Geologic Inheritance and Earthquake Rupture Processes: The 1905 M8 Tsetserleg-Bulnay Strike-Slip Earthquake Sequence, Mongolia

In 1905, 14days apart, two M similar to 8 continental strike-slip earthquakes, the Tsetserleg and Bulnay earthquakes, occurred on the Bulnay fault system, in Mongolia. Together, they ruptured four individual faults, with a total length of similar to 676km. Using submetric optical satellite images Pleiades with ground resolution of 0.5m, complemented by field observation, we mapped in detail the entire surface rupture associated with this earthquake sequence. Surface rupture along the main Bulnay fault is similar to 388km in length, striking nearly E-W. The rupture is formed by a series of fault segments that are 29km long on average, separated by geometric discontinuities. Although there is a difference of about 2m in the average slip between the western and eastern parts of the Bulnay rupture, along-fault slip variations are overall limited, resulting in a smooth slip distribution, except for local slip deficit at segment boundaries. We show that damage, including short branches and secondary faulting, associated with the rupture propagation, occurred significantly more often along the western part of the Bulnay rupture, while the eastern part of the rupture appears more localized and thus possibly structurally simpler. Eventually, the difference of slip between the western and eastern parts of the rupture is attributed to this difference of rupture localization, associated at first order with a lateral change in the local geology. Damage associated to rupture branching appears to be located asymmetrically along the extensional side of the strike-slip rupture and shows a strong dependence on structural geologic inheritance.