Earthquakes induced by fluid injections

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Science  12 Jun 2015:
Vol. 348, Issue 6240, pp. 1204-1205
DOI: 10.1126/science.aab3820

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In the early 1960s, the U.S. Army unintentionally triggered some seismic activity by injecting waste fluids into the basement rock beneath the Rocky Mountain Arsenal, near Denver, Colorado (1). It is now recognized that an increase in the pressure applied by the fluids that fill fractures and faults at depth balances progressively the normal stress exerted through the rock on these fractures but leaves the shear stress supported by these surfaces unchanged. According to friction laws, when the fluid pressure gets too high and the effective normal stress gets too low, shear motion starts. This motion is generally considered to be mostly seismogenic, that is, to be the source of earthquakes that may reach in some localities magnitudes larger than 5, or even 6, as was observed upon filling the Koyna Dam in India in the early 1960s. Today, the phenomenon of fluid-induced seismicity has become a societal concern wherever injection of large quantity of fluids at depth is involved. On page 1224 of this issue, however, Guglielmi et al. (2) report that water injected in a natural fault at a depth of 282 m generates nonseismic motion. That is, the ground displacements take place at slow velocities (≈4 µm/s), with only very small microseismic activity.