If there is no plate boundary in the middle of the
United States, why did these earthquakes take place? Geologists are
beginning to understand the answer. The New Madrid Fault Zone is part of
an ancient plate boundary. In this area, the North American Plate
tried to form a divergent plate boundary about 500 million years ago.
The splitting stopped before new plates could form. The faults in the
New Madrid Zone are remnants of this old event. Earthquakes occur
because the North American Plate is still "settling down".
The faults in the New Madrid Zone do not reach the
Earth’s surface. They are buried beneath thousands of feet of rock and
sediment deposited by the Mississippi River. Geologists have located
them by looking at the patterns of earthquakes in the zone.
This satellite photograph shows major cities near the
New Madrid Fault Zone.