[S19-2] Apollo seismic data re-processing and future lunar seismology project

[S19-2-03] Scattering attenuation profile of the Moon : implications for shallow moonquakes and the structure of the megaregolith

^○Kevin Gillet, Ludovic Margerin, Marie Calvet, Marc Monnereau (IRAP - University of Toulouse, Toulouse, France)

We report measurements of the attenuation of short period seismic waves in the Moon based on the quantitative analysis of envelope records of lunar quakes. Our dataset consists of waveforms corresponding to 62 events, including artificial and natural impacts, shallow and deep moonquakes, recorded by Apollo missions 12 to 16. To quantify attenuation and distinguish between elastic and inelastic mechanisms, we measure the time of arrival of the maximum of energy tmax and the coda quality factor Qc. We employ diffusion theory in spherical geometry to model the propagation of seismic energy in depth-dependent scattering and absorbing media. To minimize the misfit between predicted and observed tmax for deep moonquakes and impacts, we employ a genetic algorithm and explore a large number of attenuation profiles quantified by the scattering quality factor Qsc or equivalently the wave diffusivity D, and the absorption quality factor Qi. The profiles that best fit the data display very strong scattering attenuation (Qsc ≤ 10 ) or equivalently very low wave diffusivity (D ≈ 2 km2/s) in the first 10 km of the Moon. These values correspond to the most heterogeneous regions on Earth, namely volcanic areas. Below this surficial layer, the diffusivity rises very slowly up to a depth of approximately 80 km where Qsc and D exhibit an abrupt increase of about one order of magnitude. Below 100 km depth, Qsc increases rapidly up to approximately 2000 at a depth of about 150 km, similar to Earth's mantle values. By contrast, Qi ≈ 2400 on the Moon, which is about one order of magnitude larger than on Earth. Our results suggest the existence of an approximately 100-km thick megaregolith, much larger than what was previously thought. Using our best attenuation model, we invert for the depth of shallow moonquakes based on the observed variation of tmax with epicentral distance. On average, they are found to originate from a depth of about 50 ± 20 km.