Concluding there could be a tie to oil and natural gas production activity, U.S. and Oklahoma Geological Survey officials jointly reported Monday that 145 3.0-magnitude or stronger earthquakes have occurred in the state so far this year.
As part of the U.S. Geological Survey (USGS) and Oklahoma Geological Survey (OGS) joint assessment of the 2011 5.0-magnitude quake in Prague, OK, the two agencies have concluded that the quake was "human-induced by fluid injection," and more recent seismic activity also was due to "fluid injection" used in oil/gas drilling processes (see Shale Daily, March 10).
"The rate of earthquakes in Oklahoma has increased remarkably since October 2013 -- by about 50% -- significantly increasing the chance for a damaging 5.5-magnitude or greater quake in central Oklahoma," the USGS and OGS said in a joint announcement.
Oklahoma's previous annual record was set last year with 109 quakes, while the long-term annual average between 1978 and 2008 was two. All of that has been eclipsed in four months so far this year.
Since 2009, Oklahoma has experienced 20 4.0- to 4.8-magnitude quakes, along with the largest in the state's history, the 5.6-magnitude quake Nov. 5, 2011 in Prague, which damaged a number of homes and the historic Benedictine Hall at St. Gregory's University in Shawnee. Prior to that, the state's biggest quake was in 1952, a 5.5-magnitude quake near El Reno that damaged state buildings in Oklahoma City.
"The analysis suggests that a likely contributing factor to the increase in earthquakes is triggering by wastewater injected into deep geologic formations," the USGS's statistical analysis suggests, according to the joint state-federal announcement. "This phenomenon is known as 'injection-induced seismicity', which has been documented for nearly a half century with new cases identified recently in Arkansas, Ohio, Texas and Colorado."
USGS analysis indicated that the big increase in small temblors has not been due to "typical random fluctuations in natural seismicity." Thus, these do not fit the model of "natural" quakes.
The joint research is seeking to better understand quake rate increases in central Oklahoma, including quantifying the changes in earthquake rates, assessing the implications of the increase in small to moderate quake activity for large earthquake hazards, and evaluating the possible links to wastewater disposal from oil/gas production activity.
OGS said it is also focusing on seismicity in north-central Oklahoma. OGS has a network of 15 permanent and 17 temporary monitoring stations spread around the state, many on loan from the USGS.