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The Moon's Crust and Upper Mantle Discontinuities Revealed by Seismic Interferometry Methods Applied to Apollo Seismic Data.

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Title: The Moon's Crust and Upper Mantle Discontinuities Revealed by Seismic Interferometry Methods Applied to Apollo Seismic Data.
Authors: Torrent Duch, Arnau1 (AUTHOR) Arnau.TORRENT-DUCH3@isae-supaero.fr, Garcia, Raphael F.1,2 (AUTHOR), Drilleau, Mélanie1,3 (AUTHOR)
Source: Journal of Geophysical Research. Planets. Dec2025, Vol. 130 Issue 12, p1-18. 18p.
Subject Terms: Moon, Seismic waves, Crust of the earth, Regolith, Structural geology, Seismic reflection method, Seismic surveys, Scattering (Physics)
Company/Entity: Apollo program (U.S.)
Abstract: Seismic determination of the Moon's crustal thickness is crucial for understanding its formation, composition, and evolution. Additionally, crustal thicknesses below seismic stations serve as anchoring points for 3D crustal thickness maps derived from gravity and topography field variations. However, strong seismic scattering inside the Moon can prevent seismic waves reflected on, or converted at, crust and upper mantle discontinuities from reaching the surface without interacting with scatterers. This study estimates Auto‐Correlation Functions (ACF) of all channels of Apollo seismometers during the coda of high‐signal‐to‐noise seismic events. As expected for a strongly scattering medium, Apollo stations S12 and S15 do not detect late arrivals in the ACF indicative of waves reflected on internal discontinuities. However, station S16 and, to a lesser extent, station S14 show stable late arrivals in the ACF. Arrivals at 14.2 and 27.1 s and at 23.9 and 50.6 s in the ACF of vertical and horizontal mid‐period channels of station S16 are interpreted as P $P$ and S $S$ waves reflected at mid‐crustal and crust‐mantle interfaces. A seismic crust and upper mantle model beneath station S16 fits the arrival times of P $P$ and S $S$ waves with realistic seismic velocities and VP/VS ${V}_{P}/{V}_{S}$ ratios ranging from 1.65 to 1.88. The crustal thickness is non‐uniquely estimated at 53 km below station S16. Our results are also compatible with significant variations in the properties of the interfaces, or in strength of seismic scattering between Apollo stations. Plain Language Summary: Understanding the total volume of the Moon's crust is essential to reconstruct its formation and geological evolution. Although measurements of the Moon's topography and gravity field, such as those from the GRAIL mission, have revealed the relative lateral variations in crustal thickness, they do not provide direct estimates of the total crustal volume. Seismic methods, which could help resolve this, are complicated by the fact that the Moon's interior strongly scatters seismic waves, making traditional analysis difficult. In this study, we turn this challenge into an opportunity by using the scattered wavefield to search for seismic reflections from the base of the lunar crust. Among all the Apollo seismic stations, station S16, located in the lunar highlands, shows the clearest evidence of such reflections. These signals suggest a crustal thickness of about 53 km beneath that site. Our findings also indicate that the seismic properties of the Moon's crust vary significantly across different regions. This approach demonstrates how scattered seismic waves can be harnessed to improve our understanding of the deep crustal structure of the Moon. Key Points: Auto‐correlation of seismic coda records and their stability are computed for all Apollo passive seismic sensorsOnly station S16 is presenting stable late arrivals along vertical and horizontal components being compatible with a seismic interpretationA non‐unique crustal seismic model is estimated below station S16 by fitting our two‐way travel time estimates in the upper and lower crust [ABSTRACT FROM AUTHOR]
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Abstract:Seismic determination of the Moon's crustal thickness is crucial for understanding its formation, composition, and evolution. Additionally, crustal thicknesses below seismic stations serve as anchoring points for 3D crustal thickness maps derived from gravity and topography field variations. However, strong seismic scattering inside the Moon can prevent seismic waves reflected on, or converted at, crust and upper mantle discontinuities from reaching the surface without interacting with scatterers. This study estimates Auto‐Correlation Functions (ACF) of all channels of Apollo seismometers during the coda of high‐signal‐to‐noise seismic events. As expected for a strongly scattering medium, Apollo stations S12 and S15 do not detect late arrivals in the ACF indicative of waves reflected on internal discontinuities. However, station S16 and, to a lesser extent, station S14 show stable late arrivals in the ACF. Arrivals at 14.2 and 27.1 s and at 23.9 and 50.6 s in the ACF of vertical and horizontal mid‐period channels of station S16 are interpreted as P $P$ and S $S$ waves reflected at mid‐crustal and crust‐mantle interfaces. A seismic crust and upper mantle model beneath station S16 fits the arrival times of P $P$ and S $S$ waves with realistic seismic velocities and VP/VS ${V}_{P}/{V}_{S}$ ratios ranging from 1.65 to 1.88. The crustal thickness is non‐uniquely estimated at 53 km below station S16. Our results are also compatible with significant variations in the properties of the interfaces, or in strength of seismic scattering between Apollo stations. Plain Language Summary: Understanding the total volume of the Moon's crust is essential to reconstruct its formation and geological evolution. Although measurements of the Moon's topography and gravity field, such as those from the GRAIL mission, have revealed the relative lateral variations in crustal thickness, they do not provide direct estimates of the total crustal volume. Seismic methods, which could help resolve this, are complicated by the fact that the Moon's interior strongly scatters seismic waves, making traditional analysis difficult. In this study, we turn this challenge into an opportunity by using the scattered wavefield to search for seismic reflections from the base of the lunar crust. Among all the Apollo seismic stations, station S16, located in the lunar highlands, shows the clearest evidence of such reflections. These signals suggest a crustal thickness of about 53 km beneath that site. Our findings also indicate that the seismic properties of the Moon's crust vary significantly across different regions. This approach demonstrates how scattered seismic waves can be harnessed to improve our understanding of the deep crustal structure of the Moon. Key Points: Auto‐correlation of seismic coda records and their stability are computed for all Apollo passive seismic sensorsOnly station S16 is presenting stable late arrivals along vertical and horizontal components being compatible with a seismic interpretationA non‐unique crustal seismic model is estimated below station S16 by fitting our two‐way travel time estimates in the upper and lower crust [ABSTRACT FROM AUTHOR]
ISSN:21699097
DOI:10.1029/2025JE009090