Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbaatar (Mongolia)

Abstract

We present first constraints from tectonic geomorphology and paleoseismology along the newly discovered Sharkhai fault near the capital city of Mongolia. Detailed observations from high-resolution Pleiades satellite images and field investigations allowed us to map the fault in detail, describe its geometry and segmentation, characterize its kinematics, and document its recent activity and seismic behavior (cumulative displacements and paleoseismicity). The Sharkhai fault displays a surface length of ∼ 40 km with a slightly arcuate geometry, and a strike ranging from N42 to N72∘. It affects numerous drainages that show left-lateral cumulative displacements reaching 94 m. Paleoseismic investigations document faulting and depositional/erosional events for the last ∼ 3000 years and reveal that the most recent event occurred between 775 and 1778 CE and the penultimate earthquake occurred between 1605 and 835 BCE. The resulting time interval of 2496 ± 887 years is the first constraint for the Sharkhai fault for large earthquakes. On the basis of our mapping of the surface rupture and the resulting segmentation analysis, we propose two possible scenarios for large earthquakes with likely magnitudes of 6.7 ± 0.2 or 7.1 ± 0.7. Furthermore, we apply scaling laws to infer coseismic slip values and derive preliminary estimates of long-term slip rates. Finally, these data help build a comprehensive model of active faults in that region and should be considered in the seismic hazard assessment for the city of Ulaanbaatar.