Earthquakes related to hydraulic mining and natural seismic activity in western New York State

Abstract

We have monitored the seismic activity of western New York since 1970 using a combination of up to eight permanent seismograph stations and several portable stations. Our investigation centered on the Attica-Dale area, the site of several damaging earthquakes in this century. Although the background level of seismicity was found to be extremely low, less than one event per month in the first 12 months of our study, this near quiescence was broken in 1971 by a sharp increase in seismicity at our site near Dale following the initiation of fluid injection under high pressure (120 bars tophole) at a hydraulic mining operation nearby. This facility, which mines salt from the Vernon Formation of Silurian age, is centered near the Clarendon-Linden Fault, a major north-south trending system of high-angle thrust faults that extends for over 100 km from Lake Ontario to Allegheny County, New York. Although the seismic events were small (none were recorded at our station 30 km to the northwest), as many as 80 occurred per day, and many were felt locally. The marked increase in seismic activity after attaining high pressures, the closeness of these events to the bottom of the injection well, and the near cessation of activity within 48 hours of the shutdown of injection strongly suggest that this activity was caused by the triggering of tectonic strain on or near the Clarendon-Linden Fault by the high fluid pressures of the mining operation. The minimum pressure (41–48 bars) at which seismic activity occurred is consistent with predictions made by applying the Hubbert and Rubey theory of effective stress to hydrofracturing stress measurements from Alma, New York. Since 1972, five other wells at Dale have been hydrofractured at pressures of about 110 bars, but none had abrupt changes in seismic activity associated with them during the high-pressure phases of pumping. The well used in 1971 (0.43 km deep) is the closest (about 50 m) of the six to the Clarendon-Linden Fault. It was hydrofractured near the base of the salt layer, whereas the others were hydrofractured well within the salt layer. Thus fluids under high pressure appear to have reached the Clarendon-Linden Fault in 1971 but not in the other five cases. A focal mechanism involving thrust faulting on a plane nearly parallel to the Clarendon-Linden Fault was obtained from events that continue to occur but at a much reduced level in the brine field in 1974 and 1975. An earthquake of magnitude 2.7, which occurred about 7 km to the west of the brine field in 1973, is apparently unrelated to the injection operation and is of natural origin. This and other nearby natural events appear to be associated with the western and southwestern branches of the Clarendon-Linden Fault. Hydraulic diffusivity values calculated from the space-time relationship of earthquakes triggered by fluid injection at Denver and Rangely, Colorado, Matsushiro, Japan, and Dale are about 104 and 105 cm2/s and are similar to those obtained from precursory anomalies of earthquakes. The similarity of the diffusivity values suggests that the precursory changes do involve the movement of water. In each of these four places the wells used for fluid injection bottomed into or very close to major fault zones.