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

Abstract

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.

Tectonic and gravity-induced deformation along the active Talas–Fergana Fault, Tien Shan, Kyrgyzstan

“…The recent deformation along this fault has been studied by several scientists over the past few decades, with a number of works appearing in English (Simpson et al, 1981;Burtman et al, 1987Burtman et al, , 1996Makarov 1989;Trifonov et al, 1990Trifonov et al, , 1992Korzhenkov et al, 2013). There is clear evidence of surface faulting events with river channel offsets (in the order of 25-100 m), along tens of km long faults, characteristics that are typical of large magnitude earthquakes (McCalpin, 1996).…”

“…However, although all authors agree on significant Holocene offsets along the TFF (review in Simpson et al, 1981), only a couple of studies have been published, in Russian, on the distribution of landslides along this fault (Korjenkov, 1997;Strom, 2009), despite recognition that large active faults in mountain settings may be associated with both shallow and deep-seated slope failures. Several researchers infer a generic relationship between active faults and slope failures (e.g.…”

“…The maximum depth of the reservoir is 200 m. From the dam, the reservoir follows the Naryn River northeastwards until it crosses the TFF and opens into the main part of the reservoir. Along the SW shore, the reservoir abuts directly against the trace of the TFF (Simpson et al, 1981) (Fig. 2).…”

Seismotectonic study of the Fergana Region (Southern Kyrgyzstan): distribution and kinematics of local seismicity

“…One of the major historic earthquakes, which occurred in 1946 with an Mw = 7.3 is located in the Chatkal region. Simpson et al (1981) suggested that this earthquake was associated with the complex zone at the oblique intersection of the Talas-Fergana fault and Chatkal range. (2) The Alai region with the Alai Valley (see Figure 1; SW Tien Shan), the Alai range north of the Alai Valley, and the Trans Alai Range south of the Alai Valley, which are all located to the south of the Fergana basin.…”

Tectonics of Asia and Waveform Studies of the Velocity and Q Structure of the Mantle.

“…In many parts of the Tien Shan, concentrated zones of seismicity can be related to edges of zones of Hercynian deformation, e.g., along the Gissar-Kokshal fault in the southern Tien Shan (Gubin, 1967) and along the Talas-Fergana and main Tien Shan faults in the northern Tien Shan (Simpson et al, 1980). Scattered exposures of Hercynian age continue in outcrops northwest of the southern Tien Shan but become increasingly covered by Neogene and Quaternary sands (Markovskii, 1975;Yanskin, 1966).…”