A Relationship Between Joint Intensity and Induced Seismicity at Lake Keowee, Northwestern South Carolina

Abstract

In January and February 1978, an earthquake swarm occurred within an elliptically shaped epicentral area at Lake Keowee, South Carolina. Lake Keowee is located in the Inner Piedmont geologic belt of the southern crystalline Appalachians and is underlain by interlayered granitic gneiss and hornblende gneiss/amphibolite. The epicentral region is approximately 2 × 3 km and trends northwest–southeast, perpendicular to local geologic structure. The events were shallow (<3 km), low level (ML ≤2.2), and frequent (up to 200 events per day). Renewed activity in the same general epicentral area began in February 1986 and has continued to the present (August 1989) with approximately 280 events recorded. This study was initiated to test the hypothesis that areas of reservoir-induced seismicity (RIS) may have different fracture characteristics than areas that do not exhibit seismic activity. Joint intensity (a measure of joint surface area per unit volume), measured at surface exposures, was selected to quantify fracture characteristics of the epicentral and surrounding regions. The available data on fracturing with depth in crystalline rocks supports the extrapolation of the surface measurements to the hypocentral depths of the seismic events. Comparison of a contour map of joint intensity with the locations of the events show that a spatial relationship exists between the epicentral zone of RIS at Lake Keowee and a zone of low joint intensity (wider spacing between joints). The association of low joint intensity with the seismicity suggest the RIS is influenced by and is related to low joint intensity. Two factors involved in RIS that can be related to low joint intensity are permeability and strain energy. The permeability of the rock mass controls the migration of water (pore pressure) away from the reservoir and thus controls the temporal relationship of RIS onset to reservoir filling. The rock mass in the low joint intensity zone has a higher in situ stress field (closer to failure conditions) and therefore higher strain energy than the high joint intensity region. The spatial location of RIS at Lake Keowee is controlled by the low joint intensity zone. The correlation of the seismic activity with jointing in the low joint intensity zone suggest the potential maximum earthquake at Lake Keowee is probably the maximum that has occurred (MLg =3.8). The potential failure surfaces are relatively small and shallow and thus cannot accumulate enough strain energy for a large event. Two models for the RIS activity at Lake Keowee are proposed: a) release of applied (tectonic) strain which reaccumulates following an earthquake swarm, and b) release of residual strain which cannot be renewed. Both models incorporate the mechanical and chemical effects of increased pore pressure as a triggering mechanism. Continued monitoring of the epicentral region will supply data that may allow one of the above models to be accepted.