Abstract
AbstractThe 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.
Highlights
We call this method deep‐moonquake seismic interferometry applied to S‐wave coda (DMSI‐S)
After randomly selecting four out of the seven clusters recorded at Apollo station 14, DMSI‐S is repeatedly carried out
We proposed a method that gives a new perspective of seismic azimuthal anisotropy on the Moon
Summary
Driven analyses have aided to broaden our knowledge of numerous aspects of the Moon's interior: wave propagation in the near surface (e.g., Dal Moro, 2015; Sens‐Schönfelder & Larose, 2010), regional‐scale Poisson's ratio (Zhao et al, 2008), mechanism of the moonquakes (e.g., Frohlich & Nakamura, 2009; Kawamura et al, 2017; Toksöz et al, 1977), and velocity profiles (e.g., Nakamura, 1983; Toksöz, 1974; Toksöz et al, 1974) including investigations which focused on the crustal thickness (e.g., Chenet et al, 2006) These seismic analyses, which used moonquakes and ambient noise recorded during the National Aeronautics and Space Administration (NASA)'s Apollo missions from July 1969 to September 1977, were directly or indirectly based on the identifications of the moonquakes. Event detection is still ongoing as can be found from a recent example by Dimech et al (2017), who preliminary succeeded in identifying an additional ~50,000 moonquakes (near‐surface local events) using a pattern‐recognition algorithm at the Apollo 17 site.
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