By Dr. Robert Thorson
Seismologists are hearing a new snap, crackle and pop from terra firma.
Fracking? Geothermal drilling? Coal gas? They are changing the planet in ways we’re not used to thinking about. Having changed Earth’s atmosphere by injecting carbon through tailpipes and smokestacks, we’re now changing Earth’s crust by injecting fluids in deep wells. A new national issue has arrived. I’m passing this baton to you.
Let’s start with the north-central United States. It’s flat, stable and ancient, a far cry from young and restless California. Beneath corn and soybeans is moist glacial soil, and below that a layer-cake of firm strata resting calmly on rock half the age of planet Earth. Yet even there, seismologists are finding a “remarkable increase” in earthquakes strong enough to rattle dishes, “almost certainly manmade.” These quotes are from a U.S. Geological Survey seismologist William Ellsworth, in a review article by Richard Kerr in the March 23 issue of Science.
Why inject fluids? In the bad old days, it was to dispose of them, especially nuclear-contaminated military wastes at Colorado’s Rocky Mountain Arsenal. Decades ago, we learned that deep-well injections there triggered earthquake swarms. We’ve now got 144,000 wastewater injection wells in the U.S.
Many of the newer wells are associated with energy extraction and drilled to depths of a mile or more. Some re-inject the chemically-tainted brines of “fracking,” the hydraulic fracturing of shale. Others inject special fluids into deep coal beds to release methane. In New England, and other places with ancient crystalline rock, the main purpose of deep drilling is to develop geothermal energy reservoirs.
I’ve felt such small earthquakes in strong ancient crust. Each reminds me of a large truck backing up too quickly to the loading dock, and sending an audible jolt, rattle and scare through a building. Millions of Americans felt one last August when an unexpected magnitude 5.8 earthquake struck Mineral, Va., and vibrated up the East coast. How quickly we forget.
The main concern with the new issue is that localized damage might be severe. Even worse is the possibility that quakes triggered by fluid injection might trigger larger interior earthquakes like those that that reversed the flow of the Mississippi River near New Madrid, Mo., in 1811 and 1812, and which wrecked Charlestown, S.C., in 1886.
These were a complete surprise relative to those in the active tectonic margin of the U.S. West Coast where the plates move past each other along known boundaries releasing differential stresses. Elsewhere on the continent, however, the crust is mostly being squeezed, with earthquakes happening only rarely and unexpectedly. And within that squeeze are millions of tiny fractures and hundreds of old fault zones left as a legacy from previous tectonic regimes.
When fluids are injected into near-surface rocks that are porous, or under limited stress, very little happens. But when the bulb of fluid pressure migrates downward or outward to older rocks that are tighter and stronger, the added hydraulic pressure can “frack” old fault zones apart and let them slip.
This isn’t fiction. It just happened in the small city of Greenbrier, Ark., where the injection of fracking fluids began in April 2009. By the spring of 2011, nearly 1,000 earthquakes had been recorded, the largest being a city-jolting shock of magnitude 4.7. All the big ones were aligned along an ancient fault zone more than a mile below the depth of injection. The alarm was raised. The injections were halted. The earthquakes went silent. The residents relaxed.
Fracking of shale and coal will not stop. And the associated fluids are too nasty to be released near the surface where they would contaminate our aquifers and streams. Geothermal drilling won’t stop either, because we need it.
To develop these resources, we must be cautious. We must array seismic monitors before injection and then watch what happens. If the crust goes snap-crackle-pop, we must inject somewhere else.
Out of sight is not always out of mind.