Injection‐Induced Moment Release Can Also Be Aseismic

The cumulative seismic moment is a robust measure of the earthquake response to fluid injection for injection volumes ranging from 3,100 to about 12 million m3. Over this range, the moment release is limited to twice the product of the shear modulus and the volume of injected fluid. This relation also applies at the much smaller injection volumes of the field experiment in France reported by Guglielmi et al. (2015, https://doi.org/10.1126/science.aab0476) and laboratory experiments to simulate hydraulic fracturing described by Goodfellow et al. (2015, https://doi.org/10.1002/2015GL063093). In both of these studies, the relevant moment release for comparison with the fluid injection was aseismic and consistent with the scaling that applies to the much larger volumes associated with injection‐induced earthquakes with magnitudes extending up to 5.8. Neither the microearthquakes, at the site in France, nor the acoustic emission in the laboratory samples contributed significantly to the deformation due to fluid injection. Plain Language Summary Injection of fluid into the Earth's crust sometimes results in a sequence of earthquakes. The deformation associated with these earthquakes is proportional to the volume of injected fluid. This relationship between injected volume and induced earthquakes applies for volumes ranging from 3,100 m3 up to volumes exceeding 10 million m3, for induced earthquakes with magnitudes as high as 5.8. It turns out that this simple relationship is also useful at much smaller injected volumes. At a field experiment in southern France, injection of 0.95 m3 of water into a preexisting fault zone, cutting through a limestone terrain, caused a “slow earthquake” with a magnitude of 1.17. At a much smaller scale, laboratory experiments to simulate hydraulic fracturing revealed that injection of approximately 1 ml of water into samples of granite resulted in slow sample expansion equivalent to the deformation of earthquakes with magnitudes of about minus 3. Key Points Crustal deformation in response to fluid injection can be seismic or aseismic Cumulative moment release, a measure of deformation, varies linearly with injected volume The range of linearity extends from volumes of 1 ml in the laboratory up to more than 10 million m3 for sequences of induced earthquakes