Poster of the Oklahoma Earthquake of 13 October 2010 - Magnitude 4.3
Today's earthquake is not unprecedented in central Oklahoma. The area has had earthquakes at least since it was settled. Most of them were small. Since 1974 more than 200 earthquakes have been detected within about 80 kilometers (50 miles) of today's shock. Since 1974 earthquakes roughly the size of the one this morning occurred in 1995 and on last February 27. Earthquakes large enough to cause damage are rare. Since 1882, all of Oklahoma has had 11 damaging earthquakes. The largest of these caused moderate damage in and near El Reno in 1952.
Most of North America east of the Rocky Mountains has infrequent earthquakes that can strike anywhere at irregular intervals. Here and there earthquakes are more numerous, but central Oklahoma is far from the most active area in the central U.S. That honor goes to the New Madrid seismic zone in southeastern Missouri and neighboring states. The causes of earthquakes are not understood well enough for us to predict earthquakes reliably. Because most earthquakes are small, it is possible but unlikely that any subsequent shock would be much larger than this morning's earthquake. East of the Rockies, an earthquake the size of the one this morning can be felt as far away as roughly 100 km (60 miles).
Earthquakes occur on faults. Most earthquakes occur miles deep. At well-studied plate boundaries like the San Andreas fault system in California, often seismologists can determine the specific fault on which an earthquake occurred. East of the Rockies, far from plate boundaries, that is rarely the case. Most of the known faults are deep, and probably there are other faults that have not been discovered. It is hard to link an individual earthquake to an individual fault. In most areas, the best guide to earthquake hazards is the earthquakes themselves.
Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).
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