North American Plate
Source: USGS World Plate Boundaries Animation
Virginia and the eastern side of the North American continent are in the middle of a tectonic plate. The North American Plate is one of the 15 or so major "chunks" of crust that float on top of the hot mantle. The plate includes both continental crust and heavier (iron- and magnesium-rich) oceanic crust.
The eastern coast of the United States marks the boundary between continental and oceanic crust, but the North American Plate includes both continental and oceanic crust. The Eastern Shore/Virginia Beach are at the edge of the continent, but are not located at the edge of the continental plate. Instead, the eastern edge of the North American Plate is in the middle of the Atlantic Ocean, at the Mid-Atlantic Ridge. At that ridge, magma rises slowly and pushes Virginia (and the rest of the North American Plate) towards China, at the rate of about 2-3 centimeters/year.
In contrast, the far western edge of the North American Plate is near the edge of the North American continent. In Southern California, the western edge of the North American Plate rubs against the eastern edge of Pacific Plate. That boundary is marked by the San Andreas Fault and many other named faults. The North American and Pacific plates are moving in different directions. Earthquakes occur when the pressure to move exceeds the capacity of the rocks to resist motion. Intermittently, the plates "break free" of each other and the edges spring into a new alignment. When the land moves into the new alignment, everything shakes nearby - and buildings, highway bridges, etc. may collapse.
Virginia is not on an edge of a continental plate where sudden, jerking movement occurs regularly; Virginia is in the relatively-stable middle of a plate. That's why Virginia does not experience the large-magnitude earthquakes that affect Los Angeles, Alaska, Haiti, Japan, Chile, or other places that are on a tectonic plate's edge.
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.
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...
A seismometer is a device that records how the earth moves during an earthquake. When Dr. Charles F. Richter of the California Institute of Technology was studying Southern California geology in the 1930's, his seismometers marked the motions with a pen that drew lines on a roll of paper. Normally, the lines were horizontal. When a quake hit, the pen would zig and zag, documenting the tiny motion of the earth. The bigger the shift in the pen's lines from the normal horizontal, the bigger the earthquake - or the closer it was to his seismometer.
Richter was frustrated by the number of small earthquakes that cluttered up his data. He needed a way of quickly describing the significance of the motion, so he created his Richter scale to identify the big quakes and screen out the little ones. He used the terminology of astronomy to label earthquakes by "magnitude."
His scale is based on logarithms; for each increase in the number of the scale, the earthquake is 10 times more powerful. A 6.0 magnitude earthquake is 10 times more powerful than a 5.0 magnitude earthquake , 100 times more powerful than a 4.0 magnitude earthquake , and 1000 times more powerful than a 3.0 magnitude earthquake.
Richter measured how much a pen moved on a Wood-Anderson torsion seismometer, which recorded the amplitude of seismic waves, and then calculated the logarithms of the distance the pen moved. The exactness of Richter's scale was based on the pen's motion in that instrument. (He also used a formula to reflect the seismometer's distance from the quake epicenter.) The pen on the seismometer moved more for a powerful earthquake compared to a minor shock.
Using Richter's scale, it was possible to correlate the numbers on his scale with impacts of earthquakes. Quakes moving the Wood-Anderson torsion seismometer pen enough to be classified 4.0 were also quakes that rattled the dishes in the china cabinet, while quakes below 2.0 were rarely felt.
Richter's measurements ended up with a maximum size of about 7.0 for earthquakes, so scientists now use a different scale now to assess how much earth moved for quakes larger than 3.5 on the Richter scale. The "moment magnitude scale" uses the motion recorded by seismometers, but also includes how far chunks of the earth moved and how big were the chunks that moved. Incorporating the size and distance numbers for how much earth moved enables seismologists to assess the energy in a quake, in addition to the motion. If a lot of earth moved, or if a slice of earth moved a long distance, the energy released by the quake will be larger and the "moment magnitude" of that earthquake will be bigger.
So far, the biggest earthquake recorded in modern times was a 9.5 event in Chile, in 1960.
NOTE: The Modified Mercalli Intensity (MMI) scale measures how the earthquake is felt, rather than the amount of energy that is released. In the MMI scale, intensity of I is so mild the earthquake is felt only by instruments, VIII would be destructive, while an even classified at XII would be catastrophic.
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,
