The good news is that the number of injection-triggered earthquakes hitting Oklahoma is dropping. However, it is unclear what share of the drop is caused by state-mandated reductions in injection volumes and how much reflects the — temporary — drop in oil prices and production. Another question hanging over Oklahoma oil and gas operations is whether some of the earthquakes are caused by hydraulic fracturing.

Oklahoma had less than 200 earthquakes (greater than magnitude, M 3.0) in June. That is a monumental drop from the high of 761 earthquakes recorded in June 2015, although it is still a huge number. These earthquakes are generally tied to injection of wastewater, and wastewater disposal volumes have fallen with the drop in oil production that started in 2014. In addition, 2015 directives from the Oklahoma Corporation Commission (OCC) have reduced injection volumes. Injection volumes in Oklahoma in mid-2016 are 1 million barrels/day less than in 2015.

South-central Kansas has seen a similar reduction in earthquakes, but from a lower baseline, as the state has restricted wastewater injection. The U.S. Geological Survey (USGS) recorded 11 quakes in south-central Kansas in the first half of 2016 compared to 48 quakes in the last half of 2014.

Hydraulic fracturing induced earthquakes though rare are documented in Oklahoma, Ohio, Pennsylvania and Canada. Perhaps a handful of earthquakes caused by hydraulic fracturing are documented in Oklahoma, whereas the number of quakes attributed to wastewater injection is in the hundreds. However, earthquakes recently occurred in an area near hydraulically fractured wells but more than 20 miles from any high-volume wastewater injection wells. Given that fluids or pressure pulses — from injection or hydraulic fracturing — can rapidly travel long distances additional studies are needed to clarify the source of the seismicity.

Another indication of the hydraulic fracturing –earthquake connection comes from a recent report by G. M. Atkinson and others in Seismological Research Letters, which found some earthquakes that show a strong spatiotemporal correlation to hydraulic fracturing in the Western Canada Sedimentary Basin of Alberta and British Columbia. More than half of earthquakes with M≥3 were associated with hydraulic fracturing and 39 of 12,289 hydraulically fractured wells correlated with earthquakes of M ≥3 (highest magnitude earthquake associated with these wells was M=4.6). The 39 wells represent only 0.2 percent of hydraulically fractured wells and include about a half dozen previously documented earthquake-hydraulic fracturing correlations.

Over the past several years, states and the USGS have installed dozens of new sensors to enhance the coverage and precision of existing seismic networks. These improved systems allow scientists to more accurately pinpoint the origin of the earthquakes and work toward explaining how fluid injection causes earthquakes.