Effects of Lunar Near-Surface Geology on Moonquakes Ground Motion Amplification

Lunar seismology has focused on exploring the Moon's seismic activity and its internal structure. However, less attention has been given to site specific, near-surface geology-related amplification of seismic waves on the Moon. Here, we quantified ground-motion amplifications of seismic waves due to local site conditions at the Apollo landing sites. We analyzed Apollo Passive Seismic Experiments seismometer recordings of 30 artificial and natural impacts and estimated the mid-period (0.1-3 Hz) ground-motion amplification at each site. We applied techniques commonly used in engineering seismology to understand the local site effects, such as the Horizontal over Vertical Spectral Ratio (HVSR) and the Reference Site Method (RSM) on lunar seismic data. The HVSR and the RSM show convergent spectral ratios specific for each landing site. To correlate these empirical results estimated from lunar artificial and meteoroid impact events with the local site geology, we simulated ground motion amplification using 2D finite difference modeling of lunar geological models and estimated the best geological setting capable of producing a similar frequency response to those observed at each Apollo landing site. Correlation results from the empirical methods and the simulated models show similarities ranging between 79% and 92%, a correlation more than 90% between the RSM and the HVSR results, and similarity exceeding 96% between RSM estimations using different reference sites. These results demonstrate the prominence of a local site geology dependent amplification for frequencies between 0.1 and 3 Hz.