Azimuthal Anisotropy of the Megaregolith at the Apollo 14 Landing Site. Issue 5 (4th May 2020)

• Record Type: Journal Article
• Authors: Nishitsuji, Yohei; Ruigrok, Elmer; Draganov, Deyan
• Is Part Of: Journal of geophysical research. Volume 125:Issue 5(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-05-04
• Subjects: Moon -- deep moonquakes -- Apollo 14 -- exploration; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2019JE006126
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The characterization of the megaregolith on the Moon has been investigated in various ways including analysis of lunar meteorites, remote sensing of mineralogy and gravity, and deriving a seismic velocity profile. In this study, we propose a method for analyzing azimuthal anisotropy of the megaregolith. We call this method deep‐moonquake seismic interferometry applied to S ‐wave coda (DMSI‐S). DMSI‐S can turn the records of deep moonquakes into recordings from virtual active sources. The retrieved virtual sources coincide with the station positions, and thus, we obtain virtual zero‐offset (pulse‐echo) measurements. DMSI‐S is applied to seven clusters of deep moonquakes recorded at the Apollo 14 landing site, resulting in virtual zero‐offset measurements at the Apollo station 14. We use the S ‐wave recordings retrieved from DMSI‐S to analyze azimuthal anisotropy. We find weak anisotropy at the layer where the megaregolith is assumed to be present. We interpret our result to show that the megaregolith at this site is characterized by a layer (or layers) of impact material, following the Imbrium impact, with internal alignment of the crushed material. Plain Language Summary: Characterization of the Moon's subsurface helps understand how the Moon has evolved. Past studies of the Moon that made use of data from the National Aeronautics and Space Administration's Apollo missions found that the outermost rock layer is fragmented due to meteorite impacts. This outermost layer is called the megaregolith. Because this layer's depth is too great to sample directly, its characteristics, like alignment of fragments, existence, and alignment of fractures, are currently not known well. To describe some of the characteristics of this layer, we apply a seismic method that makes use of deep moonquakes recorded by an Apollo station to turn the station into co‐located seismic source and receiver. We apply this method to the Apollo 14 landing site. From this survey, we found that the uppermost layer below the Apollo 14 landing site seems to be composed of blocky material. Our application could be used for the other celestial bodies (e.g., Mars) if there are quakes in their subsurface. Key Points: We apply seismic interferometry to S ‐wave coda of deep moonquakes Weak azimuthal anisotropy is found between 0‐ and 1‐km depth at the Apollo 14 site We interpret a layer (or layers) of impact material, following the Imbrium impact, with internal alignment of the crushed material