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Seismic Hazards More Widespread, Maps Show
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Damaged Washington National Cathedral after the 5.8-magnitude earthquake, centered in Mineral, Virginia
After the 5.8-magnitude earthquake centered in Mineral, Virginia, that struck on August 23, 2011—damaging the Washington National Cathedral and the Washington Monument, among other structures—scientists determined that the region could experience even larger seismic events. USGS/William Leith

Maps suggest that the risk of an earthquake in the United States is more widespread than previously known.

August 5, 2014—Engineers who work in the United States may want to brush up on their seismic design expertise. Newly updated seismic hazard maps produced by the U.S. Geological Survey (USGS) indicate that the potential earthquake risk is more widespread across the country than previously thought.

Research reveals that the East Coast has the potential for stronger and more damaging earthquakes, the Central United States has a greater range of potential earthquake magnitudes and locations, and the seismic hazard in California extends over a wider area than prior maps indicated. Still, while “there are incremental changes to the hazard in the maps ... there are no drastic changes on a regional scale,” said Robert Williams, the central and eastern U.S. regional coordinator for the USGS, in written responses to questions posed by Civil Engineering online.

The updated U.S. National Seismic Hazards Maps were released in July as part of the USGS’s ongoing earthquake hazard research. The agency has updated the maps approximately every six years since the 1990s as part of its contribution to the National Earthquake Hazards Reduction Program—a partnership of four federal agencies that is dedicated to reducing the risks to life and property as a result of earthquakes. The other participating agencies are the Federal Emergency Management Agency (FEMA), the National Institute of Standards and Technology (NIST), and the National Science Foundation (NSF). 

The maps help inform FEMA-sponsored seismic design provisions, which states and localities incorporate into their building codes. The codes serve as the minimum standard to which buildings and other structures must be designed, constructed, and maintained. “The expanded high-risk zones in the new USGS maps will definitely broaden the relative importance of seismic loads in building design across the country, trickling down to even ordinary industrial and residential development,” said Robert Nigbor, Ph.D., P.E., M.ASCE, a research engineer in the Department of Civil and Environmental Engineering at the University of California Los Angeles (UCLA), in written responses to Civil Engineering online.

According to the USGS, all 50 states have some potential for seismic activity, but 42 states have a reasonable chance of experiencing damaging ground shaking from an earthquake during a 50-year period—the average design life of a building. Sixteen of those states have experienced earthquakes of magnitude 6 or greater in the past and are considered to be at a relatively higher risk for destructive ground motions: Alaska, Arkansas, California, Hawaii, Idaho, Illinois, Kentucky, Missouri, Montana, Nevada, Oregon, South Carolina, Tennessee, Utah, Washington, and Wyoming.

While the states with the highest probability of damaging ground shaking are located in the central and western United States, the new maps show that the East Coast could see larger and more destructive earthquakes than previously recognized. After studying the 5.8-magnitude earthquake centered in Mineral, Virginia, that struck on August 23, 2011—one of the largest to hit the area in the past century—scientists have determined that the region could experience even larger events. Research has shown that the earthquake hazards near the historic city of Charleston, South Carolina, in particular, are greater than formerly predicted.

  USGA map which reveals that the East Coast has the potential for stronger and more damaging earthquakes than previously known 

 Research reveals that the East Coast has the potential for stronger
and more damaging earthquakes than previously known, and the
central United States has a greater range of potential earthquake
magnitudes and locations. Additionally, the seismic hazard in
California extends over a wider area than prior maps indicated.
USGS

Yet, despite the increased potential for larger and more damaging earthquakes in the eastern U.S., the maps suggest that the hazard for New York City’s skyscrapers is lower than previously stated. Scientists predict that ground motions near the city will be more rapid than originally thought, and fast shaking is believed to cause less damage to high-rise buildings than slower shaking, according to the USGS.

Nigbor, who is also the c-principal investigator and operations manager of nees@UCLA—the university’s component of the NSF-funded George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES)—said that he was quite surprised by the East Coast hazard changes and thinks more research must be done to better understand the seismic risks in the region. “We still do not know enough about East Coast seismic sources,” Nigbor said. “New knowledge from EarthScope-related studies should better inform the next revision of the mapping in this area.” EarthScope is an expansive NSF-funded geoscientific project aimed at better understanding the structure and evolution of the North American continent and its processes—including those that cause earthquakes and volcanic eruptions.

The USGS maps also reveal that the seismic hazard is broader than previously indicated in the central United States. On the basis of data from the U.S Nuclear Regulatory Commission, research suggests that the New Madrid Seismic Zone—the epicenter of which is New Madrid, Missouri—could produce temblors across a wider range of locations and in a wider range in magnitudes than previously known. “In the New Madrid Seismic Zone, there are several new faults added to the model around the perimeter of the zone that broaden the area of active faults,” Williams explained. “The new faults include the Commerce Fault, Marianna Seismic Zone, and East Rift Margin Fault. These faults are expected to have long time intervals between earthquakes compared to the main New Madrid faults, so the hazard is not altered significantly.”

One of the most seismically active states in the nation, California has seen trade-offs as the result of new seismic data. The seismic hazard estimates for the cities of San Jose, Vallejo, and San Diego increased due to newly discovered faults, while the risk decreased for Irvine, Santa Barbara, and Oakland because of new research. Similarly, the earthquake hazard increased for some parts of the San Francisco Bay Area and near Los Angeles and decreased in others on the basis of a new Uniform California Earthquake Rupture Forecast Model, which incorporates more potential fault ruptures than previous assessments.

While the seismic changes are not dramatic, people living in the seismic hazard zones and those designing and constructing buildings and other structures there must be vigilant about the potential dangers and take appropriate precautions. “We hope citizens, government agencies, and the private sector take note of these maps and then take steps to mitigate possible damage and losses, thereby increasing resilience and speeding recovery,” Williams said.


 

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