Comparing Aftershocks

Many San Fernando Valley residents have remarked that the aftershocks of the Northridge earthquake seem to be stronger and more numerous than the aftershocks of the 1971 Sylmar-San Fernando earthquake. But scientists say this is not the case.

The epicenter of the 1971 earthquake was under the San Gabriel Mountains, several miles north of the populated areas of the Valley, and about two-thirds of all its aftershocks were centered outside these populated areas.

By contrast, the Northridge quake was directly underneath a thickly populated area, and almost all aftershocks except a few at the far western end of the aftershock zone north of Chatsworth have been centered under populated areas.

There have been 8,000 aftershocks from the Northridge quake, about 400 of them strong enough to be felt. The latest struck Wednesday at 8:27 p.m., when an 3.7-magnitude aftershock hit a mile southwest of San Fernando.

Although the Northridge earthquake was somewhat stronger than Sylmar-San Fernando, 6.8 as against 6.5 magnitude, the director of the U.S. Geological Survey’s Pasadena field office, Jim Mori, said that the total number of aftershocks was about the same. He acknowledged that instrumentation was not as extensive in 1971, so many of the tiny 1- and 2-magnitude quakes were not recorded. But the number of 3s and larger was about the same, he said.

Caltech Seismologist Egill Hauksson, who was among those who plotted aftershocks for this map, noted that the Northridge aftershock zone is on a subterranean plane that slopesupward to the north and never intersects the surface. The Sylmar-San Fernando aftershock zone was on a plane sloping upward toward the south that did come to the surface.

The two zones differ in depth by about five miles. So even where the two aftershock zones seem to intersect on the map, they actually do not. In this area, the Northridge aftershocks actually underlie the Sylmar-San Fernando aftershocks. Still, the northeast Valley is the one area where aftershocks from both earthquakes were occasionally strongly felt.

Magnitude For both quakes and their top five aftershocks Jan 17: Northridge-6.8 Feb. 9: Sylmar-6.6