Sunday, November 6, 2011

Oklahoma Earthquake

On Saturday, November 5, at 2:12 am CDT, a 4.7 magnitude earthquake hit central Oklahoma (about 45 miles east of Oklahoma City between the towns of Prague and Sparks).  It was a foreshock followed by 7 others ranging from magnitude 2.7 to 3.6 and then culminating in a 5.6 magnitude quake at 10:53 pm CDT.  As of 8:00 am CST, there have been a dozen aftershocks ranging from 2.7 to 4.0 on this fault according the the National Earthquake Information Center (only quakes above magnitude 2.5 are reported).

On February 27, 2010, there was a magnitude 4.3 earthquake near this location as well.  The largest earthquake in Oklahoma, previous to Saturday's, was a 5.5 magnitude quake near El Reno just west of Oklahoma City back in 1952.  Moderate earthquakes, while not common, do occasionally occur in Oklahoma (as they do in other places in the continental interior - remember the 5.8 magnitude Virginia earthquake in August?).

All of these recent earthquakes were relatively shallow at 5 km (3 miles) or so of depth.  According to the Oklahoma Geological Survey, these earthquakes appear to have occurred on the Wilzetta Fault (line with red squares in the above map).  For those of you who know something about geology, the focal mechanism seems to indicate dextral strike-slip movement.

Not much information online regarding the Wilzetta Fault and associated Seminole Uplift (it's an area that's been studied, however, since there's oil and gas there).  Looks to me like strike-slip reactivation of an older basement thrust fault associated with the Pennsylvanian/Permian Alleghanian-Ouachita Orogeny.

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