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Friday, March 31, 2023

10-Year Anniversary of the United States’ Most Widely Felt Earthquake

Ten years ago, in the early afternoon of August 23, 2011, millions of people throughout the eastern U.S. felt shaking from a magnitude 5.8 earthquake near Mineral, Virginia. No lives were lost, something experts called “lucky” given the extent of shaking, but property damage was estimated to be in the range of $200 to $300 million.

Although not the strongest earthquake to have occurred in the eastern U.S., let alone the western U.S., the Virginia earthquake was likely felt by more people than any earthquake in North America’s history. This is due to the large distances at which people felt ground shaking and because of the density of the population in the eastern U.S.

“The 2011 earthquake in Virginia was significant in raising awareness of east coast earthquake hazards and the importance of research to understand when and where earthquakes can occur,” said David Applegate, the U.S. Geological Survey’s associate director for natural hazards, currently exercising the delegated authority of the director. “Damaging earthquakes do not strike the eastern U.S. often, but the potential consequences of not understanding and planning for such events with their widely distributed shaking could be severe. The next significant earthquake on an eastern U.S. fault may not occur for hundreds of years, yet there is a small chance it could happen at any time.”

Scientific studies by the USGS and partners played a critical role in helping prepare for and respond to this earthquake. The USGS is the federal government’s lead agency for assessing seismic hazards before, during and after earthquakes, providing critical information used to reduce risks to lives and property nationwide. Over the past decade, scientists have continued building on their knowledge of eastern U.S. earthquakes, specifically focusing on what can be learned from this Virginia earthquake.

“One of the fascinating things we discovered was heightened ground shaking in Washington, D.C., resulting in damage to buildings in the city at distances that would not ordinarily be expected,” said Thomas Pratt, who is a USGS research geophysicist and expert in eastern earthquakes.

Science Since the Earthquake

Heightened Shaking in Washington, D.C.

Amplified shaking was initially documented from the Mineral earthquake through detailed analyses of USGS “Did You Feel It?” reports. Each of those reports is from a member of the public describing their earthquake experience. Analyses showed that people in Washington noted stronger shaking than scientists would have expected.

“Subsequent research identified that the underlying sediment is what led to amplified shaking,” Pratt said. “We were familiar with that phenomena on the West Coast of the U.S. and internationally, but the Mineral earthquake showed the significance of this effect in the eastern U.S. The areas on sediment received significantly stronger shaking than nearby locations on firmer rock.”

The USGS and partners installed temporary seismometers in the city following the earthquake to better understand the variation in the strength of shaking and its relationship to sediments beneath the city. Sediments are weaker than the underlying bedrock and therefore move more, while also trapping more energy.

Amplification by sediments has been seen in earthquakes in other areas, for example in the concentration of damage in the Marina district of San Francisco during the 1989 Loma Prieta earthquake, but the strength of this effect was not as well known for the sediments along the East Coast. It is now recognized that this amplification can be severe in the eastern U.S.

Direction of Shaking

USGS scientists found that the strength of ground shaking from the Mineral earthquake was substantially greater to the northeast than in other directions. This direction is nearly parallel to the orientation of the Appalachian Mountains and the eastern edge of the continent, which shows the influence of large-scale features like mountain ranges on ground shaking. A similar pattern was observed in the magnitude 5.1 earthquake in Sparta, N.C. in 2020, which also showed stronger shaking along the trend of the Appalachians.

Informing Decisions

Knowing the amplification caused by these sediments and the direction of shaking will help emergency managers identify communities that may be more vulnerable to shaking. This knowledge will help the USGS refine its seismic hazard maps, which estimate the strength of ground shaking that can be expected during earthquakes in each area of the country.

These insights can also be used by emergency managers when planning for and responding to disasters; state and local governments as they refine building codes; and architects and engineers as they design and renovate buildings to mitigate the effects of future earthquakes. In addition, the science helps inform planning for major infrastructure investments such as dams and reservoirs.

Read more at USGS

Homeland Security Todayhttp://www.hstoday.us
The Government Technology & Services Coalition's Homeland Security Today (HSToday) is the premier news and information resource for the homeland security community, dedicated to elevating the discussions and insights that can support a safe and secure nation. A non-profit magazine and media platform, HSToday provides readers with the whole story, placing facts and comments in context to inform debate and drive realistic solutions to some of the nation’s most vexing security challenges.

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