The Long-Lived and Recent Seismicity at the Lunar Orientale Basin: Evidence From Morphology and Formation Ages of Boulder Avalanches, Tectonics, and Seismic Ground Motion

A detailed geologic mapping of tectonic features and co-seismic boulder avalanches at the 930-km-diameter lunar Orientale basin is performed. We traced 6869 boulder falls at 141 sites along the basin rings, impact crater walls, and volcanic features. The boulder fields also occur on the graben walls and wrinkle ridges. The boulder trail length and width, and slopes of trail and source region of fallen boulders provide insights into formation processes and transport dynamics. Tens of sites have anomalously larger number of boulder falls and trail density. The cumulative size-frequency distributions of superimposed impact craters present on the boulder trails provided their model formation ages varying between present and 7.5 Ma with at least eight formation episodes, suggesting the timings of seismic activities along the Cordillera and Outer Rook Rings. Many boulder falls are devoid of superimposed impact craters suggesting their recent origins, including some possibly triggered by the 1972 (MW 2.7) shallow moonquake that occurred near the Cordillera Ring. The formation rates of shallow moonquakes and impact craters along with the seismic ground motion simulations suggest that shallow moonquakes are more dominant sources of ground shaking for triggering the boulder falls in the last 30 million years. Therefore, it is inferred that the Orientale basin has been seismically active for long time and it may be seismically hazardous locally for the future landed missions.

Citation

Mohanty, R., Kumar, P. S., Raghukanth S. T. G., & Lakshmi, K. J. P. (2020).
The long-lived and recent seismicity
at the lunar Orientale basin: Evidence from morphology and formation ages of boulder avalanches, tectonics, and seismic ground motion. Journal of Geophysical Research: Planets, 125, e2020JE006553. https://doi. org/10.1029/2020JE006553