Potential record of large earthquakes from lacustrine sedimentary archives along the Bulnay fault system (Mongolia)

On July 1905, two M~8 earthquakes occurred 14 days apart along the Bulnay Fault system, in northwestern Mongolia. These seismic events are among the largest recorded earthquakes in intracontinental regions. However, our current understanding of the earthquake behavior of the Bulnay Fault is quite limited due to the scarcity of paleoseismic data. Additionally, the geographic and climatic conditions of the region play a major key in permafrost development, posing challenges in the excavation of paleoseismological trenches and causing cryoturbation. Lacustrine environments, conversely, are isolated depositional systems that minimize the influence of external factors and provide high temporal resolution with continuous sedimentation. Here, we present our findings on earthquake-triggered turbidites of eight sedimentary cores collected from three lakes around the Bulnay Fault. These cores were analyzed using X-ray tomography, X-ray fluorescence, and hyperspectral imaging. We found that prior to the 1905 event, three large earthquakes ruptured the Bulnay Fault, with recurrence intervals of 1.5 to 3 kyr. By integrating our observations with previous paleoseismic trench investigations, we proposed that strain is primarily accommodated through large earthquakes along the Bulnay fault, and major events involving both the Bulnay and Tsetserleg faults, potentially analogous to the 1905 doublet.