At 4:09am PDT on Sunday, April 27, 1997, a M4.9 earthquake occurred 8 miles NNE of Simi Valley (4 miles SW of Castaic Junction, 6 miles W of Newhall), with a depth of approximately 14 km. This event is approximately 1 mile NE of yesterday's M5.0 aftershock. Similar to yesterday's event, the focal mechanism indicates a thrust faulting mechanism.

Since January 17, 1994 the Southern California Seismic Network has recorded over 13,750 aftershocks from the Northridge Earthquake .

Aftershocks themselves have aftershocks, and today's event is producing several dozen small aftershocks. Yesterday's M5.0 had 55 aftershocks recorded in the M1 to M3 range.

Ground Motions

The ground motions produced by this earthquake were recorded at over 50 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. The ground accelerations recorded for this event were 12%g at Solamint, 11%g at Sylmar, and 5.9%g at Granada Hills. The ground accelerations recorded for yesterday's M5 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.

Southern California Earthquake Center Data Center Home Page

U.S. Geological Survey - Pasadena Home Page