecting the Landers rupture is its width--much wider than expected, and in some cases spanning the area between faults.

Rick Wilson, an engineering geologist with the Division of Mines and Geology, said Wednesday that the Landers earthquake seems to have caused rupture across at least three faults--the Camp Rock Emerson, the Johnson Valley and the Homestead Valley.

“Alquist-Priolo basically told us to find what faults we could that were active and well defined,” Wilson said. “But in this earthquake we found the faults are more zones than lines--and they have many strands.”

In Sacramento on Wednesday, officials said they are studying ways to strengthen the law, and a spokesman for state Sen. Alfred Alquist (D-San Jose), a co-author of the act, said Alquist intends to introduce a bill to do that next year.

James Davis, the Department of Conservation’s state geologist, said Alquist-Priolo is directed at reducing property losses from surface rupture within fault zones.

“This type of surface damage is generally considered to constitute approximately 5% of property loss in earthquakes,” Davis said. “A new law passed in 1990, the Seismic Hazards Zoning Act, is directed at reducing the other 95% of losses by zoning areas where soil conditions cause especially severe ground shaking, ground failure such as liquefaction and earthquake-induced landslides.

In the first reports of the Landers earthquake surface rupture, attention was focused on the horizontal displacement of what was mainly a strike-slip earthquake.

Kerry Sieh, a Caltech seismologist, has put the horizontal offset at as much as 22 feet, meaning that two points directly across the fault from each other before the quake would be separated by as much as 22 feet.

But further exploration of the rupture zone has found many scarps, where one side of the fault rose one to three feet above the other, causing a vertical disruption in the surface.

Williams, the UC Berkeley scientist, explained Wednesday that where the faults bend, even horizontal slipping can cause earth on one side of the fault to pile up against earth on the other side, causing the scarps.

Meanwhile, state geologists said in a report that the Landers temblor had a duration of 30 seconds--a long time, as quakes go.

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