Aftershock : Latest Sequence of Tremors May Not Be Over, Scientists Say

By KENNETH REICH

Feb. 12, 1988 12 AM PT

Times Staff Writer

Thursday’s earthquake was the largest event yet in an unusual seven-week resurgence of aftershocks of last October’s Whittier Narrows earthquake, according to seismologists at Caltech and the U.S. Geological Survey’s Southern California field office in Pasadena.

The new sequence of aftershocks began Dec. 24 and may not yet have spent itself, said Lucille Jones, a senior seismologist with the Geological Survey.

Scientists said the resurgence of aftershock activity, considered unusual but not unprecedented in this region, is centered in a largely uncharted maze of subterranean thrust, or “dip-slip,” faults that lie beneath the Los Angeles Basin. This fault zone has been of increasing interest to scientists since the Oct. 1 quake.

Jones and Caltech seismologist Kate Hutton told reporters Thursday that there were fewer aftershocks than were expected in the two or three days immediately following the Oct. 1 earthquake. Then there was the stronger than expected 5.3 aftershock of Oct. 4.

Between Nov. 10 and Dec. 24, seismic activity dropped off and there were no aftershocks as powerful as magnitude 2, a quake that can barely be felt by people.

Then, on Dec. 24, there was a 2.5 aftershock and the resurgence was under way. The Dec. 24 shock was followed by 3.2 jolt on Jan. 2 and a moderately strong magnitude 4 temblor on Jan. 19. There have been 15 aftershocks of at least magnitude 2 since Jan. 2.

Thursday’s aftershock was recorded at 5.0 at Caltech and 4.7 at the National Earthquake Information Center in Golden, Colo. Preliminary measures of magnitude frequently disagree and are revised later.

All of these events have been in the San Gabriel Valley epicenter area of the Oct. 1 earthquake, leading to the conclusion that they are aftershocks rather than a new quake sequence.

Caltech seismologists Hutton and Clarence Allen said the long delay between the Oct. 1 quake and Thursday’s strong aftershock is unusual but not unheard of in Southern California.

“There is no such thing as a typical aftershock sequence,” Allen said.

Allen said that while all the aftershocks have been in the same general area, they have been on different faults. Because these faults run in varying directions, the areas of strongest surface shaking have varied.

For example, some of the strongest shaking in Thursday’s aftershock took place in Orange County areas where the shaking had not been particularly strong in the original quake.

Jones added that the Geological Survey, which issued an advisory Oct. 4 predicting that an aftershock sequence was “likely to extend over several days and possibly weeks,” decided Thursday morning not to issue another advisory.

She said she discussed the situation by telephone shortly after Thursday’s aftershock with John Filson, chief of the Geological Survey’s Office of Earthquakes, Volcanoes and Engineering in Reston, Va.

Filson, who wrote the earlier advisory, said later: “We don’t have any scientific basis to predict what’s going to happen in the future with this sequence. All we can do is watch it carefully.”

AFTERSHOCK ACTIVITY

Scientists suggest that the pattern of aftershock activity associated with the Oct. 1 Whittier Narrows earthquake is unusual. Aftershocks tapered off, then began again in December. The resurgence was later and stronger than had been anticipated.

Aftershocks of 3.0 or better

Oct. 1 Whittier Narrows main shock: 5.9

Oct. 1: 4.4*

Oct. 3: 3.2, 3.1

Oct. 4: 5.3, 3.7

Oct. 5: 3.4, 3.2

Oct. 6: 3.0

Oct. 8: 3.3

Oct. 11: 3.0

Oct. 16: 3.0

Oct. 19: 3.2

Nov. 6: 3.0