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The State

Landers Quake Gave Experts New Insights

Geology: The event 10 years ago showed that big jolts can span multiple faults and can trigger temblors hundreds of miles away.

June 28, 2002|KENNETH REICH | TIMES STAFF WRITER

Ten years ago today, the Landers earthquake set scientists on some new courses, proving that the seismic process was more complicated than they had thought and showing that a big enough jolt could span more than one quake fault at once.

The magnitude-7.3 Landers temblor and the 6.5 aftershock just hours later near Big Bear remain the most powerful quake sequence in Southern California since the magnitude-7.7 Tehachapi quake of July 21, 1952. As such, the 1992 combination was the third most powerful seismic event in the state in the 20th century.

The Landers quake occurred in the Mojave Desert, centered in sparsely populated territory 100 miles east of Los Angeles, causing only one death and fairly minimal damage.

But scientists consider the event to be of first-rank significance for what it taught them about the earthquake process. A few aftershocks of this major quake still occur, and research into what happened and what may follow in the future continues to be intensive.

The most important finding from Landers, according to Tom Henyey, deputy director of the Southern California Earthquake Center at USC, is proof not only that a big earthquake can span more than one fault--Landers directly occurred on three--but that it can trigger quakes hundreds of miles away.

So-called "sympathetic" quakes quickly followed the Landers event as far away as the volcanic areas in Yellowstone National Park and Mammoth Lakes, in addition to near Mojave, Calif., and in southern Nevada.

"So we began to think there could be correlations of quake activity over large distances and several faults," Henyey said. "And when the [7.1] Hector Mine quake occurred in the fall of 1999, we learned more about stress transfers and how one big quake could bring about stress changes that would trigger another nearby in a relatively short amount of time."

Another example of one quake perhaps leading to more was seen in the 1994 Northridge quake, which followed by 23 years the 1971 Sylmar-San Fernando, both about magnitude 6.7. Their epicenters were only a few miles apart.

Both the Hector Mine, following the nearby Landers temblor by only seven years, and the Northridge events weakened the argument that a big quake necessarily foreshadows an extended period of little seismic activity nearby. The 23 years between the two San Fernando Valley earthquakes might seem a long time to some, but in geologic terms, it is a very short period.

Lucy Jones, the scientist in charge of the U.S. Geological Survey's Pasadena office, said that after the Landers event, scientists went back and were able to show that the great San Francisco quake of 1906 also triggered many quakes long distances from the epicenter.

Jones said a graduate student at San Diego State, Aaron Metzner, has researched the 1906 newspapers and found reports of quakes in many California locales after the San Francisco disaster. Two of the largest were a magnitude-6 quake in the Imperial Valley and a magnitude-5 in Santa Monica Bay.

But it was not until after the Landers quake that scientists were able to confirm the direct correlation in time and stress transfers.

Also, Jones said, the Hector Mine quake occurring parallel to the Landers one and fairly close, to the northeast, helped to further research about stress transfer.

Tom Heaton, an earthquake engineering professor at Caltech, said the fact that the Landers temblor spanned three separate faults and that ruptures occurred along all three "really started us thinking about fundamental issues of how earthquakes rupture in the first place."

There were certain discontinuities, or gaps, between areas that were shaken in the Landers quake and its aftershocks that showed scientists how the process is more complicated than they had thought. And the fact that the quake was well recorded provided data supporting that concept, Heaton said.

The Landers event also gave support to the work of geophysicist Amos Nur at Stanford, who in 1989 wrote a paper with an Israeli and an Italian scientist contending that a new fault system was emerging in what he called the Eastern California Shear Zone. That extended in a line from the epicenter of the magnitude-8 Owens Valley quake of 1872 south through Barstow, Landers and Yucca Valley to the Palm Springs area.

Both the magnitude-6.1 Joshua Tree quake that preceded the one in Landers by two months, and the Landers quake, tended to confirm Nur's reasoning. He had already pointed out in 1989 that there had been four or five sizable quakes in the 20th century along this general line.

Other research into the Owens Valley-Landers orientation of quakes has been done recently by Gilles Peltzer, a UCLA scientist who specializes in the new satellite radar analysis known as "interferometry," which can show minute geologic slippage along faults.

Peltzer and several colleagues have found that in a gap with few if any large earthquakes between the southern end of the Owens Valley rupture and the northern end of the Landers rupture, there still seems in recent years to have been a slippage rate two or three times higher than the normal of past centuries. That means stress has been accumulating.

This may indicate that a magnitude-7 quake could occur in the gap, roughly between Barstow and Coso Junction along U.S. 395, a span of about 100 miles, in the years ahead, although no precise time frame is evident, Peltzer said this week.

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