Fracking for natural gas, whereby gas-trapping rock formations are blasted open with high-pressure water and chemicals, has prompted serious concerns over the safety of groundwater supplies. But another risk is gaining profile: the potential for inducing nerve-rattling microseismicity or, potentially, unleashing a quake of truly destructive magnitude. Like the magnitude-5.6 quake that rocked Oklahoma last weekend.
As Spectrum documented this spring, human activity can and does induce earthquakes. To quote myself:
Tectonic pressures cause the vast majority of earthquakes, but geophysicists also recognize the existence of human-induced seismicity. Hydropower reservoirs, for example, frequently cause small, shallow quakes as shifting water levels change the strains on the rock layers below. Such microseismicity—up to magnitude 4 on the Richter scale—is also caused by wells that inject hazardous waste and wastewater into deep rock formations at high pressure.
And yet, this connection between us humans and something as vast and primordial as an earthquake is hard to accept, much as many (particularly in the U.S.) continue to dismiss the link between such things as cars, coal and climate change.
Take this week's report by West Virginia's Intelligencer / Wheeling News Register: Experts Doubt Fracking Linked to Quakes. The reporter stitches together isolated quotes from reputable geophysicists to present two arguments against a connection between extensive fracking in Oklahoma and this weekend's earthquake, both of which are fallacious.
Argument 1: Microseismicity from fracking is of too little force to unleash a magnitude-5.6 quake that had "the power of 3,800 tons of TNT, which is nearly 2,000 times stronger than the 1995 Oklahoma City bombing."
Argument 2: This and other recent quakes in Oklahoma must be "natural" because they followed "the lines of a long-known fault."
Where these arguments fall down is where they meet. The risk of microseismicity is, in fact, in the very presence of natural faults. Microseismicity can, over time, unlatch the fault like a spring, releasing tectonic strain built up over many years, centuries or even millennia to produce a major shake. As we reported in April, a textbook case occurred in 1967 at India's hydroelectric Koyna Dam (pictured above). Filling of the reservoir behind the then six-year-old dam unleashed a magnitude-6.3 quake on a previously unknown fault, killing 180 people and leaving thousands homeless.
The Oklahoma quake could well have been similarly "triggered" according to Paul Earle, a U.S. Geological Survey scientist quoted by the Intelligencer / Wheeling News Register. As Earle told me this morning: "I believe the earthquake *could* be natural but more studies need to be done to rule out the possibility of an anthropomorphic trigger."