A study just published in the journal Science (subscription required) by researchers from the California Institute of Technology and other institutions finds that injection of water into a subterranean fault induces earthquakes; further, the study illuminates the process by which seismic activity is induced.
The study, "Seismicity triggered by fluid injection–induced aseismic slip," was spearheaded by researchers Yves Guglielmi of France's Aix-Marseille University and Jean-Philippe Avouac of England's Cambridge University and Cal Tech.
From the Cal Tech press release on the study:
The research team drilled a hole into the fault at a depth of about 925 feet. They then lowered a five-foot-long canister outfitted with sensors called the Step-Rate Injection Method for Fracture In-Situ Properties, or SIMFIP, into the hole. The SIMFIP was designed to measure pressure, water flow rate, rock movement, and other key data while suspended in the fault zone . . .
After inserting the SIMFIT probe, the researchers injected 250 gallons of water into the fault zone. The SIMFIP recorded an initially slow, creeping slip of the rocks as fluid was pumped in; this type of movement on a fault is referred to as aseismic slip since no measurable microearthquakes occur. As rock on the two sides of the fault separated during this minor movement, however, the rate of water flow into the fault increased dramatically. About 18 minutes into the experiment, the slip rate increased, generating the seismic waves and dozens of measurable microearthquakes.
The experiment revealed that fluid injection itself did not directly provoke an earthquake. Instead, the aseismic slip likely built up stress at the edges of the creeping zone of rock. Eventually, the stress overcame the friction between the rock faces within the fault, triggering earthquakes.
And, in the dry, scientific language of the study itself, published in Science:
Our results prove unambiguously that fluid injection can trigger primarily aseismic slip, with seismicity induced as a secondary effect . . . Thus, our results may be of particular relevance to seismic activity triggered at shallow depth by human induced injections and shallow aseismic slip.
Well, frack me - you could knock me over with a magnitude 4.3 . . .
