On August 23, 2011 at 1:51 pm Eastern time, a magnitude 5.9 earthquake erupted from central Virginia, unleashing seismic waves up and down the entire East Coast. The earthquake took everyone by surprise, including local geologists who were aware of the seismic zone, but were not expecting an earthquake of this magnitude. The depth of the earthquake was about 3.7 miles (6 km), which is relatively shallow.
The Source of the Earthquake
The epicenter of the earthquake was located within the Central Virginia Seismic Zone, which has produced small and moderate magnitude earthquakes since the 1700's. None of these historical quakes have been studied using scientific instruments; instead geologists determine the size and frequency of these earthquakes using historical records of personal accounts. For example, the dispersed locations of people who felt an earthquake in 1875 suggest that the earthquake was around a magnitude 4.8. Historical accounts of this 1875 earthquake list chimneys falling down, broken plaster and windows, and furniture getting knocked over at several locations.
The epicenter of the August 23, 2011 earthquake was very near the town of Mineral, VA. Although an exact fault line has yet to be assigned, the earthquake did occur in a region that is known for seismic activity. Previous activity in this region has not usually been associated with strong earthquakes or known geologic faults, although, like the August 23 event, seismic activity is commonly shallow (average depth of 8 km).
East Coast Bedrock Conducts Seismic Waves
Immediately after the earthquake, the most common question that geologists received was, "Why were the waves felt so strongly, and so far away?" The reason has to do with the type of rock through which the seismic waves travel. The crust that makes up the eastern portion of the United States is very old, dense and cold. The crust that makes up the western portion of the United States is fresher and warmer. This affects the way in which seismic waves propagate through the crust, both in distance and strength.
Earthquakes of similar magnitude can be felt 10 times farther away in the eastern U.S. than in the western U.S. For example, Colorado also experienced a shallow earthquake (2.5 miles or 4 km deep) on August 22, but the cases reported to the "Did You Feel It?" USGS website was limited to a distance of about 300 km from the epicenter. For the East Coast earthquake, responses showed that shaking was felt as far as 1000 km away from the epicenter. People located at the epicenter reported strong to moderate shaking for both earthquakes.
Source:
US Geological Survey Earthquake Hazards Program, "2011 Significant Earthquake and News Headline Archives", earthquake.usgs.gov
Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
