Tectonics and seismicity of the Toktogul Reservoir Region, Kirgizia, USSR

The 214 m high Toktogul Dam will impound a 19.5-km³ reservoir on the Naryn River in central Kirgizia. The reservoir lies along the Talas-Fergana fault, a major tectonic feature of central Asia. The nature of Cenozoic movements along the fault is in debate; however, there is considerable evidence of present-day right-lateral movement. The region is one of moderate to high seismicity. A magnitude 7.6 earthquake occurred in 1946, approximately 65 km from the dam, near the intersection of the Talas-Fergana fault and Chatkal ranges. Construction of the dam was completed in 1975 and by late 1979 the water level in the reservoir reached three fourths its final planned depth. Low-magnitude, induced earthquakes began as the water level first exceeded 100 m in late 1977. A microearthquake survey using portable instruments was carried out in July-August 1978 and a seven-station telemetered network was installed in October 1978. The largest of the induced earthquakes has been of magnitude 2.5; the activity is in the immediate vicinity of the dam at very shallow depths (2-5 km) and is related to water level. Preliminary analysis of data gathered during 1978-1979 shows a zone of activity, extending to 5 km depth, supporting geological evidence for an active reverse fault beneath the dam and parallel to the canyon of the Naryn River. Where the reservoir crosses the Talas-Fergana fault, 15 km upstream from the dam, the water level has not yet reached 100 m and seismicity along the Talas-Fergana has not shown any significant change related to filling of the reservoir.