Artificial Impacts on the Moon: Modeling 3D Seismic Propagation Effects with AxiSEM3D

Abstract During the Apollo missions, used spacecraft were deliberately crashed into the Moon to provide large signals for the seismic experiments. We use AxiSEM3D and a variety of 1D and 3D models to model these impacts and propagate seismic waves across the Moon in 3D. Simulations using a smoothly varying background model, which include scattering heterogeneities to 50 km depth, reproduced some of the observations, especially farther from the source. These simulations have an emergent onset and an increase and decay in the seismic envelope. However, close to the source (<15°), the simulations include a large-amplitude surface wave, which is inconsistent with the observations. Traditionally, artificial and meteoroid impacts on the Moon have been considered to have P and S phases. However, a true S phase may be absent. Previous authors have treated artificial or meteoroid impacts on airless bodies as explosions. A shear wave traveling directly from the source is not expected from an explosion. Instead, the small change of the slope of the seismic envelope may be explained by either a PS phase or scattered phases of P-to-SV conversions that start to dominate the seismogram. The locations of the artificial impacts are well known in comparison with other types of events. Due to their shallow paths, they are also ideal for constraining the near-surface layers. Understanding how the seismic waves propagate within the highly complex near-surface layers will also lead to a better understanding of how the waves propagate beneath them, as well as better results from future missions.