| A network of seismometers emplaced on the Moon during the Apollo missions recorded many types of seismic signals during the lifetime of the experiment (1969--1977). The research presented in this thesis focuses on understanding a subset of those signals: deep moonquakes---natural lunar seismic events that appear to be related to tides, and occur at great depths (800--1000 km) in the Moon. We investigate when, where, and why deep moonquakes occur by: (1) establishing an extended catalog of deep moonquakes using a cross-correlation technique to identify events missed in earlier visual examination of printed seismograms, (2) investigating the tidal periodicities present in moonquake occurrence times through spectral analysis, (3) estimating the uncertainties in deep moonquake source locations using revised seismic phase arrival picks from stacks which include our new events, (4) developing an orbital model which describes the position of the Earth with respect to the Moon, and examining these parameters at the times of moonquakes, and (5) using our orbital model to compute the gravitational tidal potential within the Moon (due to the Earth) and determining how the Moon deforms, and what stresses are generated within the Moon, in response to this potential.; We observe tidal (∼27-day, 206-day, and 6-year) periods in moonquake occurrence times, suggesting a relationship between deep event occurrence and tidal forcing. Investigation of the relationship between the position of the Earth and the times of moonquakes from 51 source regions reveals that while individual moonquake clusters sometimes produce events at specific values of Earth position, relatively few groups of nearby clusters respond similarly. This indicates that local structure is possibly important to failure at individual clusters. To investigate, we compute tidal shear and normal stresses on a range of failure plane orientations at the same 51 locations. While linear combinations of stresses can fit moonquake occurrence at many clusters quite well, for some clusters, the fit is not heavily dependent on plane orientation. This suggests that the process responsible for generating moonquakes has two components: one which is dependent on plane orientation, and another which is independent of orientation. |
