EARTHQUAKE: THE LONG ROAD BACK : Burst of Temblors Normal, Experts Say

Saturday’s magnitude 5.0 aftershock, centered above Chatsworth in Browns Canyon, was the strongest jolt in the Northridge earthquake sequence since Jan. 19 and the third-largest aftershock to date.

Striking near the western end of the aftershock zone, it was also part of a cluster, or what seismologists termed a “burst,” of nine aftershocks greater than 3.0 that occurred in the five hours between 2:03 a.m. and 7 a.m. Most of those were also near Chatsworth.

According to a list provided by Caltech, the quakes were a 3.3 at 2:03 a.m., a 3.3 at 3:13, the 5.0 at 3:20, a 3.4 at 3:37, a 4.2 at 4:16, a 3.4 at 4:47, a 3.1 at 4:59, a 3.4 at 6:03 and, finally, a 3.3 at 7 a.m.

The aftershocks occurring in recent days have on several occasions come in such clusters, sometimes within minutes of one another, rattling nerves and adding to public concern.

But U.S. Geological Survey seismologist Lucile M. Jones said Saturday that the aftershocks following the Northridge quake are within “a normal range,” even though they have been a little more numerous and powerful than are usually expected from a magnitude 6.6 temblor.

Jones, in a briefing at the Caltech Seismological Laboratory, said the chances of another magnitude 5 aftershock in the next week are about 1 in 7, and the chances of another one in the next year about 50-50.

There is little immediate concern that an earthquake larger than the original main shock of magnitude 6.6 will grow directly out of the Northridge earthquake, she said.

“This is just the earth settling into its new position,” she said of the aftershocks. “Large blocks have moved, and the earth is still readjusting.

“If we had one of these 5s every day, then it would be abnormal,” she said. “Or if we were not having any aftershocks at all, we would be concerned.”

Caltech seismologist Egill Hauksson said that unlike almost all the other quakes felt since Jan. 17, the largest one Saturday was a strike-slip, rather than a thrust earthquake. This means the two sides of the unnamed fault moved horizontally. In a thrust quake, the fault is on a dipping plane, and the movement is up and down.

The aftershocks Saturday were within three miles of the surface, and therefore strongly felt.

Jones said it has become clear, through the overlapping aftershock zones of the Northridge quake and the 1971 magnitude 6.4 Sylmar-San Fernando earthquake, that the two temblors are related.

On Saturday night, an aftershock of the 1992 Landers and Big Bear earthquakes joined the rash of aftershocks. The 3.5-magnitude aftershock struck at 5:06 p.m., a Caltech spokesman said.