Last week the Wall Street Journal had an article that looked at the jump in magnitude 3 and above quakes that have been registered in Oklahoma.
Temblors in Oklahoma used to be rare. Before 2008, the state experienced just one earthquake of magnitude 3 or larger each year. So far this year the state has already witnessed 230 quakes of that size-more than the number recorded in California.
The article further reported that the USGS has been looking into the increased number of magnitude 3 and above earthquakes in the midcontinent of the United States.
In the central and eastern U.S., the number of quakes has jumped over the past few years, according the U.S. Geological Survey. The agency says that more than 300 earthquakes above a magnitude 3.0 occurred in the three years from 2010-2012, compared with an average annual rate of 21 from 1967-2000. These earthquakes were large enough to be felt, but they rarely caused any damage.
This graph from the USGS web page illustrates the dramatic shift in the occurrence of magnitude 3 and above quakes in the midcontinent of the US.