Oklahoma, Where It's Complicated Deep Beneath The Plains

The rate of earthquakes has increased in central Oklahoma since 2009, culminating in the damaging quake of November 5, 2011 near Prague, OK.

The US Geological Survey and the Oklahoma Geological Survey are studying the problem, as there's a correlation between the quakes and human activity associated with the petroleum industry.

“While it’s been known for decades that Oklahoma is ‘earthquake country’, we hope that this new advisory of increased hazard will become a crucial consideration in earthquake preparedness for residents, schools and businesses in the area,” said Dr. Bill Leith, Senior Science Advisor for Earthquakes and Geologic Hazards at USGS. “Building owners and government officials should have a special concern for older, unreinforced brick structures, which are vulnerable to serious damage during sufficient shaking.”

USGS statistically analyzed the recent earthquake rate changes and found that they do not seem to be due to typical, random fluctuations in natural seismicity rates. Significant changes in both the background rate of events and earthquake triggers needed to have occurred in order to explain the increases in seismicity, which is not typically observed when modeling natural earthquakes.

The analysis suggests that a likely contributing factor to the increase in earthquakes is triggering by wastewater injected into deep geologic formations. This phenomenon is known as injection-induced seismicity, which has been documented for nearly half a century, with new cases identified recently in Arkansas, Ohio, Texas and Colorado. A recent publication by the USGS suggests that a magnitude 5.0 foreshock to the 2011 Prague, Okla., earthquake was human-induced by fluid injection; that earthquake may have then triggered the mainshock and its aftershocks. OGS studies also indicate that some of the earthquakes in Oklahoma are due to fluid injection. The OGS and USGS continue to study the Prague earthquake sequence in relation to nearby injection activities.

With the large increase in petroleum and gas extraction in the eastern US, there has been an increase of earthquakes larger than M3 since 2010. The vast majority of these quakes have occurred in Oklahoma, Arkansas, Colorado, Texas, and Ohio. A curious question comes to mind: Why these states and not North Dakota, Pennsylvania, or other states where fracking is occurring? The answer may reveal a complex picture.