abstract

Six weeks after the start of filling of the Manic 3 reservoir a long sequence of induced earthquakes began. The main shock, mbLg = 4.1, occurred on October 23, 1975, preceded by 1 month of foreshock activity and followed by more than 1,000 aftershocks in the next 4 months. After 2 years, the activity still persists, although the microshocks are substantially reduced in frequency and magnitude. Station MNQ, about 80 km from the activated source, served as an efficient monitor providing data from which increasing induced seismicity was predicted and for evaluating the subsequent activity. Two portable networks were deployed around the source, one in 1975 and the other in 1976. The activity was very shallow, with an average value of 1.5 km, and the cluster of microearthquakes spread over an area of 4 by 4 km. No systematic spatial migration was observed. The activity appeared to have been triggered by the water percolating through joint systems and along northwest oriented planes. The presence of regional stresses and local inhomogeneities, structural and lithological, are suggested as the principal causes, and not the dimensions of the reservoir and the water height.

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