A new study has added powerful new support to the growing evidence that deep underground injections of wastewater from hydraulic fracturing or other drilling activity has made some areas of the U.S. vulnerable to tremors triggered by large earthquakes thousands of miles away.
Wastewater injected near several existing faults left them in a critical state that allowed seismic waves from distant quakes to set off temblors that occurred hours to months after the triggering event, the researchers wrote in a study published last week (July 11) in the journal Science. The minor earthquakes may serve as early warnings that injection activity is pushing faults “to their tipping point” and making larger shocks more likely, they said.
Using data from the Advanced National Seismic System (ANSS), a team at Columbia University led by Nicholas van der Elst discovered three locations close to wastewater injection wells where large but distant earthquakes had triggered an upswing in local seismic activity. The towns of Trinidad, Colo., Prague, Okla., and Snyder, Texas all had decade-long histories of deep wastewater injection yet relatively few seismic events until the remote quakes struck.
- In February 2010, a magnitude 8.8 quake that hit Maule, Chile, killing 500 people, triggered a series of earthquakes in the Colorado and Oklahoma towns, where injection wells had been in use for years.
- The magnitude 9.0 earthquake off Japan that set off a devastating tsunami and the Fukushima nuclear disaster in March 2011 was followed by a swarm of seismic events that hit Trinidad and the West Texas town of Snyder, home to an injection well for wastewater used in nearby oil fields.
- n the aftermath of a 2012 magnitude 8.6 earthquake in Sumatra, both Snyder and Trinidad again experienced surges in seismic activity, although milder than after the Chilean earthquake.
The early swarms of minor seismic activity turned out to be precursors of larger, more damaging quakes that occurred in the three towns 6-to-20 months after the triggering events. These included a magnitude 5.7 temblor in Prague in November 2011, one of the largest ever associated with wastewater disposal. It destroyed 14 homes and injured two people. The researchers said the occurrence of small, remotely triggered earthquakes can be an indicator that an area is “critically stressed” and is likely to experience a more severe quake in the near future.
The suggestion that distant earthquakes can set off seismic activity thousands of miles away is not new.
“We’ve known for at least 20 years that shaking from large, distant earthquakes can trigger seismicity in places with naturally high fluid pressure, like hydrothermal fields,” said Geoffrey Abers, a coauthor of the study, in a release issued by Columbia’s Lamont-Doherty Earth Institute. But the study breaks new ground by suggesting that human activity, in this case wastewater injection, was the factor that caused the fault systems to become critically stressed and vulnerable to remote triggering.
Hydraulic fracturing involves pumping a high-pressure mixture of water, chemicals and sand deep underground to create fractures and extract natural gas and oil from impermeable rock formations. The water that returns to the surface is contaminated with chemicals from the fracking fluid and naturally occurring elements from the rock. Conventional drilling techniques also sometimes pump fluid into the ground to force more oil or gas from depleted fields.
Injecting this contaminated wastewater deep underground is a common way to dispose of it, the researchers noted. There it increases the pressure in the pores, or gaps, in the soil or rock. The fluid acts as a lubricant, making the rock along a fault more likely to slide and eventually fail, causing an earthquake. In some areas the increased seismicity does not appear until the faults have been gradually saturated with fluids and reach a critical threshold, sometimes months or even years later. It is very difficult to predict when a fault is reaching this point, the Columbia team said.
Heather Savage, physicist and another coauthor of the study said in Columbia’s release, “If the number of small earthquakes increases, it could indicate that faults are becoming critically stressed and might soon host a larger earthquake.”
It would make sense to increase seismic monitoring around injection sites to help detect this change and indicate that it was time to stop pumping wastewater into the ground in that area.
