Neotectonic and Modern Stresses of Southeastern Altai.

On September 27, 2003, the destructive Chuya (Altai) earthquake (magnitude 7.3), the strongest earthquake among those recorded in 1991–2022 in continental Russia, occurred near the southwestern border of the Chuya and Kurai basins in southeastern Altai. We have analyzed the seismic activity of this area and compiled the scheme of neotectonic stresses of the Chuya–Kurai Basin and adjacent structures. The modern and neotectonic stresses of southeastern Altai have been compared. The neotectonic stresses are retrieved using the structural-geomorphological method of shear stress reconstruction. It has been found that the orientation of the subhorizontal compression axis of the first rank changes from meridional to NNE and NE. The earthquake occurred in the region within which the largest faults of rank I (Kurai and Shapshal), SSW part of the prerecent Charysh–Terekta fault (we combine it at the neotectonic stage with the fault bounding the Chagan–Uzun block from the southeast in a common regional fault of rank I) were activated at the neotectonic stage in an setting of additional compression. A larger-scale stress field has been also reconstructed for the Chagan–Uzun rigid block that delineates the Chuya and Kurai basins. The neotectonic fault of the NW strike bounding the Chuya and Kurai basins from the southwest has been identified. The fault, we notionally called Bezymyannyi, is divided by smaller faults of SE orientation into three segments and is included in the large South Chuya fault zone. According to the data on strong earthquake focal mechanisms in the investigated region, a stable subhorizontal position of the compression axis has been established (as well as at the neotectonic stage). It is accepted that strong earthquakes occur in a compression setting. The mechanisms of earthquakes (mostly, aftershocks) related to the Chuya event indicate the concentration of thrust mechanisms within the Chagan–Uzun elevated block. This may be due to the ongoing uplift of the Chagan–Uzun block at the current stage. Along the Bezymyannyi fault, the earthquake focal mechanisms are mostly shear-type. This is the result of formation of a shear field under the shear displacement along the fault during the earthquake. Major seismic events occur mainly in a compression setting. Thus, the Chuya earthquake is caused by a subhorizontal compression setting (both regional and local), which is determined by comparing the neotectonic and modern stresses. [ABSTRACT FROM AUTHOR]