Human-induced Seismicity

Contrary to previous strategies thought to mitigate human-induced seismicity, injections into basement rock result in more confined and smaller-magnitude earthquakes (left). Injection into sedimentary rock leads to more distant and larger earthquakes (right).

Thomas Göebel, CC BY-ND

Fluids injected deep underground at high pressure for hydraulic fracturing (fracking) or for wastewater disposal can cause moderate-magnitude earthquakes, and the rate at which these earthquakes occur has been increasing since 2006. Scientists have now determined that the type of rock into which fluids are forced can affect the distance of tremors from injection wells.

Thomas Goebel and Emily Brodsky of the University of California, Santa Cruz, studied eighteen fluid injection sites and seismic activity on three continents. They observed only isolated injection wells in order to correlate cause and effect definitively. Data were eliminated from such places as Oklahoma—where earthquakes now outnumber those in California—because injection sites there are so numerous that individual wells cannot be associated with specific quakes.

The researchers discovered that injection wells that extend deep into basement rock cause quakes that tend to occur close to a well site. When fluids are pumped into sedimentary rock layers above the basement level, the resulting quakes tend to have higher magnitudes and are spread out over a much wider area, sometimes more than ten kilometers away.

The wider distribution of quakes from sedimentary rock injections, the authors say, is a function of that rock’s poroelasticity—the interaction between the rock’s solid mass and its pore spaces, or the gaps into which fluid is forced. Increased pressure in the pores causes stresses in the rock at greater distances. According to Goebel, each pore behaves like an inflated balloon inside a box. If one steps on the balloon, the pressure inside it increases, and it pushes out against the walls of the box—or adjacent rock—which may move as a result.

It was previously thought that injection into shallower levels of sedimentary rock, well above basement formations, would reduce the earthquake hazard. Actually, says Goebel, this technique likely increases the probability of earthquakes and their geographic extent because sedimentary rock has higher poroelasticity than stiffer basement rock. But, he adds, operators and regulators must confer to develop recommendations for individual sites, and no one blanket rule may apply. (Science)