Low-amplitude brittle deformations revealed by UAV surveys in alluvial fans along the northwest coast of Lake Baikal: Neotectonic significance and geological hazards

Many large deltas and other areas, underlain by unconsolidated sediments, are heavily populated but impacted by various natural deformational processes. The causes and mechanisms of the deformation are often obscure because of difficulties in the identification of their geological source. We used an unmanned aerial vehicle (UAV) to survey an area of up to several km² to detect and map surface discontinuities with displacements of a few centimeters, which can indicate the presence of initial geological deformations. We encountered such offsets in the alluvial fans on the northwestern coast of Lake Baikal, overlying the damage zones of several faults. These deformations at the different sites are referred to as primary or secondary co-seismic ruptures, brittle creep, or cryogenic fissures, which have specific origins. They are located a few hundred meters from a principal fault plane, from junctions of faults, and in the peripheral parts of alluvial fans, making these areas potentially hazardous. We analyzed the digital surface models of local segments of one of the delta plains between 2020 and 2021 and established that its periphery has subsided by an average of 5–10 cm. In areas of accumulation of the river sediment, the thickness of coarse clastic deposits has increased by approximately the same amount. Locally and in the coastal zone, the vertical surface changes are larger. In the axial parts of some seismically induced gravitational failures, the subsidence reached 33 cm over a period of 11 months and 19 days. Our results show that sediments of alluvial fans are very susceptible to various tectonic and exogenous deformational processes. The interpretation of the ultra-high resolution UAV's images can help the recognition the low-amplitude brittle deformations at an early stage of their development. Therefore, such UAV surveys are critical in the discernment of neotectonic activity and its related hazards over short observation periods.