01/13/1995 08:00:00

Northridge Earthquake Facts, a One-Year Anniversary Summary

The Earthquake

* The magnitude 6.7 Northridge earthquake started on January 17, 1994 at 5 seconds before 4:31 a.m.

* There were no immediate foreshocks. No systematic change in strain above the background noise occurred during the hours to milliseconds before the event.

* The fault ruptured by the Northridge earthquake rises from a depth of about 19 kilometers (12 miles) at its southern edge to a depth of about 5 kilometers (3 miles) at its northern edge. The fault is blind—it does not break the surface—and was previously unknown.

* The rupture started at the southern, deepest edge and spread up to the northwest, north, and northeast. The final dimension of the fault plane broken in the earthquake is about 16 by 19 kilometers (10 by 12 miles).

* The actual rupture of the fault only lasted about 8 seconds, but because of amplification and reverberation of the seismic waves through the complex of faults, sediment, and mountains, most people felt shaking for 20 to 30 seconds.

* Geodetic measurements (those made by satellite) show permanent changes in the topography of the San Fernando Valley of up to 40 to 50 centimeters (16 to 20 inches) of vertical gain, and up to 20 centimeters (8 inches) of horizontal displacement.

* Geophysicists calculate that most of the slip took place in concentrated patches of the fault. The greatest amount of slip at any spot was about 4 meters (13 feet).

* The earthquake caused very large ground motions with peak accelerations of 0.5 to 1.0 g in the Northridge area, decreasing to 0.1 g at distances of about 50 kilometers (31 miles). (A "g" of acceleration is equivalent to the acceleration of gravity. There were a few sites near the Northridge earthquake that recorded over 1 g of vertical acceleration. These ground movements would have been capable of throwing objects of any size into the air.)

* The pattern of damage and strong ground shaking was irregular, with severe damage in places like Sherman Oaks and Santa Monica. These effects were caused by the complexity of the earthquake source and the wave propagation through complicated geology.

Aftershocks

* Through December 31, 1994, 11,030 aftershocks, most of which were too small to feel, were recorded by the Southern California Seismic Network, which is operated jointly by the United States Geological Survey and Caltech.

* More than 400 aftershocks have been large enough to feel, including

8 between magnitude 5.0 and 5.9 48 between magnitude 4.0 and 4.9 367 between magnitude 3.0 and 3.9

These numbers and sizes are typical for a magnitude 6.7 earthquake.

* Based on the the statistical pattern of Northridge aftershocks and on the behavior of other past earthquakes, in 1995 people should expect to feel about 17 magnitude 3.0 to 3.9 aftershocks, and about two magnitude 4.0 to 4.9 aftershocks. There is also about a 25 percent chance of another magnitude 5.0 to 5.9 aftershock.