One Year On From The Virginia Earthquake

Damage is seen on the street outside a library at Euclid and 15th Street NW where part of the roof crumbled during a 5.9 magnitude earthquake that struck the East Coast of the US in Washington DC, USA, 23 August 2011. Image Corbis.

On August 23, 2011, tens of millions of people in the eastern U.S. and southeastern Canada were startled by sudden ground shaking from a rare, magnitude 5.8 earthquake in central Virginia. Several small earthquakes occur every month in the eastern U.S., but this earthquake was among the largest to occur in this region in the last century. 

It is estimated that approximately one third of the U.S. population could have felt this earthquake, more than any other earthquake in U.S. history. Around 148,000 people reported their ground-shaking experiences caused by the earthquake on the USGS “Did You Feel It?” website. Shaking reports came from southeastern Canada to Florida and westward to locations near the Mississippi River.

This image illustrates how earthquakes are felt over much larger areas in the eastern United States than those west of the Rocky Mountains. USGS “Did You Feel It?” data from the magnitude 5.8 earthquake on August 23, 2011 in central Virginia (green) and from one of similar magnitude and depth in California (red).

Start with Science

There is much still unknown about the earthquake, including details of the fault that produced it and possible relationships to older faults and other geologic features. Although it was a rare event for the east coast, the earthquake was not a surprise, in that it occurred within the Central Virginia seismic zone. This zone has been identified on USGS seismic hazard maps for decades as an area of elevated earthquake risk. However, it is the largest known earthquake to have occurred in that zone.

“Every large earthquake is a learning experience, but it is particularly the case for this Virginia earthquake because of the rarity of such events in the eastern U.S.,” said USGS Director Marcia McNutt. “For example, what are we doing so right that a record setting number of east coast residents know the value to science of submitting their experiences on ‘Did You Feel It?,’ and yet not enough appropriately responded with ‘duck and cover’ during the seconds of most intense ground shaking?”

Further studies to better understand this earthquake will help ensure public safety in Virginia and other areas of the eastern U.S.

The USGS is actively involved in studying last year’s earthquake in Virginia, as well as earthquake hazards worldwide. The President’s requested FY13 budget includes a proposed increase in funding to expand USGS efforts to assess eastern U.S. earthquake hazards. USGS expertise includes earthquake monitoring and notification, earthquake impact and hazard assessments, geologic mapping and targeted research on earthquake causes and effects.

So what have scientists been up to? Take a glimpse below at some new insights and projects currently underway.

Rapid Response to Record Aftershocks

Since the earthquake, more than 450 aftershocks have been recorded. These events were calculated based on analysis by the USGS National Earthquake Information Center (NEIC) using data from portable seismographs that were deployed by several organizations immediately after the earthquake.

“The speed with which the USGS and our state and university partners got networks of seismometers into the field to capture the aftershock sequence from this earthquake defined the causative fault at depth, a first for a major eastern US earthquake,” said McNutt. “Unlike the typical situation in the western U.S., faults in this part of the country do not have a surface expression, making it far more difficult to estimate the maximum possible magnitude of earthquake that the fault can generate or the expected repeat time of the earthquakes.”

This careful effort has produced the best recorded aftershock sequence in the eastern U.S. A complete catalog of the number, size and timing of all the aftershocks is being compiled. Aftershock monitoring is valuable for locating and characterizing the dimensions of the causative fault, recording data useful for ground-motion investigations, and characterizing the aftershock-sequence.

View a map of the aftershocks and watch a time lapse video of the aftershock sequence.

Damage Assessments and Impacts
The Virginia Department of Mines, Minerals and Energy is leading an effort to map associated property damage from last year’s earthquake to inform community preparedness plans for future earthquakes in the region. Fortunately, the event last year was centered in a rural area and did not cause widespread severe damage or serious injuries, but that would not be the case in future events if they occur close to urban centers like Richmond or Charlottesville.

The earthquake was far enough (about 40 miles) from the densely populated Richmond, Virginia, area that there was no loss of life or serious property damage despite the presence of a large number of old, unreinforced masonry buildings. However, moderately heavy damage did occur to schools, businesses, and homes in rural Louisa County southwest of Mineral, Virginia. Widespread light-to-moderate damage occurred in the area from central Virginia to southern Maryland. In the nation’s capital, there was damage to several landmarks including the Washington Monument and Washington National Cathedral. The North Anna nuclear power station, located about 12 miles from the main shock epicenter, was shut down as a result of strong shaking from the earthquake.

USGS scientists also recorded changes to groundwater levels within minutes to 24 hours after the earthquake, as far away as 350 miles from the epicenter. Changes in groundwater levels have been observed from other earthquakes around the world and are a reminder of the wide-ranging impacts of an earthquake.

Mapping Underground Faults

Scientists are mapping faults and other geologic features to help refine their knowledge of the Central Virginia seismic zone. This will help determine the potential sizes of future earthquakes in the region and the likelihood of their occurring.

As part of this effort, USGS scientists conducted airborne geophysical surveys across parts of Louisa, Goochland and Fluvanna Counties from July 15 – 25, 2012. These data will be used to help develop 3D imagery of underground faults responsible for the earthquake. The instruments in the airplane took magnetic, gravity, and radiometric readings across the region. Subtle changes in the Earth’s magnetic and gravity fields can help scientists map contrasts in rock types and thus underground faults.

This map shows earthquakes greater than magnitude 3.0 since 1974 in the central and eastern United States.