Last year, the surprise 5.8-magnitude earthquake that struck central Virginia was actually worse than previously thought. Detailed analyses of the ground motions triggered by the event indicate that Washington DC and other affected regions could be at higher risk of major ground movement.
The event triggered landslides over a wider area than any other recorded quake anywhere in the world. The earthquake was centered 130 km south-southwest of Washington DC, near Mineral, Virginia. The August 23, 2011 quake was the strongest to hit the eastern USA since 1897.
The findings were reported by scientists of the US Geological Survey at the Geological Society of American’s annual meeting in Charlotte, North Carolina. The area affected by the quake was about the same as a much larger 7.2 tremor that struck Baja, California in 2010 because of a variety of factors, including the age, type, temperature and density of the rocks underlying the eastern United States.
The quake was strong enough to do serious damage to some of the landmark buildings in Washington. The National Cathedral needed $20 million in repairs. US cities on the eastern seaboard could be more affected by future earthquakes. A large fraction of the seismic energy was directed towards the northeast, where several major population centers are located.
This preferential concentration and amplification of seismic energy has been noted in other fault-ridden regions in the world, but hadn’t been measured in the eastern USA because it is rarely hit by severe earthquakes.
Seismologists suggest that more seismometers should be deployed around Washington and other cities to asses to what degree seismic waves generated in the region might be amplified.
The large ground motions also triggered rockfalls over an unexpectedly large area. They were reported as far as 245 km away. Quakes this size shouldn’t trigger landslides more than 60 km away from its epicenter.