Intraplate Boundaries

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. No one seems quite sure how long it will take to repair the damage, but the monument is still closed. Other sites in Washington DC, like the National Cathedral, were also damaged.

Intraplate Earthquakes

About 5% of earthquakes take place within a plate, away from plate boundaries. Theseintraplate 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.

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

Vocabulary

  • intraplate earthquakes : Earthquakes that are caused by stresses within a plate.

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.

Practice

Use the resource below to answer the questions that follow.

  • The New Madrid Fault at
  1. How many active earthquake fault zones are there in the United States?
  2. What is the second most active fault zone in the United States?
  3. When did the New Madrid earthquake occur?
  4. What was the magnitude of the New Madrid earthquake?
  5. How many aftershocks occurred? What was the highest magnitude of an aftershock?
  6. How long is the New Madrid fault?
  7. How many people would be affected by another quake?
  8. Where is the Ramapo fault?
  9. Why is quiet along an active fault considered a problem?

Review

  1. What caused the New Madrid earthquakes?
  2. What causes intraplate earthquakes?
  3. Why did an earthquake strike Virginia in 2011? Why did it strike at that location?
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