The tenth anniversary of the most widespread US earthquake

(USGS/WOWK) — Ten years ago, in the early afternoon of August 23, 2011, millions of people across the eastern United States felt the shaking from a 5.8-magnitude earthquake near Mineral, Virginia. No lives were lost, which experts called “lucky” given the extent of the shaking, but property damage was estimated at $200-300 million.

Although it was not the most powerful earthquake to have occurred in the eastern United States, let alone the western United States, the Virginia earthquake was likely felt more by people than by any other earthquake in North American history. This is due to the large distances at which people felt the shaking of the earth and due to the density of the population in the eastern United States

“The 2011 earthquake in Virginia was important in raising awareness of East Coast earthquake hazards and the importance of research to understand when and where earthquakes can occur,” said David Applegate, associate director of the USGS Natural Hazards. from the outlet. “Destructive earthquakes do not strike the eastern United States very often, but the potential consequences of not understanding and planning for such events with their widely spread vibrations may be severe. The next major earthquake may not occur on the eastern United States fault for hundreds of years, but there is little possibility happen at any time.”

Damage to Washington’s National Cathedral in D.C. from the earthquake in central Virginia on August 23, 2011 (Credit: William Leith, USGS. Public domain.)

Scientific studies by the USGS and its partners played an important role in helping to prepare for and respond to this earthquake. The USGS is the federal government’s lead agency for assessing seismic risk before, during, and after earthquakes, and provides critical information used to reduce risks to life and property nationwide. Over the past decade, scholars have continued to build on their knowledge of earthquakes in the eastern United States, with a particular focus on what can be learned from the Virginia earthquake.

“One of the remarkable things we’ve discovered is the intense shaking of the ground in Washington, D.C., which has caused damage to buildings in the city at normally unexpected distances,” said Thomas Pratt, a USGS geophysical researcher and expert on eastern seismicity. .

Science since the earthquake

Severe shaking in Washington, DC

The initially amplified vibration from the mineral earthquake was documented by USGS’ detailed analyzes of “Have you felt it?” reports. Each of these reports from a member of the audience describes their earthquake experience. Analyzes showed that people in Washington noticed stronger shaking than scientists had expected.

“Later research determined that it was underlying sediment that amplified the vibration,” Pratt said. “We were aware of this phenomenon on the West Coast of the United States and internationally, but the mineral earthquake demonstrated the importance of this effect in the eastern United States. Areas on the sediments received much stronger shaking than nearby sites on more solid rock.”

Photo of chimney damage to a home in Louisa County, Virginia. This was the result of a 5.8-magnitude earthquake in Virginia on August 23, 2011 (Credit: Francis Ashland, USGS. Public domain.)

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

Amplification by sediment was seen in earthquakes in other regions, for example in the damage focus in the Marina District of San Francisco during the 1989 Loma Prieta earthquake, but the strength of this effect was not well known for sediments along the East Coast. It is now recognized that this amplification can be severe in the eastern United States

vibration direction

USGS scientists found that the ground shaking force from the mineral quake was much greater in the northeast than in other directions. This trend is roughly parallel to that of the Appalachian Mountains and the eastern edge of the continent, which indicates the influence of large-scale features such as mountain ranges on the Earth’s vibration. A similar pattern was observed in the magnitude 5.1 earthquake in Sparta, North Carolina in 2020, which also showed stronger shaking along the direction of the Appalachian Mountains.

inform 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 improve its seismic hazard maps, which estimate the strength of ground shaking that can be expected during earthquakes in each region of the country.

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

Earthquakes of magnitude 3.5 and greater occurred in eastern North America from 1971 to August 2, 2021. The yellow star is the epicenter of the August 23, 2011 earthquake of magnitude 5.8 near Mineral, Virginia. (Public domain.)

Understanding Eastern Earthquakes

The earthquake occurred in what scientists know as the Central Virginia Seismic Zone, which has a long history of episodic earthquakes. Determining the locations and magnitudes of past earthquakes and the likelihood of future earthquakes in the region is an area of ​​active research.

The USGS and its partners deployed portable seismographs immediately after the mineral quake to supplement existing seismometers in the area to record aftershocks. More than 450 aftershocks, including six greater than magnitude 3, were identified in the year following the earthquake, and nearly 4,000 subsequent analyzes have been identified to date. This knowledge helps to better understand fault lines and earthquake sources and to improve estimates of the number of aftershocks to expect.

Within minutes of the earthquake, “Did you feel it?” The website has recorded nearly 150,000 responses ranging from Florida to most of southeastern Canada and west to the Mississippi River Valley. The locations and strength of ground shaking were also measured at seismic stations established across the United States. This data informs an understanding of the distances at which earthquake ground shaking can occur in this seismic zone.

Earthquakes east vs west coast

The mineral earthquake showed how far ground shaking can extend in the eastern United States compared to the western United States. The eastern United States contains older, more solid and often denser rocks, and faults of those older rocks had a longer recovery time, allowing seismic waves to recover. Cross it more efficiently when an earthquake occurs. Another factor that increases the potential risk in the eastern United States is that many older structures, such as buildings and bridges built before the 1970s, were not designed to withstand strong vibrations from earthquakes.

Map showing where the ground shaking was felt from the August 23, 2011 earthquake of magnitude 5.8, near Mineral, Virginia, compared to the earthquake of similar magnitude of magnitude 6 on September 28, 2004, near Napa, California. Stars show epicenters of earthquakes, blue and red dots show ground shaking reported from the public to USGS “Did you feel it?” website. (Public domain.)

Effects and damage of the Virginia earthquake

The damage caused by the earthquake was extensive. In Louisa County, Virginia, near the epicenter, several homes and buildings were damaged, with two schools damaged hard enough to be replaced. Homes and other buildings were damaged as far away as Washington, about 80 miles from the epicenter. Several well-known historic structures have been severely damaged, notably the Washington National Cathedral, the Washington Monument, and the Smithsonian Institution Building, known as the Citadel.

In the aftermath of the earthquake, many buildings were temporarily evacuated across the Mid-Atlantic region. The North Anna Nuclear Power Plant, located about 12 miles from the main epicenter, was closed as a result of strong shaking from the earthquake. The shaking caused rocks to fall 150 miles away and changes in the water level in wells up to 340 miles away.

I look ahead

The eastern United States is not immune to devastating earthquakes. This danger has been known since colonial days when devastating earthquakes occurred off Cape Ann, Massachusetts, in 1755 and near Petersburg, Virginia, in 1774. The largest earthquake in the eastern United States was the magnitude 7 earthquake in 1886 near Charleston, South Carolina, and major earthquakes have occurred in recent years, such as in Delaware in 2017 and North Carolina and Puerto Rico in 2020, confirming that earthquakes can strike unexpectedly in many areas of the eastern United States and its territories.

There is clear geological evidence of large earthquakes in the ancient past in eastern North America. For example, an ancient earthquake probably of magnitude 6-7 caused a crack at the surface below Washington, D.C., leaving still visible geologic evidence in a cut road near the National Zoo. This earthquake may have occurred nearly 400,000 years ago, and the chances of it recurring in the near future are very small, but an earthquake of similar or even smaller size today can greatly affect the city and the nation.

While earthquakes in this region occur less frequently than other natural hazards such as severe storms and floods, the impact of a major earthquake can equal or exceed the damage caused by other natural disasters. See FEMA’s Ready campaign for earthquake preparedness tips.

historical precedent

August 2021 also marks the 135th year of the month since the largest historic earthquake in the eastern United States struck Charleston in the late evening of August 31, 1886. This earthquake was felt throughout the central and eastern United States. More than 100 people and many buildings collapsed or were severely damaged. Then, the newly established USGS conducted one of its first post-earthquake investigations, publishing a scientific description of the extensive damage across the region. The 2011 Mineral earthquake raises the question of whether a repeat of the 1886 earthquake would occur near Charleston, or more accurately, or whether a similar earthquake might strike another major city in the central or eastern United States.

The Next East Coast Earthquake – Are You Ready?

Mark your calendar and take part in The Great SouthEast ShakeOut, part of one of the largest earthquake drills in history. On October 21, 2021, at 10:21 am, “Participants fall, cover, and stop.” This will be an opportunity to better prepare and practice what to do in the event of an earthquake in your community.

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