Earthquake Maps Reveal Higher Risks for Much of U.S.

07/17/2014 20:55

A photo of a stone angel dislodged from the Washington National Cathedral following a 2011 earthquake.

Look out, South Carolina, Illinois, and Missouri. Although California is well known for earthquakes, new federal government maps extend the high-risk zones for temblors across much more of the country.

On Thursday, the United States Geological Survey (USGS) announced updated U.S. National Seismic Hazard Maps, which reflect the most current scientific views on where future earthquakes will occur, how often they will occur, and how hard the ground will shake.

Since the agency's previous maps were released in 2008, "the general patterns of earthquakes across the U.S. have not changed significantly, but lots of the details have changed," says Mark Petersen, who leads the USGS's mapping efforts from Denver as chief of the National Seismic Hazard Project.

The maps are widely used by engineers and planners to design buildings and infrastructure to withstand earthquakes, and Petersen says his agency will be working with that sector to decide if building codes need to be updated. (Watch "Earthquake 101.")
Map of earthquake ratios

New Data

The maps were drawn from new seismic data collected over the past several years as well as improved computational modeling done at the University of California, Berkeley and elsewhere.

The maps also draw from GPS data of movement along fault lines, the first time such data has been used by the USGS in this way, says Petersen. GPS data has allowed scientists to monitor much wider areas than was previously possible with limited laser studies.

In a report accompanying the maps, the USGS points out that while all U.S. states have some potential for earthquakes, 42 of the 50 states "have a reasonable chance of experiencing damaging ground shaking from an earthquake in 50 years," which is generally considered the typical lifetime of a building. Sixteen of those states have a "relatively high likelihood" of damaging shaking.

Based on historic trends, the regions most at risk remain the West Coast, the Intermountain West, and several known active regions in the central and eastern U.S., including near New Madrid, Missouri, and Charleston, South Carolina.

The 16 states at highest risk of quakes are Alaska, Arkansas, California, Hawaii, Idaho, Illinois, Kentucky, Missouri, Montana, Nevada, Oregon, South Carolina, Tennessee, Utah, Washington, and Wyoming.
Map of earthquake hazards.

Expanded Risks

Petersen says several areas have a higher potential for bigger earthquakes than previously thought. Among them is much of the eastern U.S.

Scientists historically hadn't had a lot of earthquake data from the eastern part of the country, but the magnitude 5.8 earthquake that struck Virginia in 2011 was felt by tens of millions of people and was recorded by numerous data centers. The quake caused structural damage, including to historic monuments in Washington, D.C., and taught scientists a lot about the regional geology by "enhancing our data set immensely," says Petersen.

The new maps also reflect expanded earthquake risk around the New Madrid Seismic Zone in southwestern Missouri; the zone stretches into Illinois, Indiana, Kentucky, Tennessee, Mississippi, and Arkansas. A series of earthquakes up to magnitude 8.1 devastated that area in 1811 and 1812.

Higher risk is also seen across much of the West Coast, thanks to new data from California and the Northwest's Cascadia Subduction Zone.

In a statement, Jim Harris, a member and former chair of the Provisions Update Committee of the Building Seismic Safety Council, said the building industry is reviewing the new maps with an eye toward updating codes. "The committees preparing those standards welcome this updated USGS information as a basis for making decisions and continuing to ensure the most stable and secure construction," he said. NationalGeographic

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