A map of eastern
earthquakes in 1977-97 shows that earthquakes in Virginia are rare in the Coastal Plain, but
are not restricted to just one region. Two zones in Virginia are more susceptible to earthquakes
than others, and can be identified by the rivers which follow those faults. The James River
follows the Central Virginia Seismic Zone between Charlottesville and Richmond, while the
New River follows the other zone from Radford to the West Virginia border.
Virginia and the eastern side of the North American continent are in the middle of a tectonic plate. Contrast the maps from the Council of the National Seismic System (CNSS) showing locations of large (greater that 6.0 magnitude) earthquakes in the 50 years between 1946-96, for the eastern vs. the western half of the continent. The US east of the Mississippi has many fewer earthquakes than does the west, and obviously western quakes are stronger, but eastern earthquakes can cause more damage away from their origin. In the east the underlying bedrock is pretty well-connected (like a concrete slab). Eastern earthquakes can travel farther that in the west, where the underlying topography is so chopped-up (like a brick patio) that the energy of a quake is dissipated closer to the epicenter.
Virginia is pretty stable, but just about any place in the state can experience an earthquake. Manassas was hit by a surprise tremor in 1997, and an equivalent earthquake was felt in Culpeper (which is in the same Triassic basin as Manassas) two months earlier.
The last "big one" in Virginia (about a 5.8 on the Richter scale) was on May 31, 1897, in Pearisburg, the county seat of Giles County. The judge in the courthouse adjourned a trial, jumped over the railing, and fled outside with everyone else as the courthouse rattled, brick walls cracked, and chimneys fell over. It was Virginia's most powerful recorded earthquake - but our recorded memory extend back only a few centuries, and the geologic history of the state extends back hundreds of millions of years. In 1959, Giles County was shaken again by a 3.8 temblor. More recently, windows were broken in a Veterans Day, 1975 earthquake in Blacksburg.
Since 1977, there have been about 200 earthquakes in Virginia.
Virginia is classified as a "moderate" seismic risk, and has a 10-20% chance to experience a 4.75 quake every century or so. In quakes above 4.5 on the Richter scale, buildings begin to fall...
An earthquake in Northern Virginia struck Manassas on a Monday afternoon on September 29, 1997 at 1:45pm. It was centered at the Manassas Battlefield and Interstate 66.
It was tiny one, just 2.5 on the Richter scale -- about as slight as they bother tracking. If you're a James Bond fan, you'll appreciate one local person's description as having been "shaken, not stirred" after hearing what sounded like an unusually large sonic boom. [He did check to see if a tree had fallen on the roof.] But another resident, whose house may have been right above the epicenter, was "stirred" from a deep sleep. Ironically, she had just returned the previous day from San Francisco. For years she had avoided traveling there, from a fear of earthquakes. Sure enough, after visiting California, she experienced an earthquake... while sleeping in her own bed in Manassas.
As reported in the Washington Post the next day,
