Researchers of an international team have observed for the first time how fluid injection sets off microearthquakes on a sizable, subterranean fault.
The new findings sheds light on earthquake physics in general, as fluids are ubiquitous in the subsurface.
The study, published on the June 12 issue of the journal Science, could lead to better seismic risk management through improved understanding of fluid flow on faults, while also illuminating the mechanics of natural earthquakes.
“At the moment, a major issue for industry is that there is no established theory to evaluate the seismic hazard associated with fluid injections,” said paper coauthor Jean-Philippe Avouac, a professor of geophysics at the University of Cambridge.
“With experiments such as ours, we can build much-needed models that would help assess the possible location, magnitude, and likelihood of earthquakes,” said Avouac, who is also the Earle C. Anthony Professor of Geology at California Institute of Technology.
Earthquakes typically occur when segments of the Earth’s crust slip along faults due to the built-up pressure. However, many smaller faults branch off these major faults, extending down to microscopic cracks that exist in most if not all rocks.
Researchers ran a fluid-injection experiment on a fault running more than a quarter of a mile (1,610 meters) through limestone. The fault is accessible because of its location adjacent to a former underground military facility in southeastern France now available to scientists.
The research team drilled a hole into the fault at a depth of about 281.9 meters and then lowered an about-1.5 meter-long canister outfitted with sensors called the Step-Rate Injection Method for Fracture In-Situ Properties, or SIMFIP, into the hole.
After inserting the SIMFIP probe, which was designed to measure pressure, water flow rate, rock movement, and other key data while suspended in the fault zone, the researchers injected 250 gallons of water into the fault zone.
Around 18 minutes into the experiment, the slip rate increased, generating the seismic waves and dozens of measurable microearthquakes.
The research showed, based on dynamic modeling, that injecting fluids at high pressure promotes aseismic, or non-seismic, fault creep, and subsequently earthquakes occur as a secondary result.
In other words, fluid injections can cause faults to creep silently, and initially relieve pressure or elastic strain near the point of injection. However, as a secondary effect, earthquakes can then occur at the periphery of the creeping zone, where stress is now concentrating, due to the creeping aseismic fault.
“Let’s first fully understand the risks, hazards and mechanisms of fluid injection before we actively engage in these activities as a standard practice,” researchers warned. Enditem