ESTIMATING THE DECADAL-LEVEL SEISMIC HAZARD IN OKLAHOMA

The rate of earthquakes across the United States mid-continent has dramatically increased since 2009. The historically high seismicity rates across the mid-continent have been largely driven by substantial increases in seismicity occurring within Oklahoma, including several magnitude 5.0+ earthquakes. Prior to 2009, background tectonic rates were about 2 M 3.0+ earthquakes per year and increased to 579 and peaking at 903 M 3.0+ earthquakes in 2014 and 2015, respectively. The increase in seismicity in Oklahoma was roughly coincident with production from formations containing substantial amounts of co-produced formation brines. Common disposal practices involved disposing of wastewater in deep underground injection wells completed into upper parts of the basement and the karst Arbuckle Group, which directly overlies the basement. From 2010 to late 2014, statewide disposal rates increased from ~30 million bbls/month to ~90 million bbls/month. The increase in seismicity rate roughly corresponded to the increase in monthly injection rates, though with sometimes a lag greater than a year or so in many sub-regions of Oklahoma. As of late 2018, the statewide disposal is down to ~35 million bbls/month and the earthquakes have also declined (184 M 3.0+ earthquakes in 2018 as of December 4, 2018), due to economic factors, Oklahoma Corporation Commission targeted and statewide reductions in permitted disposal, and changes in disposal practices, i.e. injecting into shallower units. In light of the observed time-lagged coherence between wastewater injection and seismicity rate, several groups have strived to estimate the future seismic hazard in Oklahoma and consensus has yet to be reached. However, it appears that the seismic hazard likely remains high in the next several years and elevated above background levels over the next several decades. I plan to present our latest findings relevant to measuring the underlying elevated background seismic hazard.