"Oklahoma is a seismically active region that has always had a lot of earthquakes in the magnitude 5 range." I don't think the historical record supports this

"Oklahoma is a seismically active region that has always had a lot of earthquakes in the magnitude 5 range." I don't think the historical record supports this assertion, for any reasonable interpretation of "a lot". I found this table: http://paleoseismicity.org/m5-6-earthquake-in-oklahoma/ http://paleoseismicity.org/m5-6-earthquake-in-oklahoma/ showing only one earthquake of >= M5 in Oklahoma in the last 70 years (an M5.5 in 1952). That is, excluding the ramp-up since 2008 and in particular the Prague, OK M5.6 (2011). This sparse historical record of M5 earthquakes is also supported by the linked graph from USGS (http://earthquake.usgs.gov/research/induced/ http://earthquake.usgs.gov/research/induced/) that shows about 20 events/year of M3 or larger in the entire central-eastern US (pre-2009 -- the rate increases after 2005-2009). The simple version of the Gutenberg-Richter relation predicts 10x fewer earthquakes for each unit jump in M. So, very roughly, that would imply 0.2 earthquakes per year of M5 or above across the central-eastern US, a region much larger than Oklahoma. The actual rate observed in the 20th century appears significantly lower than this.