Preliminary study on the characteristics and slip rate of the Quaternary fault in Mogod, Mongolia

A comprehensive, interdisciplinary study was carried out to investigate the characteristics of the 1967 magnitude 7.1 earthquake along the right-lateral strike fault in Mogod, Mongolia. This fault consists of three segments—two strike faults and a reverse fault spanning from north to south. Recent research revealed a 25 ka cycle in the movement of the reverse fault segment located in the south (Bollinger et al., 2021).

To understand two remaining faults, four excavation surveys (T1, T2, T3, T4) were conducted along the two northern segments. Optically Stimulated luminescence (OSL) was used to track the deposition period in unconsolidated sedimentary layers where surface ruptures occurred, aiding in estimating the recent earthquake of the fault. An additional excavation survey was conducted near the river crossing the fault (at location T4) to determine the thalweg for evaluating geological displacement over the geological timescale.

The excavation results revealed Quaternary surface ruptures in three trenches with OSL sampling. A total of 51 samples were respectively collected from Trench T2 (24 samples), T3 (18 samples), and T4 (9 samples). The Quaternary sediment layers have been deposited since the Last Glacial Maximum (LGM), around ~20 ka. Excavation sites (3 upstream and 4 downstream) intersecting the fault line (T4) aimed to assess displacement caused by seismic activity.

In summary, seismic movements resulting in surface ruptures were detected in the northern two segments around the 20 ka. Further analysis would provide a more precise earthquake recurrence cycle, potentially revealing the timing of subsequent seismic events. Furthermore, completion of the identifying the thalweg, it is expected to reveal the slip rate over geological time scales.