Since the 1960s, scientists have begun to understand that small earthquakes can actually be caused by humans. This discovery was made when the U.S. Army drilled a 12,000-foot well at the Rocky Mountain Arsenal, near Denver, to dispose of waste fluids. This activity generated a series of earthquakes in the area.
Later that decade, at an oil field in Rangely, Colo., tests were done and seismologists found that changes in the number of earthquakes recorded per year correlated with changes in the quantity of fluid injected into the ground.
In Texas, there are about 50,000 injection wells and only a handful of earthquakes a year. It stands to reason that if hydraulic fracturing posed a significant public threat, this number would be much higher.
Although typically the world's strongest earthquakes occur along the major fault lines, there are smaller, pre-existing faults all over. The common injection of water during the frac'ing process can change the pressure along these faults, causing them to slip and triggering small earthquakes, scientists say. In this way, frac'ing might cause an earthquake sooner than it would have occurred naturally.
"My preliminary studies suggest you almost never get induced earthquakes that are bigger than the natural earthquakes in an area," said Cliff Frohlich, a researcher at the University of Texas at Austin's Institute for Geophysics.
The goal researchers have is to identify and prevent a situation in which frac'ing might cause severe seismicity problems. This is where microseismic technology comes into play, using imaging technologies to look at subsurface rock formations, and to monitor frac'ing of wells in real-time.