Rupture propagation inferred from damage patterns, slip distribution, and segmentation of the 1957 MW 8.1 Gobi-Altay earthquake rupture along the Bogd fault, Mongolia

A detailed analysis of the rupture patterns and slip changes along the surface rupture associated with the 1957 Gobi-Altay earthquake (M-W = 8.1) in Mongolia, which occurred along the Bogd left-lateral strike-slip fault, was carried out to better understand segmentation and propagation. The major surface ruptures show a simple linear pattern, whereas minor ruptures along the major ruptures, which are concentrated at rupture step overs and tip zones, show complex patterns. In the latter case, their patterns strongly resemble the geometric and kinematic characteristics of previously reported mesoscale fault damage patterns. The geomorphologic offsets show that left-lateral slip dominated with an average displacement of 3.5 to 4.0 m, but it decreased or transferred to dip-slip components mainly at rupture step-overs and tip zones. Abrupt changes of rupture patterns and slip distribution indicate that the coseismic surface rupture along the Bogd fault comprises three major segments from west to east, namely, the North-Ih, East-Ih, and North-Baga Bogd segments, and a highly damaged eastern tip zone. Based on the location of the epicenter, as well as the asymmetrically distributed damage structures and slips, we infer that the surface rupture propagated eastward unilaterally through several segments and reached the easternmost step-over, which acted as a tough barrier. The rupture finally terminated, producing a highly deformed tip damage zone developed to accommodate released stress. We argue that detailed analysis of damage patterns, slip distribution, and slip transfer can help us to better understand fault segmentation and rupture propagation.