Paleoseismological study at the Bayan-Agt fault, Mongolia

Abstract

The Bayan-Agt fault is located in west-central Mongolia. Along the fault zone, which extends south of Bayang-Agt soum in Bulgan Province, multiple fault segments and associated scarps with clear morphological expression have developed. Well-defined linear scarps are particularly evident on several segments, especially where they are preserved within the volcanic basalt series that are widely distributed across the study area. Satellite imagery, combined with geological field survey data, indicates that the fault follows the southern piedmont of Bayang-Agt soum, trends predominantly east–west, and can be mapped continuously for approximately 98 km. Guided by field observations and supported by remote sensing analysis of satellite imagery, sites were selected for paleoseismological trenching. Trenches were excavated approximately 550 m south of the Bayang-Agt soum center, at the piedmont of the Dalan-Tolgod hills. Radiocarbon dating of sediments exposed in the Bayan-Agt fault trenches yielded ages ranging from 904 to 28,051 years BP within a lateral distance of ~100 m between trenches. These results indicate a complex depositional history involving reworked Late pleistocene deposits. Although the data do not permit direct estimation of earthquake recurrence intervals, they confirm long-term seismic activity and highlight the challenges of establishing reliable paleoseismic chronologies in active fault zones. Based on the 98 km long surface rupture of this left-lateral fault and the area affected by surface deformation, it is  possible to infer the occurrence of a potential earthquake of approximately magnitude 7 in the region.