[P-PS09] 月の科学と探査

[PPS09-P11] アポロ地震データの再解析から推定される月浅部の弱い構造不均質性と非弾性

*熊谷 博之¹ (1.名古屋大学大学院環境学研究科)

I reanalyzed seismic waveforms generated by artificial impacts on the Moon during the Apollo missions. Long-lasting seismic waves recorded in these seismograms have been interpreted as waves generated by strong heterogeneity like that at Earth’s active volcanoes. I performed envelope waveform simulations using a diffusion model and Monte Carlo method to reproduce the observed waveforms in a 1-Hz frequency band. I searched for the scattering mean free path (l₀) and the quality factor of medium attenuation (Q_i) for S waves by fitting waveforms to observed and calculated envelopes. I used the relative amplitudes corrected by momentums of the individual artificial impacts. The results indicated that l₀ is 10⁴–10⁶ m and Q_i is ~10⁶ down to a depth of 500 km, which reasonably explained the waveform shape and amplitude relationship of the seismograms observed at various distances over 1000 km. These estimates of both l₀ and Q_i are larger than previous estimates (l₀ = 10² –10⁴ m and Q_i of ~10³ at depths shallow than 100 km), indicating weak heterogeneity comparable to that of Earth’s lithosphere and intrinsic attenuation smaller than previously thought. My results point to the existence of a thin surface layer with a thickness of 1 km characterized by a low velocity and relatively strong heterogeneity, but they provide no supporting evidence for highly fractured layers called megaregolith. My estimated small intrinsic attenuation is consistent with a cool but wet Moon model with a water content of the lunar mantle like that of Earth’s asthenosphere. This study provided a new perspective on the internal structures of the Moon by showing that they were characterized by weak heterogeneity and low intrinsic attenuation.