February 7 – The Earth Moves

Today’s Factismal: The fourth and final large earthquake in three months hit New Madrid in 1812.

Ask most people where earthquakes happen and odds are they’ll say “California”. And that is right; California happens to sit along a plate boundary. Part of California sits on the North American plate and another part sits on the Pacific plate, which is moving to the northwest at about two inches per year. This means that everything that stretches across the boundary slowly stretches until it snaps, creating an earthquake.

One year of earthquakes across the world. (Image courtesy the USGS)

One year of earthquakes across the world. (Image courtesy the USGS)

But California isn’t the only part of the world with earthquakes. They happen where the Pacific plate dives under the North American plate in Alaska. They happen where the Pacific plate goes under the Eurasian plate in Japan. They happen where the Indian plate goes under the Eurasian plate in Sumatra. And they happen where the Nazca plate goes under the South American plate in Chile. What all of these earthquakes have in common is that they happen where two plates meet.

But not all earthquakes happen at plate boundaries. About 20% of the earthquakes happen in the middle of a plate (what we call “intraplate”). Because they break the rules, those earthquakes are among the most interesting. And if you ask any seismologist what was the most interesting intraplate earthquake, odds are the answer will be “New Madrid”.

The New Madrid earthquake series started on December 16, 1811, with an earthquake that shook buildings for miles around and sent a small wave upstream on the Mississippi.That was followed six hours later by another earthquake of about the same strength. The area had a series of small aftershocks that had mostly died away by the time of the next major earthquake on January 23, 1812. That event caused widespread minor damage, mostly by jiggling the ground so hard that the soil turned to quicksand, tilting buildings and collapsing chimneys. But the worst was yet to come. On February 7, 1812, the last major earthquake hit the area. It completely destroyed the town of New Madrid and caused the Mississippi to run backward for a few hours. The earthquake was strong enough to ring the Liberty Bell in Philadelphia, over 900 miles away.

Because the seismometer hadn’t been invented yet, seismologists have to estimate the strength of the event based on eyewitness reports. They believe that the earthquakes had an energy between magnitude 7 and 8. That means that they were around sixteen times more powerful than the Loma Prieta earthquake in 1989.

The New Madrid Seismic Zone (Image courtesy USGS)

The New Madrid Seismic Zone (Image courtesy USGS)

The area remains seismically active today. Though most of the events are magnitude 3 to 4 (about 1/1,000,000th as strong as the 1812 events), there is concern that another large event could happen. As a result, the area is monitored by seismometers and has been investigated by many seismologists from around the world.

You can help seismologists learn more about earthquakes in New Madrid and elsewhere. You can find earthquakes using the Rapid Earthquake Viewer or the USGS Earthquake Monitor. And please contribute to science by telling the USGS if you felt the earth move!
http://earthquake.usgs.gov/earthquakes/dyfi/

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