New Madrid Earthquake (EC) EarthCache

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves.

The New Madrid Earthquake, the largest earthquake ever recorded in the contiguous United States, centered near here on February 7, 1812. It got its name from its primary location in the New Madrid Seismic Zone, near New Madrid, Missouri.

This earthquake was preceded by three other major quakes: two on December 16, 1811, and one on January 23, 1812. These earthquakes destroyed approximately half the town of New Madrid. There were also numerous aftershocks in the area for the rest of that winter.

There are estimates that the earthquakes were felt strongly over 50,000 square miles, and moderately across nearly one million square miles.

Some sections of the Mississippi River appeared to run backwards for a short time. Church bells were reported to ring in Boston, Massachusetts and sidewalks were reported to have been cracked and broken in Washington, D.C.

As a result of the quakes, large areas sank into the earth, new lakes were formed (notably Reelfoot Lake in Tennessee), and the Mississippi River changed its course, creating numerous geographic exclaves, including Madrid Bend, along the state boundaries defined by the river. Many houses were thrown down, but fatalities and damage were low, because the area was sparsely settled.

Geology

750 million years ago

The New Madrid Seismic Zone is made up of reactivated faults that formed when North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era. The rift failed but remained as a scar or zone of weakness. The area was then flooded by an ancient ocean, depositing layers of sediment on the rift.

200 million years ago

Geological structure of Reelfoot Rift During the Mesozoic Era. As the Atlantic Ocean was opening in the east, rifting was once again re-activated and intrusive igneous rocks were emplaced. But again the rifting failed and the continent remained intact, although with a significant zone of weakness.

This rift is known as the Reelfoot Rift and coincides with the northernmost portion of the Mississippi embayment where the Madrid Bend now exists.

Modern seismic activity

Since the 1970s, thousands of earthquakes have been recorded in the New Madrid seismic zone.

Most of the seismicity is located from 3 to 15 mi beneath the Earth's surface.

Probability of future earthquakes

The probability of magnitude 6.0 or greater in the near future is considered significant. A 90% chance of such an earthquake by 2040 has been given. The odds of another 8.0 event within 50 years are estimated to be between 7 and 10 percent.

Because of the unconsolidated sediments which are a major part of the underlying geology of the Mississippi embayment, as well as the river sediments along the Mississippi and Ohio River valleys to the north and east (note the red fingers extending up these valleys in the image above), large quakes have the potential for more widespread damage than major quakes on the west coast. Additionally, the area affected will be larger since beyond the rift zone itself there are few other faults to attenuate the seismic waves.

The coordinates (should have) brought you to a marker (missing) near the head of the Madrid Bend.

To log this earthcache, post a photo of yourself at this marker with your GPSr visible and include your gps coordinates.

1. How many major earthquakes occurred here during 1811 and 1812?

2. What is the probability that another major earthquake will occur along this fault by 2040?

3. In what year did the nearby village erode into the Mississippi river?

4. What is the tallest manmade object that can be seen NW of the listed coordinate?