Abstract
AbstractThe Lepsy fault of the northern Tien Shan, SE Kazakhstan, extends E‐W 120 km from the high mountains of the Dzhungarian Ala‐tau, a subrange of the northern Tien Shan, into the low‐lying Kazakh platform. It is an example of an active structure that connects a more rapidly deforming mountain region with an apparently stable continental region and follows a known Palaeozoic structure. Field‐based and satellite observations reveal an ∼10 m vertical offset exceptionally preserved along the entire length of the fault. Geomorphic analysis and age control from radiocarbon and optically stimulated luminescence dating methods indicate that the scarp formed in the Holocene and was generated by at least two substantial earthquakes. The most recent event, dated to sometime after ∼400 years B.P., is likely to have ruptured the entire ∼120 km fault length in aMw7.5–8.2 earthquake. The Lepsy fault kinematics were characterized using digital elevation models 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 ∼110°, 50°S, and 317–343°, respectively, which is consistent with predominant N‐S shortening related to the India‐Eurasia collision. In light of these observations, and because the activity of the Lepsy fault would have been hard to ascertain if it had not ruptured in the recent past, we note that the absence of known active faults within low‐relief and low strain rate continental interiors does not always imply an absence of seismic hazard.
Highlights
This paper looks at seismic hazard in regions that may contain active faults, but which have no documented record of earthquakes
The Lepsy fault kinematics were characterized using digital elevation models 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 ∼110∘, 50∘S, and 317–343∘, respectively, which is consistent with predominant N-S shortening related to the India-Eurasia collision
Due to the sparse seismicity, and overall absence of obvious geomorphic evidence of late Quaternary active faulting, the foreland of the northern Tien Shan is usually considered as a stable continental regions (SCRs) or low strain rate continent interior, and it is not clear whether these rare foreland earthquakes occur on faults that connect with the active structures of the mountains and whether the Kazakh platform may be susceptible to larger, more damaging, earthquakes than those present in instrumental catalogues
Summary
This paper looks at seismic hazard in regions that may contain active faults, but which have no documented record of earthquakes. Due to the sparse seismicity, and overall absence of obvious geomorphic evidence of late Quaternary active faulting, the foreland of the northern Tien Shan is usually considered as a SCR or low strain rate continent interior, and it is not clear whether these rare foreland earthquakes occur on faults that connect with the active structures of the mountains and whether the Kazakh platform may be susceptible to larger, more damaging, earthquakes than those present in instrumental catalogues. We describe remote sensing and field observations, and Quaternary dating results, which show that the Lepsy fault has generated at least two substantial earthquakes in the Holocene, the latter of which may have been Mw 8 and is dated to within the last ∼400 years B.P. We characterize the geometry and kinematics of the Lepsy fault, as well as its late Holocene seismic activity, and discuss the role of the fault in accommodating regional N-S shortening related to the ongoing India-Eurasia collision
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