The number of earthquakes east of the Rocky Mountains has risen sharply since 2009, according the U.S. Geological Survey. The majority of the rise can be attributed to the increase in earthquakes in Oklahoma, where the number of earthquakes has risen from one to two per year to one to two per day since 2008.
“More earthquakes in these areas have coincided with the increase in oil and natural gas production from shale formations,” according to the U.S. Energy Information Administration. “Seismic events caused by human activity — also known as induced seismicity — are most often caused by the underground injection of wastewater produced during the oil and natural gas extraction process.”
Most of the induced earthquakes are small, between 3 and 4 magnitude on the moment magnitude scale. “These earthquakes are large enough to be felt by most people, but they do not often cause damage to structures,” according to the U.S. Energy Information Administration.” In Oklahoma, production is in areas with high water-to-hydrocarbon ratios, meaning more produced wastewater must be disposed.
Since 2014, Oklahoma has experienced one to two low-magnitude earthquakes per day and a few quakes between magnitude 5 and 6, which have caused damage. Before 2009, the state might have had one to two low-magnitude earthquakes annually, according to the EIA.
“In addition to the increased use of wastewater injection related to oil and natural gas production in the region, the geologic conditions in central Oklahoma are conducive to triggering seismic activity,” according to the EIA. “The rock underlying the formations where disposal water is being injected in the region has existing faults that are susceptible to the changing stresses caused by fluid injection. Without these geologic conditions, induced seismicity would be much less common. For example, induced seismicity in the Bakken region of North Dakota and Montana is relatively rare.”
Forecasted seismic rates for 2017 are lower than their peak in 2015 but are significantly higher than levels before 2009.