THE LOS ANGELES EARTHQUAKE : Quake Leaves a Mystery: Why So Few Hard Aftershocks?

As scientists from as far away as Colorado rushed to Los Angeles to study Thursday’s 6.1 earthquake, seismologists Friday continued to encounter surprises surrounding the jolt.

The earthquake inexplicably has been followed by an unusually low number of severe aftershocks, said Caltech seismologist Kate Hutton.

Caltech’s measurements and those of the National Earthquake Information Center have differed slightly, but they agree that there have been no more than five aftershocks registering 4 or more on the Richter scale and no more than 19 that were between 3 and 4. There has been none greater than 3 since 1:39 p.m. Thursday.

The aftershocks that have been observed, Hutton said, are progressing deeper below the earth’s surface and are migrating in a northeasterly direction from the epicenter below Rosemead.

By mid-afternoon Friday, the epicenters of the aftershocks were almost directly under Temple City. But they all registered below magnitude 3 on the Richter scale and probably were not felt, Hutton said.

Far Different in Palm Springs

The lack of significant aftershocks stands in sharp contrast to the aftermath of the 5.9 earthquake in Palm Springs in July, 1986, when 50 aftershocks greater than magnitude 3 occurred in the ensuing 24 hours--more characteristic of earthquakes of that magnitude. Even today, occasional aftershocks--some as high as 4 on the Richter scale--are felt there.

Researchers Friday were at a loss to explain why so few significant aftershocks have been recorded since Thursday morning, but they were optimistic that the lack of further seismic activity suggests that a large aftershock, say above magnitude 5, is unlikely.

“We are now down below a 1% chance of a bigger quake coming out of Thursday’s,” Hutton said.

The unusually low number of aftershocks was the second curious phenomenon associated with Thursday’s earthquake.

Caltech seismologists said after the temblor that there had been no earthquake of magnitude 4 or higher in Southern California since November, 1986--the longest such period since Caltech began monitoring geological activity 55 years ago. Again, they had no explanation.

By Friday, one team of researchers had placed sensitive new detectors along the Whittier Fault to pinpoint the location of the epicenter and to chart the progress of aftershocks.

No Ruptures Found

Another team was searching--so far unsuccessfully--for evidence that the Earth’s surface had been ruptured by the earthquake. And a third was assessing damage to man-made structures and the landscape.

“This will probably be the most thoroughly studied earthquake in history,” said Stephen Hartzell, director of the U.S. Geological Survey office at Caltech, who is coordinating the effort.

The epicenter of the quake was about eight miles below Rosemead and more than two miles beyond the point where surface signs of the fault disappear, according to Lucille Jones of the Geological Survey.

But Jones nonetheless believes that the quake occurred on the fault because the bulk of the energy released in the quake was channeled back along the fault line in a shallow plane that brought the energy to the surface near the base of the Puente Hills north of Whittier. The fault represented the path of least resistance.

As a result, Jones said, the height of those hills may have been increased. After the 1971 Sylmar earthquake, which involved the same type of vertical earth movement, the elevation of some of the surrounding San Fernando Mountains increased five to six feet, she noted.

The channeling of energy, Jones said, explained why downtown Whittier sustained more severe damage than Rosemead.

Meanwhile, most geologists who were not studying Thursday’s earthquake went about their business as usual--studying some of the hundreds of other faults in Southern California.

“We are faced with a situation where a magnitude 6 earthquake can occur on any one of hundreds of faults, and what we really have to do is focus our efforts on those faults that can generate 7s and 8s,” said Stanford University geologist Mark Zoback.

“If there is any lesson here, it’s probably that you can’t put your guard down no matter where you live in Southern California,” Hart-zell said. “You can’t discount these smaller faults. You can’t forget about them.”