(67).Intraplate earthquakes.

When can you go up in the Washington Monument again?

An earthquake in August 2011 in Virginia cracked the top of the Washington Monument. After 32 months of repair, the monument reopened to the public. Other sites in Washington, D.C., like the National Cathedral, were also damaged.

Intraplate Earthquakes

About 5% of earthquakes take place within a plate, away from plate boundaries. These intraplate earthquakes are caused by stresses within a plate. Since plates move over a spherical surface, zones of weakness are created. Intraplate earthquakes happen along these zones of weakness. The earthquakes may take place along ancient faults or rift zones.

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 an earthquake. Many didn't know what had happened!

This region is no longer part of an active plate boundary. But a few hundred million years ago, at the end of the Paleozoic, things were different. Continent-continent convergence was bringing Pangaea together. This region was being uplifted into the ancestral Appalachian mountains. There are still ancient 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. The quake even altered the course of the Mississippi River. Because very few people lived in the area 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 continues to be active.

[Figure 1]

The New Madrid seismic zone is located in the interior of the North American plate (near Missouri, Arkansas, Tennessee, Kentucky, and Illinois).

Summary

Intraplate earthquakes occur because solid lithosphere travels on a round planet.

Intraplate earthquakes hit reactivated ancient faults.

Intraplate earthquakes are not usually as large as quakes along plate boundaries. Still, they can do a great deal of damage.