Horizontal surface-slip distribution through several seismic cycles: The Eastern Bogd fault, Gobi-Altai, Mongolia
Résumé
The 1957, MW 8.1, Gobi-Altai earthquake, Southern Mongolia, produced a ~360-km-long surface rupture along the Eastern Bogd fault. Cumulative offsets of geomorphic features suggest that the Eastern Bogd fault might produce characteristic slip over the last seismic cycles.
Using orthophotographs derived from a dataset of historical aerial photographs acquired in 1958, we measured horizontal offsets along two thirds (~170 km) of the 1957 left-lateral strike-slip surface rupture. We propose a new empirical methodology to extract the average slip for each past earthquake that could be recognized along successive fault segments, to determine the slip distribution associated with successive past earthquakes.
Our results suggest that the horizontal slip distribution of the 1957 Gobi-Altai earthquake is fairly flat, with an average offset of 3.5 m ± 1.3 m. A combination of our lateral measurements with vertical displacements derived from the literature, allows us to re-assess the magnitude of the Gobi-Altai earthquake to be between MW 7.8 and MW 8.2, depending on the depth of the rupture, and related value of the shear modulus. When comparing this magnitude to magnitudes derived from seismic data, it suggests that the rupture may have extended deeper than the 15 km to 20 km usually considered for the seismogenic crust.
We observe that some fault segments are more likely than others to record seismic deformation through several seismic cycles, depending on the local rupture complexity and geomorphology. Additionally, our results allow us to model the horizontal slip function for the 1957 Gobi-Altai earthquake and for three previous paleoseismic events along 70% of the studied area. Along about 50% of the fault sections where we could recognize three past earthquakes, our results suggest that the slip per event was similar for each earthquake.