9 Cenozoic Tectonics in Asia and North America: Linking Processes Across Scales (Posters)

This session invites research integrating geologic, geomorphic, and paleoclimate data to advance understanding on orogenesis, seismic hazards, and global change.

Monday, 18 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Ulaanbaatar fault systems and its potential threat to the urban settlement in central Mongolia

Paper number: 9-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Ulaanbaatar, the rapidly growing capital and largest city in Mongolia, is located in a structural basin bound and underlain by a poorly understood network of faults with poorly unknown slip histories, but which have generated modest-sized historic earthquakes. We have completed a paleoseismic trench investigation near the mapped termini of the 50 km-long, left-lateral strike-slip Ulaanbaatar fault system that strikes from Naran in the NW to Khurel Togoot in the SE passing directly beneath the city center. The fault system is composed of at least three segments, the length of which are capable of generating Mw ~6.5 earthquakes. Constraining bends in the fault have generated local oblique-slip transpressional fault scarps with several meters of relief. A particularly well-preserved scarp at Khurel Togoot has been trenched revealing a long paleoseismic record through the late Pleistocene. Here we have discovered evidence for several ruptures in the upper ~3 m of strata that spans ~40 ka. Trench stratigraphy shows deformed crystalline rocks in a hanging wall unconformably overlain by an organic-rich wetland deposit that has been folded and thrust over several meters of red colluvium in the adjacent, subsided footwall. The entire scarp is draped by less-deformed, brown colluvial and aeolian units. The youngest rupture in the trench offsets aeolian deposits dated at ~28 ka. Other similar paleoseismic studies on the Ulaanbaatar fault system document ruptures as young as ~20 ka so collectively it is likely that the Ulaanbaatar fault system is seismogenic, and presents a seismic hazard to the city. Well-constrained slip rates cannot yet be extrapolated from these preliminary findings, but they are broadly consistent with other paleoseismic studies that report earthquake recurrence intervals on mapped faults as ranging from ~5-10 kyrs for the regional surrounding the capital in central Mongolia. Ongoing work of this nature is critical input to risk analysis models of seismic hazards that can guide public policy and infrastructural development in Ulaanbaatar and elsewhere in Mongolia.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410573

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