Orbital modulation of frequency content in Apollo deep moonquake records

The mobilization of near-surface regolith beneath Apollo seismic stations represents a significant source of noise in lunar seismic records, particularly for low-amplitude deep moonquake signals. These shallow layers strongly affect seismic wave propagation, producing detectable ground motion beneath the instruments and modulating recorded amplitudes and frequency content. In addition to seismic sources, lunar records are influenced by surface thermal expansion driven by solar illumination, tidal stresses associated with the Moon’s orbital motion, meteoroid impacts, and local surface adjustments. Deep moonquake clusters are commonly enhanced through waveform stacking to improve the signal-to-noise ratio. In this study, we analyze deep moonquake clusters previously identified in relation to lunar orbital periods, focusing on variations in frequency content across the three seismometer components. Using the Fast Fourier Transform (FFT), spectrogram analysis, and the Hilbert–Huang transform, we examine clustered seismograms recorded at Apollo stations 12, 14, 15, and 16. Our results show that the y-component consistently exhibits higher high-frequency content than the x-component. The orientation of the y-component, aligned with the lunar east–west axis and parallel to the direction of orbital motion, suggests that frequency dispersion may be sensitive to changes in the Moon’s position within the celestial plane. In contrast, the observed high-frequency variability is likely associated with surface thermal effects linked to solar exposure. These findings indicate that sensors more susceptible to orbital variability are also more sensitive to surface and regolith-related motion, emphasizing the combined influence of orbital dynamics, thermal forcing, and local geological conditions on lunar seismic records.