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USGS Response to an Urban Earthquake -- Northridge '94

Conclusions--Working Toward National Seismic Safety

Despite the injuries, loss of lives, and economic setbacks caused by the Northridge earthquake, there were many successes in the way the Los Angeles region withstood the event. Because of the awareness of earthquake hazards in southern California, tens of thousands of buildings did not collapse and the number of casualties was relatively low. The fortuitous early morning occurrence of the earthquake, when few people were on the freeways and in large structures, was another significant life-saving factor. Compared with earthquake impacts in other parts of the world, losses from the Northridge quake (especially deaths and injuries) were remarkably small for a region where millions of people live.

Ongoing research by the USGS and other NEHRP agencies continues to improve our understanding and awareness of earthquakes, and has resulted in substantive measures to strengthen our cities against damaging earthquakes. Lessons learned from the 1971 San Fernando earthquake helped to reduce damage from the Northridge earthquake. The studies of damage and ground motions from the Northridge earthquake show that a modern California urban area can be broadly resilient although locally vulnerable to the widespread shaking from a moderate earthquake.

However, many of our successes were found in areas of “moderate” shaking. Some of the important results from studies of this earthquake show that cities will suffer heavy damage in the areas of “strong” ground shaking. We saw that an earthquake of moderate size can produce intense ground motions in the immediate vicinity of the fault rupture. During the Northridge earthquake, the areas of strong shaking were relatively small and limited mainly to a few areas in the San Fernando Valley and the unpopulated mountains to the north. Southern Californians may not be so lucky in the next event. Another earthquake of similar magnitude could focus the strong shaking directly at a densely populated region, or the next earthquake could simply be larger with damage spread over a much greater area.

We are certain that more earthquakes will occur. The next moderate-to-strong damaging earthquake to affect our country may hit Los Angeles again, or it could happen in San Francisco, Seattle, Memphis, Boston, or many other urban areas that sit close to potentially active faults. In any of these regions, we can expect economic losses to be in the range of tens to hundreds of billions of dollars, with human casualties depending upon the seismic resistance built into the particular environment. In areas that are substantially less prepared than southern California, the losses could approach those witnessed in recent earthquakes in urban areas of other countries, such as Kobe, Japan.

Principal roles of the USGS are to use its scientific expertise to identify and communicate seismic hazards, and to help ensure that our society attends to them. Earthquakes like the Northridge event focus USGS efforts in one region, and scientists are able in a short time to greatly increase the base of knowledge about earthquakes with intensive post-earthquake studies. In the aftermath of an earthquake we see dramatically the successes and failures of our efforts.

The successes of the USGS response to the Northridge earthquake are many, and are exemplified by those discoveries that helped change the way we now look at earthquakes. From applying seismic recording networks to immediate disaster response to devising predictive models of the effects of future earthquakes, the USGS and its cooperators have helped create a prototype that bears review throughout the Nation.

However, the post-earthquake studies are only a small part of ongoing programs to reduce seismic hazards. It is during the years between damaging earthquakes that most of the work is done. Understanding and identifying earthquake hazards and setting the stage for mitigating them is a continuous process that transcends the immediacy of public attention to disastrous events.

Wide-ranging efforts—from studying the fundamentals of earthquake physics to developing applied products such as probabilistic seismic-hazards maps—all contribute to a comprehensive USGS program committed to reducing earthquake losses.

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