Fault segmentation and slip distribution along the 1957 Gobi-Altay earthquake rupture, Mongolia

The rupture associated with the 1957 Gobi-Altay earthquake (Mw=8.1) occurred along the E-W Gobi-Altay range in SE Mongolia. This shock ruptured about 260km along the WNW-ESE or EW-trending Bogd left-lateral strike-slip fault. Some dip-slips also occurred along the secondary thrust and normal faults. However, detailed study on slip distribution and fault segmentation along this rupture has not been carried out. In this study, geomorphologic and geological investigations including measuring the amount of horizontal- and vertical-slip are carried out to interpret rupture patterns and characteristics of slip distribution. Generally, geological faults are segmented, which are made of a finite number of large-scale segments (e.g. Manighetti et al., 2007). In other to define rupture segmentation along the Bogd fault, we analyzed 1) abrupt changes of slip amount, 2) changing points between differential rupture patterns, and 3) locations of fault linkage zones. The rupture patterns are relatively similar within some sections, even various along the whole rupture. The changing positions of rupture patterns are consistent with locations of some linkage zones. The slip profile shows relatively small amounts of slip are recorded at fault linkage zones. In most sections, the profile shapes show asymmetric patterns: the slip slowly rises from the west and sharply drops to the east. Main decreases on amounts of slip particularly occur at some linkage zones, where large-scale graben and thrust-related foreberg structures are developed. In the eastern end of the rupture, various secondary faults (conjugate faults, mole tracks, and tension cracks) are widely developed with small displacements (less than 1m). This indicates that the rupture propagation was interrupted in the eastern linkage zone, and secondary deformations occurred to release the stress in the tip damage zone. Abrupt decreases on amount of slip, changing points between differential rupture patterns and locations of fault linkage zones are well matched at five major linkage zones. In conclusion, the 1957 rupture is composed of five major fault segments and a fault tip damage zone based on the detailed rupture analyses. This result indicates that change of fault geometry and amount of slip are very important factors to define characteristics of earthquake ruptures.