A reservoir analysis of the Denver earthquakes : a case of induced seismicity

Abstract

Injection of fluid waste into Precambrian crystalline rocks at the Rocky Mountain Arsenal triggered earthquakes in the 1960s. Available data suggest that the waste fluid was injected into a reservoir composed of connected vertical fractures. Earthquakes are believed to be results of lateral sliding motions along fracture planes. A mathematical model is constructed to simulate fluid pressure build-up caused by injection. Computed pressure build-up is related to the spatial distribution of earthquake epicenters. The results show that the earthquakes are confined to that part of the reservoir where the pressure build-up exceeds 32 bars. This critical value is interpreted as the pressure build-up above which earthquakes occur. The existence of this critical pressure is consistent with the Hubbert-Rubey theory on the role of fluid pressure in fault movement. The migration of earthquake epicenters away from the injection well, a phenomenon noted by previous investigators, can be accounted for by the outward propagation of the critical pressure build-up. The analysis is extended to examining the effects of fracture widening under high injection pressure. The results show that the effect is confined to a small region within one kilometer of the injection well.