TULSA - The scientific consensus on what triggered Oklahoma’s earthquake swarm seems to be confirmed by the success of the state’s program to reduce seismicity.
Tuesday, a geophysicist with an international reputation visited Tulsa as part of aNorth American lecture tour sponsored by the Society of Exploration Geophysicists .
The topic of the tour: How widespread is human-induced seismicity in the United States and Canada?
While Dr. Mirko van der Baan’s tour began in August, and stretches into next year, it’s possible his most engaged audiences will be in Oklahoma, which saw an exponential increase in seismic activity in recent years.
He told KRMG Tuesday he’s convinced that wastewater injection wells triggered the recent quakes, not hydraulic fracturing - commonly called “fracking.”
“In Oklahoma, it’s the saltwater disposal which is the culprit, it’s not hydraulically fracturing,” he said, adding that “the good thing is that because the disposal rates are coming down, the seismicity is coming down as well.”
It happens because the water literally lubricates faults deep in the earth, allowing them to slip.
A second factor is that moving large volumes of water from one area to another can also transfer stress to faults that are more unstable.
That’s not to say that fracking might not cause some induced seismicity, he explained, but that doesn’t appear to be the case in Oklahoma.
“We have seen in other areas, in Canada for instance, where hydraulically fracturing, if those fluids interact with pre-existing faults, they may again lubricate them and therefore get induced seismicity, but it’s rather an exception than a general rule.”
Interestingly, the general rule seems to be that a sharp increase in oil and gas production - and injection wells - does not lead to a corresponding increase in seismic activity.
“Except in Oklahoma,” van der Baan explained, “where it’s just the volumes which were so large that it was easier to re-activate many parts. And so when we look at it in other areas, one of the things we find is that say, Texas is not seismically very active, and that means that there may be pre-existing faults but they’re nowhere close to being triggered. Whereas here in Oklahoma, the actual seismic hazard was already a bit higher, and so it might have been easier to get re-activation happening.”