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Damien Delvaux, Royal Museum for Central Africa (Belgium)
Kanatbek Abdrakhmatov, Kyegyz Institute of Seismology (Kyrgyzstan)
Hans-Balder Havenith, Liege University (Belgium)
Alexander Strom, Institute of Geosphers Dynamics (Russian Federation)
Eutizio Vittori, Italian Agency for Environment Protection and Technical Services (Italy)
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The 1911 M8.2 Kemin earthquake that shook the Northern Tien Shan produced a complex system of surface ruptures about 130 km long, consisting of two main zones: a ENE-trending fault zone with left lateral movement stretching along the Chon-Kemin River valley and an E- to ESE-trending reverse-thrust fault zone with north-eastern side up that crosses the Aksu and Chon-Aksu River valleys and divides in two parts the Kungey-Altau neotectonic uplift (the Aksu branch). The latter was characterized by the reverse offset up to 10 m.
Evidence of at least 4 rupturing events was found in a trench excavated across the Aksu branch at the site where it dams a small lake on the right bank of the Chon-Aksu River valley (42°49'59" N, 77°21'50" E). Geomorphic evidence, as well as geophysical data, clearly indicate reverse kinematics at this fault section, though the main rupture surface revealed in the trench demonstrates some bending up to overturning, so that it steeply dips south. The oldest identified event is marked by lacustrine sediments similar to those accumulating now in the fault-dammed lake. We assume that this unit was accumulated about 18029 - 17590 BC in the same past-event environment as at present. It is intensively deformed while approaching the rupture plane. Lacustrine sediments are overlaid by a mixture of bedrock fragments that can be interpreted as a colluvial wedge of the second event(s). This unit, in turn, is overlaid by thick uniform strata of loam, bounded from above by a buried soil dated at 1484 - 1661 AD. A sample from the middle part of the loam layer was dated at 1739 - 1611 BC. Thus, accumulation of this unit that occurred in stable conditions without any rupturing events lasted for several thousands of years. However, after formation of the well-defined buried soil layer, at least 2 rupturing events occurred that can be inferred from the successive colluvial wedges. The upper one includes several blocks of the same buried soil, some of which were overturned. The latter event could be the 1911 Kemin earthquake. It was associated with liquefaction phenomena that formed tree-like clastic dykes at the foot of the scarp.
Data available show quite an irregular earthquake occurrence: stable periods lasting for several thousand years alternating with periods when several rupturing events occurred within few centuries. Variability of the strong earthquake recurrence intervals should be taken into consideration while assessing seismic risk of the Northern Tien Shan, including densely populated Issyk-Kul and Ili depressions.
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