Earthquakes are the results of the sudden release of accumulated stress in the brittle crust of earth. This energy accumulation is caused by the friction and deformation (strain) of rocks, most pronounced where tectonic plates collide or touch. This explains the belt of earthquakes following the West Coast of the North American plate and shaking cities.
But also in the interior of apparent stable continent plates earthquakes can occur. These Intraplate Earthquakes are in general rare and weak events, however they occur in shallow (less than about 50 kilometers) regions of the crust and the seismic waves can travel long distances in the relatively cold and brittle rocks of continents, therefore these earthquakes can be felt in large areas (Why Was the Virginia Earthquake Felt So Widely?).
The interior of the North America plate is "cracked" by a network of old, near-vertical faults, some of these tectonic zones date back to the initial breakup of the last supercontinent of Pangaea 200 million years ago, some are even more than 500 million years old. These old faults can be reactivated, even if the mechanism that activates these faults is not yet fully understand:
- Sedimentation or erosion/melting ice shields remove or apply load on the continental crust. Like a wooden board swimming on water the crust reacts to this variation of weight by rising or subsiding, generating enough stress to reactivate the old faults.
- Rainfall or water infiltration can lubricate old fractures and reactivate long-dormant faults.
- Intraplate seismic activity is often connected to developing rift systems of uprising hot plumes of material in the mantle.
- The lateral motion of the North American plate to the west creates frictional stress within the plate. The large north-east to southwest oriented tectonic systems, like the Appalachian Mountains, are absorbing the stress until it is released by sudden earthquake events.
Despite the relative scarcity of earthquakes in Eastern North America (compared to the West Coast), there are various historic events recorded:
- Plymouth in Massachusetts experienced an earthquake in 1638, Boston in 1755 (the quake was felt from Nova Scotia to South Carolina) and Cambridge in 1775.
Fig.1. A pamphlet describing the earthquakes occurred in 1638 to 1755 in Boston. In November 1755 the Portuguese city of Lisbon was completely destroyed by a strong earthquake, the pamphlets affirms that god himself prevented a punishment like experienced in Europe.
- August 10, 1884 a 5.0 magnitude earthquake struck New York, it was felt in Maine and Washington D.C. The state of New York experienced again stronger earthquakes in 1929 and 1983.
- The most powerful seismic event was a series of quakes, three main shocks and more than 1.500 aftershocks, between December 16, 1811 and February 7, 1812 recorded in New Madrid, Missouri. The most powerful shake was felt from Quebec to New Orleans; it tolled the bells in Boston and stopped pendulum clocks in South Carolina. It is estimated that it was a 8.5* magnitude quake. Near the epicentre forests were flattened, large fissures opened and entire areas were uplifted or subsided, the Missouri River was dammed up and changed path.
- One of the most powerful single earthquakes ever recoded (estimated 7.0 magnitude) occurred in August 1886, hitting the city of Charleston in South Carolina it killed 60 people.
- June 10, 1987 a 5.0 magnitude earthquake was felt in Illinois and surrounding states, like Iowa and even in West Virginia.
- November 25, 1988 an earthquake with magnitude 6.0 and the epicentre 150 kilometers north of Quebec City (Canada), was felt in Washington D.C.
*the magnitude of historic earthquakes are often estimated from reported damage or historic descriptions, therefore values can vary by author and reference. The U.S.G.S. gives a magnitude of 7.0 for the New Madrid earthquakes.
And these are only some examples.
Fig.2. Simplified map with distribution of historic earthquakes in the United States and with zones of increased seismic activity/earthquake risk, see also the regional hazard maps.
The 5,8 magnitude earthquake of August 23, 2011 coincides also with a zone of past tectonic activity. The Central Virginia Seismic Zone experienced various quakes of lower magnitude since 1774 and in historic times the most powerful event happened in 1879 with a 5.9 magnitude.
The earthquake in Virginia was followed by an intense media and twitter coverage, apparently people are still interested in earthquakes (like in 1859). Using internet you also can became part of earthquake history and even help researchers learn more about the actual quake.
Video.1. Tweets mentioning "earthquake" immediately following Virginia earthquake on 08/23/2011:
Video.2. The USArray is a project to study the subsurface structure of the North American continent with a belt of seismic stations. The stations display also how the seismic waves generated from the Virginia earthquake travelled across the continent, like waves on a pond…or twitter.