Reservoir-induced seismicity at Lake Pukaki, New Zealand

Abstract

SUMMARY The Pukaki microearthquake network was installed in 1975 to monitor possible changes in seismicity accompanying the raising of the water level of Lake Pukaki for hydroelectric power generation. During the period 1976-79, the depth of the lake was increased by 37 m, thereby adding 4.9 × 109 m3 to its volume. An analysis of 8 1/2 years of data from the microearthquake network has revealed widespread changes in seismicity which correlate with groundwater changes. During the period when groundwater level was higher than any previous maximum, there was both an increase in seismicity rate within 15 km of the lake and a decrease in the background (i.e. 15-50 km from the lake). The largest earthquake of the sample, the ML= 4.6 event of 1978 December located 10 km northwest of the Pukaki High Dam, occurred during this period. When groundwater level stabilized, the seismicity rate near the lake returned to normal, while there was a significant increase in both seismicity rate and moment release in the background. Strike-slip failure currently predominates in the Pukaki region, and the water load added to the lake will tend to stabilize such a stress regime. The increased seismicity near the lake during the period when groundwater level was rising clusters around the periphery of the lake, rather than directly beneath it. This suggests that earthquakes are induced only when an increase in pore pressure has diffused beyond the region where loading stresses predominate. The close correspondence in time of the period of rising groundwater and the period of increased seismicity near the lake, and the fact that they both lag the period of rising lake level by approximately one year, indicate that pore-pressure diffusion plays an important role in inducing seismicity near the lake. However, it is difficult to explain seismicity changes in the background in terms of pore-pressure changes, and these may be a result of stress redistribution following induced seismicity near the lake.