Earthquakes triggered by human activity follow several indicative patterns that could help scientists differentiate them from naturally occurring temblors, a new study by Stanford researchers suggests.
One of the study’s main conclusions is that the likelihood of large-magnitude manmade, or “induced,” earthquakes increases over time, independent of the previous seismicity rate. A reservoir simulation model that the researchers developed found a linear relationship between frequency and magnitude for induced quakes, with magnitude increasing the longer wastewater is pumped into a well.
“It’s an indication that even if the number of earthquakes you experience each month is not changing, as you go further along in time you should expect to see larger magnitude events,” said Dempsey.
Jenny Suckale, assistant professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences, and her postdoctoral researcher David Dempsey analyzed a sequence of earthquakes on an unmapped basement fault near the town of Guy, Arkansas, from 2010 to 2011.
In geology, the term “basement” refers to rock located beneath a sedimentary cover that may contain oil and other gas reserves that can be exploited through drilling or hydraulic fracturing, also known as “fracking.”
Scientists suspected that the Arkansas quakes were triggered by the injection of roughly 94.5 million gallons of wastewater into two nearby wells that extend into the basement layer during a nine-month span. The injected water increases the pore pressure in the basement layer, adding more stress to already stressed faults until one slips and releases seismic waves, triggering an earthquake.
Other studies have found that the rate of wastewater injection into a well is more important than the total volume injected for triggering earthquakes.
But the Stanford study found that, given similar rates of wastewater injection, there is a direct correlation between the volume injected and the incidence of earthquakes. Of the two wells studied near Guy, Well 1 received four times the wastewater volume as Well 5, and induced four times as many earthquakes.
“There’s a scaling there in terms of the volume injected,” Dempsey said.
The study’s findings could have implications for both the oil and natural gas industry and for government regulators. Under current practices, extraction activities typically shut down in an area if a high-magnitude earthquake occurs. But according to Suckale, a better approach might be to limit production before a large quake occurs.