by Emil Morhardt
The seismicity induced by oil and gas operations in Oklahoma generally appears to be caused by reinjection of wastewater coming out with oil and gas rather than the increase hydraulic pressure from fracking directly. Not so in western Canada where much less wastewater is produced and injected, but there is nevertheless considerable induced seismicity tightly clustered in space and time near hydraulic fracking sites, according to Xuewei Bao and David W. Eaton (2016) writing in Science.
The figure from the paper (above) shows locations of seismicity in northwestern Alberta, anda from 1985-2016. The locations of the largest earthquakes are shown by date.
In several different shale plays in western Canada highly sensitive seismic arrays have been established to localize any induced seismicity. They have done their job, showing that seismic events occur at the time of injection of high pressure fluid during fracking, and sometimes after, in situations in which and little fracking fluid returns with the hydrocarbon products and the pressure stays high. There are apparently two different mechanisms. The first is identical to that from reinjecting wastewater; the high pressure fluid diffuses out to weaken a tectonically primed strand of a deep fault. The second occurs beyond the diffusing fluids, in which secondary stress drives fault failure.
In the Oklahoma situation, the problem can be resolved either by not reinjecting the produced water, or injecting it away from a fault zone. In the Canadian situation, the only way to avoid it apparently would be to not frack in the vicinity of faults, leaving that subset of hydrocarbons in the ground.
Bao, X., Eaton, D.W., 2016. Fault activation by hydraulic fracturing in western Canada. Science 354, 1406-1409.