The 1994 Northridge Earthquake

Photos taken by Gene Ichinose one week after the M=6.8 January 1994 Northridge Earthquake show damage to structures. The top panel shows a ground level collapse of an apartment structure next to California State University Northridge (CSUN), Northridge, CA just miles north of the epicenter. The two lower panels show the CSUN parking structure that was weakened from the mainshock. It later collapsed after a M=5.0 aftershock near Grenada Hills.

U.Cal.Berkeley CD photo album of Northridge 1994 earthquake

THREE DIMENSIONAL SIMULATION OF GROUND MOTION IN SAN FERNANDO VALLEY

We have developed a scalable, workstation-based procedure for 3D simulations of earthquake ground motion in the presence of very large seismic velocity contrasts. The method accomodates models with a large range of seismic velocity by using a form of adaptive gridding (recursive grid refinement). We have also developed a high resolution 3D geologic model for the L.A. region for use in the simulations, and this model has been distributed to other investigators engaged in ground motion prediction for the L. A. region.

We have completed 3 simulations of the Northridge mainshock to investigate the sensitivity of ground motion predictions to the rupture model. We have also completed simulations of 6 Northridge aftershocks, with the objective of investigating sensitivity of ground motion predictions to source location and mechanism. The simulations focus on low-frequency ground motion, with upper cutoff frequency of 0.4 Hz (one case had upper cutoff of 0.8 Hz). In all cases, maximum low-frequency horizontal velocity is correlated with depth to basement. In most cases, the predicted maxima are concentrated within the 10,000 ft. depth-to-basement contour. For a shallow aftershock outside the San Fernando Valley, the simulation also predicts high amplitudes along the nearest basin margin.

• Click here to see a Tres3D animation from simulating the 1994 Northridge Earthquake in the San Frenando basin. A point source was used at (lat,lon,z) = (34.286, -118.555 -16243.0) with a rise time of 4 seconds. The contours in the animation represent the crystaline basement depth in the valley. There are 240 frames at 0.125 second steps. The computation was done at dx=dy= 256 to 512 meter spaceings but then was desampled to 1024 meters. The corners of the screen are at - Upper Left: 34.3508 -118.682 and Lower Right: 34.1016 -118.239, near Dodger Stadium.