Earthquakes have been clearly linked to subsurface fluid injection in two places--near Denver at the Rocky Mountain Arsenal's waste-disposal well, and at the Rangely oil field in northwestern Colorado. The theory linking fluid pressure to earthquakes is based on the effective stress concept, i.e., that increases of pore pressure reduce the effective normal stress across existing or potential fracture surfaces. In both cases, evidence exists for substantial tectonic shearing stresses in the reservoir rock prior to injection. Although the initial shear stress was below the critical value necessary to cause failure, fluid injection relieved a fraction of the frictional resistance to shear fracture, and earthquakes resulted.

There is presently no way to determine before drilling whether injection at a given site will produce earthquakes. At Denver and Rangely the earthquakes appear to be located entirely along pre-existing faults. At Rangely, the earthquakes have been drastically reduced in frequency by reducing pore pressures in the hypocentral region. In placing injection wells, existing faults should be avoided. Seismic surveillance during injection can provide early warning of inadvertently triggered earthquakes and palliative measures can be taken. The Rangely experience suggests that seismic activity due to waterflooding in oil fields may be controlled without seriously disrupting production of oil.

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