Increasing Earthquake Frequency in Oklahoma Signals Potential for Damaging Future Quakes

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Seismic history of Oklahoma, 1970 to April 2014. Image courtesy of USGS.

Oklahoma has experienced a record-breaking number of earthquakes – a 50 percent increase over the average – since October 2013.

This heightened seismic activity greatly raises the chance of an M5.5 or greater earthquake in central Oklahoma.

OK’s Significant Quake Increase Began in 2009

Earthquakes in Oklahoma are not a new phenomenon; rather, it is the strength and the rate at which they are now occurring that is unusual.

The state’s long-term average earthquake rate from 1978 to 2008 was two M3.0 quakes or greater per year, and the largest previously recorded earthquake was an M5.5 that occurred in 1952 near El Reno, Oklahoma.

Since 2009, however, Oklahoma has seen 20 M4.0 – 4.8 quakes, and the largest in the state’s history was an M5.6 that struck near Prague, Oklahoma on November 5, 2011.

Oklahoma’s Current Seismic Activity

A joint analysis conducted by the United States Geological Survey and the Oklahoma Geological Survey found that between January 2014 and May 2, 2014 (the date of the original press release) 109 earthquakes of magnitude 3.0 or greater occurred in the state – already more than the 103 recorded in all of 2013.

To further illustrate this trend, since May 2, only one day has passed (May 10) where an earthquake was not recorded in Oklahoma. Two earthquakes of M3.6 were just recorded on consecutive days in the area, May 20 and 21, in Choctaw and Guthrie, Oklahoma. According to the USGS website, both of these events were assigned a rating of IV on the Modified Mercalli Intensity Scale – although not severe enough to cause damage, the recent Oklahoma tremors were significant enough to be felt by many people in the area.

So what is behind this surge in seismic activity? Another USGS analysis shows recent earthquake rate changes are likely not due to normal fluctuations in natural seismicity rates. Instead, the most likely cause is from wastewater being injected into deep geologic formations and causing faults to fail in a process called injection-induced seismicity.

Injection-Induced Seismicity

Injecting fluids underground can induce seismic activity by affecting subterranean water pressure that acts on faults and fractures in the bedrock. This fluid pressure found within the pores and cracks of rocks is known as its pore pressure.

When low, pore pressure has little to no effect on faults. However, as fluids are added and pore pressure increases, stresses in the faults are exposed and weakened, creating an earthquake potential that did not previously exist.

Injection-induced seismicity has been occurring for the last half century – other states affected include, Arkansas, Ohio, Texas and Colorado.

A recently published USGS study has suggested that the state’s largest ever recorded Oklahoma earthquake – the M5.6 earthquake that struck near Prague, Oklahoma on November 5, 2011 – was the result of a 5.0 foreshock. That preliminary quake, set off by fluid injection, then likely triggered the mainshock of 5.6 and the subsequent aftershocks.

Future Large Earthquake Potential

How large of an earthquake is possible in Oklahoma in the future? When Decoded Science posed this question, Robert Williams, the Eastern and Central US Coordinator of the USGS Earthquake Hazards Program, responded by stating,

We do not know what the largest possible magnitude might be in this region. From the M5.6 that occurred in 2011 near Prague, OK, we certainly know that another quake of that size is possible … The 2008 USGS National Seismic Hazard Map assumes a maximum magnitude 7.0. We cannot rule out an event of such magnitude. Considering uncertainties in this ‘maximum magnitude,’ there is even a finite chance that it could be exceeded.”

Williams also explained that the lack of a comprehensive map identifying the fault system under the state complicates matters. One fault that has been identified, the Meers fault in southwest Oklahoma, experienced an estimated M7.0 earthquake about 1,200 to 1,300 years ago.

Oklahoma Earthquakes Continue

Oklahoma’s upsurge in seismic activity is now monitored by a total of 35 seismograph stations across the state to help determine the location and magnitude of earthquakes as they occur across the region.

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