Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Richard S. Miller
Moonquakes, Seismic waves--Speed--Models, Apollo 17 (Spacecraft), Lunar Seismic Profiling Experiment (LSPE)
The origins of thermal moonquakes reveal insight into the evolution of the lunar surface through cyclic thermal stresses. While thermal moonquakes were observed during the passive Apollo 17 Lunar Seismic Profiling Experiment (LSPE), they have never conclusively been located. Using a shallow surface velocity model, thermal moonquakes can be located by minimizing the residual between the observed and theoretical travel times. The purpose of this study is to determine the source mechanism of thermal moonquakes to understand cyclic thermal stresses impacting lunar boulder breakdown. This study develops a location algorithm and explores the affects of various shallow surface velocity models. The published models differ in velocities and number of layers but do not include uncertainty values. Therefore, a new shallow surface velocity model was calculated including uncertainty in arrival time identification from using six different filters. The resulting mean velocity model consists of two layers with P-wave velocities of 198 ± 3 m/s for the upper layer and 897 ± 100 m/s for the lower layer, with a depth transition at 183 ± 15 m. In addition to the discrete shallow surface velocity model, an approach was formulated to derive a continuous velocity model using an iterative chain approach. Using a minimization of the travel time residual, events were located using the different velocity models to determine which velocity model was best to locate thermal moonquakes. The active source LSPE data was used as a calibration source to determine the accuracy of the location algorithm in a 95% confidence region. The confidence regions for the continuous velocity model were the smallest, but the new shallow surface velocity produced the most accurate locations. However, the confidence area is too large to identify individual surface features responsible for thermal moonquakes. A terrestrial analog dataset from the Cinder Lake Crater Field demonstrates additional experiments would provide insight into the seismic properties of the lunar shallow surface. While the origins of thermal moonquakes remain ambiguous, the range of velocities, produced from a robust methodology, represents a more detailed understanding of the upper kilometer of the lunar surface.
Phillips, Deanna, "Shallow subsurface seismic velocity and event location at the Apollo 17 landing site" (2023). Dissertations. 269.