Distribution of Hypocenters in a Deep Moonquake Nest - A Reevaluation with an Expanded Event List

Recent computer-based reexaminations by the UTIG and UCSD groups of the decades-old Apollo seismic data from the Moon increased the number of positively identified deep moonquakes more than a factor of five over those visually identified earlier. We have reevaluated the relative hypocenter distribution in the A1 source region (nest) using waveform cross-correlation analysis, expecting that inclusion of the machine-correlated, newly identified events may significantly alter our earlier results based solely on visually identified events. The new events, however, did not produce any significant expansion of the physical size of the nest, confirming our earlier finding that the hypocenters in the A1 nest are distributed in a nearly horizontal, planner region of no more than about 1 km in radius. This suggests that the observed low waveform correlation values for some event pairs are due not to their physical separation but to some other cause such as different orientation of source mechanisms. Events of inverted polarity are found to be slightly offset to the east from those of normal polarity, perhaps suggesting that events of opposite polarities are initiated from opposite ends of a fault.