Great Earthquakes in Low-Strain-Rate Continental Interiors: An Example from SE Kazakhstan

Abstract:

The Lepsy fault of the northern Tien Shan, SE Kazakhstan is an example of an active structure that connects a more rapidly deforming mountain region with an apparently stable continental region (SCR). The fault extends E-W ~100 km from the high mountains of the Dzhungarian Ala-tau, a sub-range of the northern Tien Shan, into the low-lying Kazakh platform. Our field-based and satellite observations reveal a ∼10 m vertical offset exceptionally preserved along the entire length of the fault. Geomorphic analysis and age control from radiocarbon and optically stimulated luminescence (OSL) dating methods indicate that this scarp formed in the Holocene and was generated by two substantial earthquakes. The most recent event, which we date to some time after ∼400 years BP, is likely to have ruptured the entire ∼100 km fault length in a Mw 8 earthquake. In spite of the long recurrence times expected for such large SCR earthquakes, they represent a potential earthquake hazard in the increasingly populated Tien Shan foreland region. To characterise the Lepsy fault kinematics we use digital elevation models (DEMs) and high-resolution satellite imagery, which indicate that the predominant sense of motion is reverse right-lateral with a fault strike, dip, and slip vector azimuth of ∼290°, 45/50°S, and 335° respectively, which is consistent with predominant N-S shortening related to the India-Eurasia collision. Finally, the geometry of the western Lepsy fault termination suggests that shortening may also be partly accommodated by counter-clockwise rotation about a vertical axis.