Résumé
Significant advancements in geomorphic mapping of earthquake surface ruptures provide along-fault variations in cumulative offsets formed by multiple successive earthquakes at the time scale of tens of thousands years. The cumulative offsets play a key role in examining earthquake cycle model in a given active fault system. In general, however, there are a few limitations in interpreting rupture behaviors over multiple past earthquakes using cumulative offsets. Firstly, it is not easy to detect cumulative offsets depending greatly on the rate of preservation of geomorphic markers. Also in many cases, segment geometry is ambiguous due to absence of well-recorded recent earthquake, particularly in intraplate regions. The most difficult thing, particularly for pre-historic earthquakes, is probably to assure the rupture extents associated with individual past earthquakes because dating results of paleo-earthquakes has much lower resolution in comparison with earthquake rupture duration. On July, 1905, two major (M = nearly equal 8) earthquakes, 14 days apart, occurred on the left-lateral Bulnay Fault System in the northern part of Mongolia. The large continental strike-slip earthquake sequence produced surface ruptures over nearly equal 670 km along the main Bulnay fault, with an average horizontal offset of nearly equal 8 m, as well as the subsidiary, but kilometric in size, branch faults. Here we present cumulative offsets, acquired by high-resolution satellite imagery reconstruction and field observations, along the main Bulnay fault and their correlation with the offset variations along the surface ruptures during the 1905 earthquake. We note that segment geometry is well defined by geometry and co-seismic offsets along the surface rupture during the most recent event. Our results indicate that surface rupture associated with the penultimate event was similar in rupture length, offset, and distribution of secondary deformations to those of the most recent event. Also we discuss offsets associated with earlier great earthquakes, and their implications for characteristic patterns of the earthquakes as well as impacts of inherited segment geometry on earthquake recurrent behaviors.