Résumé
The 1905, M > or =8, Tsetserleg-Bulnay earthquake pair, 14 days apart, produced a approximately 676 km-long surface rupture. It is one of the largest continental strike-slip earthquake sequence ever documented. Geometric complexity and co-seismic slip distribution along the 1905 rupture were examined in detail, based on high-resolution satellite imagery and fieldwork. The results indicate that the main Bulnay fault is formed of segments, on average 29-km-long, which are separated by geometric discontinuities, such as fault steps, fault bends, and fault branching points [Choi et al., accepted.]. Here, we investigate along-fault variations of cumulative horizontal offsets along the Bulnay fault, in order to understand rupture behaviors of large pre-1905 earthquakes. Offsets related to the penultimate event correspond to the 2nd smallest offset minus the 1905 co-seismic offset at multiple-offsets sites. Slip distribution of the penultimate earthquake is similar to those of the most recent event in 1905. Paleoseismological investigations at two sites (approximately 200 km apart) along the Bulnay fault show that the penultimate surface-faulting event at each site occurred respectively 2,480-3,270 cal. B.P and 2,300-3,250 cal. B.P, suggesting that it might actually be the same event [Schwartz et al., 2009; Rizza et al., 2015]. Hence, these results suggest that the Bulnay fault might at least follow a characteristic-slip pattern. At this stage time constraints on past earthquakes remain too weak to ensure unambiguously that the penultimate rupture was simultaneous along the entire 1905 rupture, as one could expect if the fault would behave following a characteristic earthquake pattern. In addition to the penultimate event, offsets related to earlier earthquakes were estimated by geomorphic markers at a number of sites, and some of these were targeted for surface exposure dating to estimate slip-rate of the Bulnay fault.