Magnitude 5.0 Northridge Aftershock, April 26, 1997

Caltech, US Geological Survey, & SCEC

April 26, 1997 10:00 A.M.

An Aftershock of the Northridge Earthquake

At 3:37am PDT on Saturday, April 26, 1997, a M5.0 earthquake occurred 7 miles NNE of Simi Valley, with a depth of approximately 16 km. The focal mechanism indicates a thrust faulting mechanism. This aftershock of the M6.7 Northridge earthquake of January 17, 1994 was the 10th aftershock of magnitude 5.0 or greater for this sequence. The previous M5.1 aftershock occurred on June 26, 1995. Today's M5.0 has so far produced approximately 20 aftershocks, of magnitude greater than M2.0.

Since January 17, 1994 the Southern California Seismic Network has recorded over 13,726 aftershocks from the Northridge Earthquake .

Aftershocks themselves have aftershocks, and can even be foreshocks. Like any other magnitude 5.0 in Southern California, this event has a few percent chance of being followed by an earthquake of the same size of greater within the next week.

Ground Motions

The ground motions produced by this earthquake were recorded at over 60 stations of the new digital seismographic network in southern California called TriNet (a cooperative project of Caltech, U.S. Geological Survey, and the State Department of Conservation Division of Mines and Geology). The ground motions are defined as a percentage of the force of gravity. An acceleration of 1%g means the ground is pushing on you with 1% of the force that the gravity of the earth pulls on you. Ground accelerations recorded in this earthquake were 14%g at Sylmar, 10%g at Solamint and 7.6%g at Granada Hills.

Peak ground values from the TriNet stations were used in an automated algorithm to produce shaking maps (ground velocity and ground acceleration) several minutes after the earthquake origin time. These maps display which regions had the strongest shaking and thus which regions should be first examined for damage. The maps are placed on the World Wide Web as soon as they are generated for general access.

List of current seismicity in southern California can be obtained by
Recent Southern California Seismicity Page

Southern California Earthquake Center Data Center Home Page

U.S. Geological Survey - Pasadena Home Page