* 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
* 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.
* 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.
Contact: Jay Aller, Max Benavidez, or Sue Pitts