M6.7 January 17, 1994 Northridge, California Earthquake

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Appreciation of the severity of earthquake hazard in the greater Los Angeles area grew slowly in the early 20^th century, cemented by the occurrence of the magnitude-6.4 Long Beach earthquake, early in the evening of March 10, 1933.  Since that time, two other moderately large earthquakes have struck within the greater Los Angeles region: the 1971 Sylmar (San Fernando) earthquake, followed by the January 17, 1994 Northridge earthquake.

Both Sylmar and Northridge were estimated 6.7s—not the dreaded “Big One”—but both damaging by virtue of their appreciable size and their locations in proximity to major population centers.  The fact that Southern California has experienced damage from “Pretty Big Ones” is not surprising: earthquake statistics tell us that Pretty Big Ones, magnitude 6-7, occur much more frequently than earthquakes closer to magnitude 8.

The Northridge earthquake took a heavy toll, killing 33 people, leaving over 7,000 injured, and 20,000 area residents homeless.  Estimates of property damage are approximately 40 billion dollars.  Damage to freeway bridges and overpasses disrupted key transportation arteries for months after the earthquake.  Detailed analyses of damage led to important updates to construction practices and building codes.  The earthquake also spurred a major program by Caltrans to retrofit the State’s bridges and overpasses, among other retrofitting programs.

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The Northridge earthquake also impacted earthquake science. The earthquake caused widespread liquefaction and soil failure, and triggered over 11,000 landslides. Ground motions recorded by local seismic instruments dramatically illustrated what seismologists had begun to understand at the time.  Although very large earthquakes release more energy overall than moderately large earthquakes, the most severe shaking from some moderately large earthquakes can rival peak shaking of larger events. At some locations in proximity to Northridge, shaking was strong enough to overcome the force of gravity.  

The earthquake also taught scientists lessons about the complexity of fault systems in the greater Los Angeles area.  Shortly after the earthquake struck, it appeared to have occurred on an “unknown” fault.  In fact, considering data carefully, scientists showed that it actually occurred on a known fault system with previously unknown complexities.  This has been a general lesson of earthquake science over the last century: the closer we look at faults and earthquake ruptures, the more complexity we see.  The Northridge earthquake also taught scientists more about “earthquake-earthquake” interactions, enhancing our understanding how the occurrence of one earthquake can sometimes influence nearby faults.

The Northridge earthquake had a major impact on earthquake monitoring.  It directly spurred further development of the USGS seismic monitoring networks and the range of products produced by what is now the Advanced National Seismic System (ANSS).  In 1994, data from the seismic network was used to pinpoint the epicenter of the earthquake and its magnitude. But technologies did not exist to rapidly characterize the distribution of shaking.  Within a few years, the USGS and its partners made key upgrades to seismic networks, and moved towards real-time development of now-familiar ShakeMaps.  These modern products have now been generated retroactively for the Northridge earthquake.  

From 1933 onwards, moderately large earthquakes have taken their toll on the greater Los Angeles area, but lessons from each event take us all one step further along the path to risk reduction.

Earthquake Science Center Seminar

The 17 January 1994 Northridge Earthquake: That was Then, This is Now