Seismology: The Northridge Earthquake and its Aftershocks

Egill Hauksson, & Lucile M. Jones

Published 1995, SCEC Contribution #138

A strong earthquake shook the Los Angeles region on January 17, 1994, at 4:30 am Pacific Standard Time (12:30 UT). The earthquake occurred beneath Northridge, a suburb in the San Fernando Valley, 30 km northwest of Los Angeles (see map on next page). With a moment magnitude of 6.7, the Northridge earthquake was the strongest earthquake to strike the Los Angeles area since the 1971 San Fernando earthquake, which also had a magnitude of 6.7. The Northridge earthquake produced extremely strong ground shaking, which caused the greatest damage in the United States since the 1906 San Francisco earthquake.

The Northridge earthquake was caused by dip slip on a thrust fault—the overriding movement of one crustal block up and over a second crustal block (see p. 1 1)—that dips down to the south-southwest beneath the northern San Fernando Valley. Although the 1971 San Fernando earthquake was centered relatively close to the Northridge epicenter, it occurred on a fault with a different orientation, a thrust fault dipping to the northnortheast instead of to the south-southwest. Although both events are rated as magnitude 6.7 earthquakes, the San Fernando earthquake caused much less damage than the Northridge earthquake because the San Fernando earthquake struck the sparsely populated San Gabriel Mountains, whereas the Northridge earthquake originated directly beneath the heavily populated San Fernando Valley.

The Northridge earthquake occurred on a blind thrust fault—a buried thrust fault that does not extend to the surface. This thrust fault and related faults make up a dense system of exposed and concealed reverse faults and thrust faults along the northern flank of the Los Angeles basin, which is contracting from south to north at a rate of about 7 mm per year. This relatively rapid rate of crustal shortening and the numerous, active thrust faults beneath the region imply that the Los Angeles region faces one of the greatest seismic hazards in California. The individual faults in the zone move more slowly than the San Andreas fault and produce smaller earthquakes than those expected on the San Andreas fault. However, in aggregate, the reverse and thrust faults represent a similar or greater threat because they produce more earthquakes and extend directly beneath densely populated, highly developed urban areas.

Hauksson, E., & Jones, L. M. (1995). Seismology: The Northridge Earthquake and its Aftershocks. Earthquakes and Volcanoes, 25(1), 18-30.