Aftershock Raises New Seismic Questions

A moderate aftershock measuring magnitude 4.7 struck six miles southeast of Yucca Valley Tuesday in an area that has had hundreds of temblors, leading a Caltech scientist to question if an even larger aftershock is yet to come there.

The latest jolt occurred at 1:47 a.m. and caused very light damage and no injuries. It was felt as far away as Los Angeles and Orange counties.

“A few drawers opened, a few dishes were broken, a television set toppled over,” said a San Bernardino County sheriff’s spokesman in Joshua Tree, one of several small towns in the vicinity.

But the frequent aftershocks near Yucca Valley are posing some new questions about the Landers-Big Bear earthquake sequence, said Caltech seismologist Egill Hauksson.

Hauksson said scientists have found that in addition to the 45-mile-long rupture north of Landers, a smaller, secondary surface rupture extends 25 miles to the south through Yucca Valley to Black Rock Springs.

Although ground slid up to 20 feet in opposite directions along the 45-mile rupture, displacement along the smaller rupture is only about four to eight inches. But it is here, scientists said, that aftershocks have occurred with more frequency than any other place in the desert earthquake sequence.

In fact, Hauksson pointed out Tuesday, the area southeast of Yucca Valley has been shaken frequently since April’s 6.1 temblor near Desert Hot Springs, which is believed to have been a precursor to the 7.5 Landers quake that occurred June 28.

“What we are asking is why was there such a large rupture, so much movement north of Landers, and so much less to the south of it,” Hauksson said in an interview. “It may be that the area to the south remains highly stressed.”

He said that a magnitude 6 to 6.5 aftershock could yet occur there, although it is conceivable that there would be only a series of smaller aftershocks over a long period.

Another area of hundreds of Landers aftershocks, a few miles northeast of Barstow, has also been pinpointed by some scientists as a potential site of a large future aftershock.