T035-09
Coseismic Slip Distribution and Earthquake Recurrence Along the Bulnay-Tsetserleg Fault Zone (Mongolia) : Insights From UAV-Based Photogrammetry and Paleoseismology

Friday, 11 December 2020: 07:32
Virtual
Yacine Nicolas Benjelloun1, Yann Klinger1, Solène Antoine1, Ganbold Baatarsuren2, Laurent Bollinger3, Youngbeom Cheon4, Jin-Hyuck Choi5, Ganzorig Davaasuren2 and Steven L Forman6, (1)Institut de Physique du Globe de Paris, Paris, France, (2)Institute of Astronomy and Geophysics, Ulaanbaatar, Mongolia, (3)CEA/DAM/DIF, Arpajon, France, (4)Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea, (5)Korea Institute of Geoscience and Mineral Resources, Geology Division, Daejeon, South Korea, (6)Baylor University, Waco, TX, United States
Recorded Presentation
Abstract:
In July 1905, two M~8 earthquakes occurred on the Bulnay left-lateral fault and the Tsetserleg oblique fault, in northwestern Mongolia. With a total length of 676 km, this is one of the largest and best-preserved earthquake sequences documented in a continental context, offering a unique opportunity to gain insights into the behavior of large strike-slip fault systems. Our study aims to (1) study in detail the rupture geometry and the coseismic slip distribution along the Tsetserleg fault, and (2) test whether similar sequences happened repeatedly through the past seismic cycles.

To better estimate the 3D coseismic slip, we acquired drone images along the Tsetserleg 1905 rupture on selected sites showing significant surface fracture development. Using the MicMac software, we produced high-resolution DEMs and orthophotomosaics and measured the geometrical characteristics of the fracture network to derive the associated surface slip.

The crack measurements provide average values ranging from 2 to 3.5 m for the horizontal coseismic slip along the Tsetserleg rupture, with no significant vertical displacement. These results agree with previous analyses of systematically offset markers. Our detailed mapping allows us to identify distinct features corresponding to successive steps of the 1905 rupture propagation. Preserved markers of previous events also suggest that the rupture path and surface expression can vary between successive seismic cycles.

To constrain the ages of the pre-1905 earthquakes, we opened two trenches along the Tsetserleg fault and we documented one new fault exposure along the Bulnay fault. We identified at least four paleo-events in the Tsetsterleg trenches, and three paleo-events on the Bulnay fault exposure. We constrained the timing of these events with the OSL dating of 23 samples. Eventually, we discuss the likelihood that both faults were activated conjointly during the two previous seismic cycles.

Our findings show that a fine analysis of the damage pattern can provide relevant estimates of the coseismic surface deformation, and help to identity the signature of successive earthquakes. We bring new constraints on the seismic recurrence of the Bulnay fault and one of its major branches, which may contribute to better assess the seismic hazard in slowly deforming continental interiors.

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