Apollo active seismic experiments reveal 2D mechanical heterogeneity of the shallow lunar subsurface

Abstract The mechanical properties of the shallow lunar subsurface remain poorly constrained, despite their importance for understanding lunar evolution and for designing future surface infrastructure. Here, we reanalyze Apollo Active Seismic Experiment (ASE) data to image the shallow subsurface beneath the Apollo 16 landing site and compare it with Apollo 14. Because conventional surface-wave analysis is challenging for the Apollo ASE records owing to their limited signal-to-noise ratio and complex waveforms, we use a continuous-wavelet-transform multichannel surface-wave method (CWT-MASW) to robustly extract surface-wave dispersion and estimate two-dimensional S-wave velocity. The resulting model reveals pronounced lateral heterogeneity within the uppermost ~ 10 m. At a depth of 5 m, S-wave velocity varies from approximately 70 to 110 m/s over horizontal distances of only several tens of meters. This lateral variation is greater than the difference between the mean shallow structures at the Apollo 14 and Apollo 16 sites, demonstrating that substantial mechanical heterogeneity can occur even within a single landing area. Importantly, the tens-of-meters scale of the heterogeneity resolved here is comparable to the dimensions of individual future lunar infrastructure sites. These findings provide new constraints on the mechanical evolution of the lunar near surface and highlight the need for site-specific subsurface characterization in planning future lunar bases and associated infrastructure.