Sunday 6 November 2011

Oklahoma shaken by series of earthquakes; 5-6 November 2011.

Slightly after 2.10 am local time (slightly after 7.10 pm GMT) he state of Oklahoma was shaken by a quake measured by the United States Geological Survey as measuring 4.7 on the Richter Scale, which occurred at a depth of approximately 5 km, 75 km to the east of Oklahoma City. Just before 11 pm local time on Saturday 5 November (slightly before 4 am on Sunday 6, GMT) a second quake measuring 5.6 on the Richter Scale, shook the same area. A little before 3.40 am local time (just before 9.40 am GMT) a third quake measuring 4 on the Richter scale struck in roughly the same place. Over the same period there have been about 10 small quakes measuring less than 4 on the Richter Scale. There are no reports of any casualties, but some reports on damage to houses in Lincoln County.

Map from the USGS showing the location of earthquakes in Oklahoma in the 24 hours prior to 13:00 on Sunday 6 November 2011 (blue squares; larger quakes get larger squares)

At 5.6 on the the Richter scale the largest of these quakes is also the largest ever recorded in Oklahoma. Prior to this the largest recorded had been a magnitude 5.5 quake in El Reno (to the east of Oklahoma City) in 1952, which caused widespread alarm and several thousand dollars worth of damage but no casualties. In 1969 a magnitude 4.6 quake hit Wewoka (southeast of Oklahoma City) without causing significant damage. In 2010 a 4.7 magnitude quake shook Cleveland in the north of the state, again causing only minor damage.

The source of these earthquakes is not completely clear, though the Oklahoma Geological Survey suggest that they may be linked to the Meers Fault in the southwest of the state, though this fault is generally thought to be inactive. In 1995 Meridee Jones-Cecil of the USGS published a paper in the Bulletin of the Geological Society of America suggesting that this fault was associated with structures in the basement rocks (i.e. rocks deep bellow the surface) and that it could become active again.

An aerial photograph of the Meers Fault.

If you felt this quake you can report it to the United States Geological Survey here, and the Oklahoma Geological Survey here; both organizations find this information very useful as more reports help them to build up a more detailed picture of how quakes have occurred.