By Laura Beans
The increased rates of earthquakes occurring in the central and eastern U.S over the past few years is a growing cause of concern. Two recent reports delve into the probability of man-made, induced earthquakes.
According to the U.S Geological Survey (USGS), more than 300 earthquakes above a magnitude 3.0 occurred from 2010-2012, compared with a much lower national average rate of 21 earthquakes per year observed from 1967-2000. The USGS also found that the increase in seismic activity coincides with the injection of wastewater in deep disposal wells. Much of this is a byproduct of oil and gas production which uses the process of fracking for extraction. The fracking wastewater is disposed of into wells specifically designed and approved for this purpose.
The journal Science published a report Thursday, concluding that powerful earthquakes thousands of miles away can trigger swarms of minor quakes near wastewater-injection well sites like those used in oil and gas recovery, sometimes followed months later by quakes big enough to destroy buildings.
What seems to happen is that wastewater injection leaves local faults “critically loaded,” or on the verge of rupture. Even weak seismic waves from faraway quakes are therefore enough to set off a throngs of small quakes in a process called “dynamic triggering.”
Geologists have known for 50 years that injecting fluid underground, by the impoundment of water in reservoirs, surface and underground mining, can increase pressure on seismic faults and make them more likely to slip. The result is an induced quake.
The “natural gas boom” of recent years has been linked to an increase in small to moderate earthquakes. According to Rueters, seismologists at Columbia University say they have identified three earthquakes—in Oklahoma, Colorado and Texas—that were triggered at injection-well sites by major quakes a far distance away.
The seismic waves from an 8.8 quake in Maule, Chile, in February 2010 triggered a 4.1 magnitude quake in Prague, OK—site of the Wilzetta oil field—some 16 hours later. Months of smaller tremors in Oklahoma ensued, and then on Nov. 6, 2011, a 5.7 magnitude quake—the largest yet associated with wastewater injection—hit the city of Prague. It destroyed 14 homes, buckled a highway and injured two people.
“The fluids [in wastewater injection wells] are driving the faults to their tipping point,” said Nicholas van der Elst of Columbia’s Lamont-Doherty Earth Observatory who led the study.
Before the advent of injection wells, triggered earthquakes were a purely natural phenomenon. Now, according to the Science paper, triggered quakes can occur where human activity has weakened faults.