Abstract Title: Recent seismic shaking and formation of boulde avalanches in Orientale basin, Moon

Abstract Submitted to: PLANETARY SCIENCES

Abstract Text:

An analysis of lunar tectonic features and co-existing mass wasting features provides significant insights into on-going and past seismic activities on the Moon. To understand the current and recent seismicity in Orientale basin, we have undertaken a detailed study of boulder fall occurrences along the basin rings. Interestingly, a MW 2.7 shallow moonquake occurred near the Cordillera ring of this basin on December 9, 1972. To further study the spatial and temporal distribution of recent and past seismic events through mapping of the co-seismic boulder falls, we mapped >6000 boulder falls at 141 sites along the Cordillera ring, Inner and Outer Rook rings, inner depression, impact crater interior walls and volcanic features. Our measurements of boulder trail length and width, and slopes of trail and source region of fallen boulders provide important information about the run-out dynamics and the triggering sources for ground shaking. In addition, measurements of diameters and aspect ratios of the fallen boulders give significant information about the nature of regolith erosion during the boulder transport. Our observations show that tens of sites have anomalously larger number of boulder falls and trail density. We have dated those trails using measurements of the cumulative size-frequency distributions of superimposed impact craters on the boulder falls. The boulder falls have a range of model formation ages ranging from 0.5 Ma to 3.5 Ma. Many boulders are devoid of superimposed impact craters suggesting their recent origins, including some triggered by the 1972 event. Our observations suggest that Orientale basin remains seismically active in the last few million years, and the on-going seismic activity may be detected by Chandrayaan-2 seismometer and other future lunar landing missions.