INDUCED EARTHQUAKES IN CENTRAL OKLAHOMA: VARIABILITY IN THE SEISMIC RESPONSE TO FLUID INJECTION

Seismicity within the past 5 years in Oklahoma has been highly concentrated in the central portion of the state near the towns of Jones, Prague, and Luther, regions of historically low seismicity. From 2009-2013, ~75% of earthquakes from the Oklahoma Geological Survey catalog occurred in this region (3035 earthquakes), including the largest earthquake recorded in Oklahoma and the largest anywhere potentially related to wastewater disposal, an Mw5.7 in November 2011 near Prague, OK. Precise relocations of aftershocks show that the tip of the initial rupture plane near Prague is within ~200 m of active injection wells and within ~1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Importantly, the Prague region showed enhanced remote triggering following the Mw8.8 Chile earthquake in 2010 – a phenomenon also observed at other sites of induced seismicity in the Midwest in 2010 and 2011. This provides additional evidence that fluid pressures were nearing critical levels as the volume of injected fluid increased after 18 years of injection. We interpret that the net fluid volume increase near Prague lowered effective stress on reservoir-bounding faults. Near Jones and Luther, OK, earthquakes began soon after the onset of injection; near Jones the earthquakes delineate multiple subparallel faults updip of high-volume wastewater disposal wells while near Luther the earthquakes define a primary fault plane. There are commonalities in the methods used for petroleum extraction from carbonate reservoirs in central Oklahoma, involving the production of high water volumes, which speculatively may explain the abundance of induced earthquakes recorded here. The differences in the timing of the Prague, Jones, and Luther sequences with respect to injection, and in the spatial distribution of seismicity, highlight the variability in seismic response to fluid injection related to local permeability structure. This variability underscores the need for careful investigation of potentially induced seismicity,as the subsurface geology can create a range of time delays and seismicity distributions that differ from historically important case studies.