The Mw 7.2 Altay (Chuya) earthquake of 27 September 2003 that occurred in Gorny Altay (southern Siberia, Russia) is the first event of this magnitude to occur in the region in historical time. The 60 km long surface ruptures of the right-lateral event follow a preexisting but previously unmapped northwest–southeast fault trace along the northern slope of the North Chuya range. Additional secondary coseismic ruptures were also observed on adjacent faults. The earthquake triggered landslides, rock falls, and liquefaction, as well as destruction of houses and other construction in the Kurai and Chuya basins. The 2003 event induced several thousand aftershocks during the years after the mainshock that formed a cloud of epicenters aligned with the main coseismic rupture trace. The local earthquake tomography of the event source shows the correspondence between the fault trace at the surface and a long narrow low-velocity zone, penetrating vertically into the crust down to a depth of 15–17 km. If the three-dimensional (3D) geometry of the aftershock cloud approximates the ruptured fault plane at first order, then the fault is mainly vertical, or slightly southwest dipping, confirming the right-lateral-reverse kinematics of the fault, compatible with northeast–southwest shortening accommodated by a combination of right-lateral and thrust faults. Together with the average surface displacement observed in the field (Δu 1–2 m), the surface of the fault plane determined by the aftershocks distribution (80×17 km) gives a magnitude Mw∼7.2, in good agreement with the Harvard determination. Natural exposures produced by the 2003 surface faulting, together with previous paleoseismic observations across the primary and secondary earthquake-induced features, have revealed the occurrence of several strong events (magnitudes about 7–8) during the last 5000 yr. The characteristics of the Altay event suggest that the Gorny Altay region, similar to Mongolia and Gobi, is characterized by large infrequent M 7–8 earthquakes along faults moving at rates of a few mm/yr or less.

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