The Hustain fault is a 100-km long, northeasterly-striking, active extensional structure that bounds the Tuul River Valley (TRV). The fault originated as a late Paleozoic compressional structure during closure of the Mongol-Okhotsk Ocean. After a period of geologic quiescence during the early Cenozoic, the fault was reactivated as an extensional structure and may be associated with the same extension that formed Baikal Lake.

The trace of the Hustain fault is clearly visible in false-color satellite images and topographic maps due to its large escarpments, truncation of Mesozoic intrusions, and a series of active springs located in the Quaternary alluvium. A southeast-northwest extensional vector (143.3/323.3±3.93° azimuth) was deduced from kinematic indicators. This fault possesses a left-lateral, strike-slip component and is capable of producing Magnitude 7.5 earthquakes that could severely impact the inhabitants of nearby Ulaanbaatar.