Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

E X T E N D E D A B S T R A C T In July 1905, two M ~ 8 earthquakes occurred 14 days apart along the two strands of the Bulnay- Tsetserleg fault system (northwestern Mongolia). The second earthquake, the Bulnay earthquake has been widely documented, both for its most recent rupture in 1905, and also about past ruptures. It has been established that return time along that rupture is about 3000 years long. Here, we focus on the first rupture in the sequence, the Tsetserleg earthquake. The surface rupture has been far less documented and no much is known about history of past earthquakes along that strand. Through fieldwork investigations we have characterized the coseismic deformation, estimated the long-term fault slip rate, and investigated the past occurrence of similar earthquake doublet sequences. We have acquired drone images along the Tsetserleg rupture, favoring sites with structural complexities and significant surface fracture development. Using the topographic dataset derived from our imagery, we mapped the fracture network associated with the rupture. The average values obtained from crack width measurements are compatible with 2 to 3 m of horizontal coseismic slip, with no significant vertical displacement. Our detailed mapping allowed us to identify distinct features such as tension cracks opening in the early stages of the 1905 rupture propagation. Cumulative deformation was documented and combined with dating of the Fig. 1: Location of the two earthquakes of the Bulnay - Tsetserleg sequence in 190. The box indicates the specific focus of that study.