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Frequency of Aftershocks Puzzles Scientists


Five years after the Landers-Big Bear earthquakes, a cluster of aftershocks, now totaling several thousand, continues a few miles northeast of Barstow, puzzling some scientists.

There was another flurry in this series Thursday night and Friday morning with magnitude 3.6, 3.5 and 3.3 temblors, all centered 13 miles northeast of the Mojave Desert city of 22,000.

That was within a mile of the strongest quake in the cluster, a 5.1 jolt, which occurred March 18. Before that, the strongest quake in the series had been a 4.8 in 1992.

Aside from minor structural damage and shattered glass in the March 18 shaker, the quakes have not caused any property losses in the Barstow area. But their frequency has caused nervousness among some residents.

"I would say the Barstow cluster is quite unusual," Caltech engineering seismology professor Thomas H. Heaton said Sunday.

"It almost has a behavior of an aftershock sequence of a considerably larger earthquake. The cluster started right after the [magnitude 7.3] Landers earthquake [of June 28, 1992], and these are Landers aftershocks. But they are certainly not typical aftershocks."

For one thing, Heaton said, they are continuing. For another, there is a gap of about 20 miles, where very few quakes have taken place, between the Barstow cluster and the main Landers rupture and aftershock zone to the south.

Caltech seismologist Egill Hauksson says there is scientific speculation about the possibility of a larger earthquake, perhaps a 6.5, eventually taking place in this gap near Pinto Mountain, south of Barstow.

But Heaton said Sunday that this could take a very long time to occur, and chances are the quakes northeast of Barstow will not grow in magnitude.

"One interpretation is that the faulting here is complex, and it is not likely that ruptures, when they occur, will run a long distance and develop into a big earthquake," he said.

Seismologist Lucy Jones of the U.S. Geological Survey, meanwhile, said the gap between the Barstow cluster and the main Landers quakes may only seem unusual. Detailed records of such large quakes as Landers are so few that it is not certain how unusual it is.

The Barstow area, about 90 miles northeast of Los Angeles, is part of the Eastern California Shear Zone. It continues to have a relatively high level of seismic activity, with quakes also persisting over the last two years in the Ridgecrest area, where a 3.9 occurred Friday night.

Amos Nur, a Stanford University scientist, has suggested that over millions of years, the shear zone--which extends northward from the Imperial Valley through the Barstow and Ridgecrest areas into the Owens Valley and along the Sierra Nevada range--may replace the San Andreas fault as Southern California's main tectonic plate boundary. In Nur's thinking, gigantic quakes would, over considerable time, tend to move farther from Los Angeles and out into lightly populated desert areas.

The tectonic boundary is between the North American and Pacific plates, with the Pacific plate moving north and the North American south.

As long as the San Andreas fault is the main boundary, the prospect is that millions of years from now Los Angeles will move past San Francisco and wind up farther north than the Bay Area.

That would not take place if the boundary shifted east of the Sierra.

Scientists stress that it would take so long to happen that present-day Californians need not worry.

Like the Landers quake, the huge, magnitude 8.0 Owens Valley earthquake of March 26, 1872, which killed 27 people near Lone Pine, was an Eastern California Shear Zone quake.

Geological Survey volcanologist David Hill and other scientists have published research showing how the Landers quake quickly triggered seismicity all the way up the Owens Valley and the Sierra front into volcanic areas extending to Yellowstone National Park.

That's a long way from the Barstow cluster. Curiosity about it focuses mainly on the persistent and intense aftershock sequence more than 20 miles from the main Landers rupture.

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