Rupture Branching and Propagation at the Eastern End of the 2021 Mw 7.4 Madoi Earthquake, North Tibet Plateau

The propagation of the 2021 Mw7.4 Madoi earthquake rupture from the central Jiangcuo fault (JCF) onto the eastern portion exhibits the most complex geometry with a series of conjugate faults, bends, and stepovers. At the east ~50 km of the 2021 epicenter, the surface rupture along the Jiangcuo eastern branch (JCEB) deviating ~12° anticlockwise from the general strike provides a valuable chance for understanding the particularly complex surface ruptures propagation and the branching behavior of the poorly known JCEB. Using sub-metric orthophotos collected by UAV with a ground resolution of 6 cm, complemented by multiple field investigations, we implemented the surface rupture mapping and coseismic slip distribution of the JCEB in detail associated with this earthquake sequence. Our mapping illuminated the sporadic breaks of the tectonic region in the dune area immediately near the branching point and eastward propagated linear rupture trace. The measurements of the high-resolution coseismic slip along the JCEB show that the slip distribution reveals an approximate dogtail shape to the eastern termination with the maximum left-lateral strike-slip offset of 2.9 m. These data might support the perspective that the rupture propagated with a supershear velocity toward the east. Combined with the accrued displacements along the JCEB, these results indicate that the poorly known divergent branch could accumulate pre-2021 surface breaks as an immature fault and bifurcated in the Madoi quake due to the matched regional stress field. We found linear surface breaks along the NW-strike geologic faults indicating triggered coseismic slip on conjugate faults. In the meantime, the intersections with conjugate faults mark discontinuities in rupture geometry and surface slip on the main fault, suggesting strong fault interaction in the eastern tip zone of the Madoi rupture.