M=4.2 Edmond earthquake on January 1, 2016, just north of Oklahoma City, continues swarm of induced seismicity

1 January 2016

Like its predecessor on 30 December 2015, the event is a strike-slip earthquake, perhaps involving left-lateral slip on a northwest-striking fault. It is somewhat shallower than it predecessor (5 km rather than 8 km), but both quakes lie below the depth of fluid injection and so might indicate fluid migration into a fault zone.

Oklahoma has become the earthquake capital of the United States

The dramatic rise in earthquake rate in Oklahoma can be traced to disposal of oil and gas field wastewater (‘produced water’) by deep injection, which intensified greatly about a decade ago (the figure below is from McGarr et al., Science, 2015). Nevertheless, the cause-effect and timing relationship between injection rates and volumes and individual earthquakes is not straightforward, and so hard to predict. The USGS is studying how to build seismic hazard assessments for states like Oklahoma, since its current hazard models—and therefore Temblor’s—do not account for this threat.

Fluid is injected into deep rock formations for many purposes

Fluid injection occurs in several US states for many reasons. Disposal of wastewater (‘produced water’) by deep (2-3 km or 1-2 mi) injection into rock formations; injection of water or CO2 into depleted oil reservoirs for enhanced oil recovery; hydraulic fracturing (‘fracking’) to enable production of oil and gas from low-permeability reservoirs; injection of CO2 into deep rock formations for permanent carbon capture and storage has been attempted as experiments to reduce the escape of greenhouse gases; and injection into geothermal reservoirs to replenish water lost to steam production or to develop enhanced geothermal systems is ongoing in places such as The Geysers in California (below).

Fluid injection can migrate to seismic depths, lubricating faults and triggering quakes

Most disposal wells inject into deep and highly permeable and porous sedimentary layers because these can accommodate fluids, with little or no earthquake response. Injection is purposely deep to prevent these highly toxic fluids from entering the groundwater supply. But if the injection is sufficiently deep, induced earthquakes become possible. Rarely, the fluid migrates to deeper still along faults in crystalline basement rock such as granite or basalt. These appear to be the conditions which trigger large induced earthquakes.

Ross Stein and Volkan Sevilgen, Temblor

Data from USGS, Oklahoma Geological Survey, and McGarr et al (Science, 2015)