Artificial Impacts on the Moon: Modeling 3D seismic propagation effects with AxiSEM3D

During the Apollo missions, the flight engineers deliberately crashed used hardware into the Moon to provide large signals for the seismic experiments. We use these impacts to constrain seismic models of the lunar subsurface. The Moon has been bombarded by meteoroid impacts throughout its history, leaving its surface fractured and dominated by craters. Its rocks also have low attenuation properties. As a result, seismic events on the Moon have long duration and are highly scattered. Impacts have an emergent waveform with a smooth increase in the seismic envelope. We use AxiSEM3D to simulate seismic wave propagation through the global Moon. Simulations using a smoothly varying background model, which include scattering heterogeneities to 50~km depth and surface and Moho topography, reproduced some of the observations, especially further 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 high-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 by scattered phases of P-to-SV conversions which start to dominate the seismogram.