What caused an earthquake in Virginia? It's not near a plate boundary.
Everyone expects earthquakes in California, but no one expects a large (okay, medium) earthquake in Virginia, but that's what happened in August 2011. This earthquake was one of the intraplate earthquakes that do not occur along plate boundaries but within plates. This crack is in the Washington Monument in the District of Columbia. The monument reopened in May 2014 after being closed nearly three years for repairs.
Intraplate earthquakes are the result of stresses caused by plate motions acting in solid slabs of lithosphere. The earthquakes take place along ancient faults or rift zones that have been weakened by activity that may have taken place hundreds of millions of years ago.
2011 Virginia Earthquake
In August 2011 the eastern seaboard of the U.S. was rocked by a magnitude 5.8 earthquake. While not huge, most of the residents had never experienced a quake and many didn't know what it was. Some people thought the shaking might have been the result of a terrorist attack.
This region is no longer part of an active plate boundary. But if you went back in time to the late Paleozoic, you would find the region being uplifted into the ancestral Appalachian mountains as continent-continent convergence brought Pangaea together. The Piedmont Seismic Zone is an area of several hundred million year-old faults that sometimes reactivate.
New Madrid Earthquake
In 1812, a magnitude 7.5 earthquake struck near New Madrid, Missouri. The earthquake was strongly felt over approximately 50,000 square miles and altered the course of the Mississippi River. Because very few people lived there at the time, only 20 people died. Many more people live there today (Figure below). A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage.
The New Madrid seismic zone is located in the interior of the North American plate (near Missouri, Arkansas, Tennessee, Kentucky, and Illinois), but many earthquakes occur there.
Like the Piedmont Seismic Zone, the New Madrid Seismic Zone is a set of reactivated faults. These faults are left from the rifting apart of the supercontinent Rodinia about 750 million years ago. The plates did not rift apart here but left a weakness in the lithosphere that makes the region vulnerable to earthquakes.
- Intraplate earthquakes occur because solid slabs of lithosphere traveling on a round planet must make some adjustments.
- Intraplate earthquakes strike at ancient fault or rift zones that are reactivated.
- Intraplate earthquakes can do a great deal of damage even though they are not usually as large as quakes along plate boundaries.
- Why do intraplate earthquakes tend to be less frequent and smaller than earthquakes at plate boundaries?
- Why do intraplate earthquakes take place at all?
- Along what sorts of features do intraplate earthquakes take place?