Classifying Thermal Moonquakes Recorded in Apollo 17 Lunar Seismic Data Using Cross-Correlation and Diurnal Temperature Variations

We refine the newly developed seismicity catalog of the Apollo 17 Lunar Seismic Profiling Experiment, containing over 12,000 thermal events, using quality control algorithms and classify these events by possible source using cross-correlation, azimuth calculation, and periodicity analysis. We first sort these events into two types—“repeating” or “isolated”—based on whether or not they share similar waveforms with at least one other event. Repeating events are characterized by minimal time between signal onset and maximum, a prevalence during early sunrise, a period consistent with the diurnal cycle, and a preferential direction to the east, coincident with the location of the Lunar Module. Isolated events, on the other hand, are distributed throughout the lunar day, originate from a wide variety of directions, and are characterized by a more gradual increase in amplitude. Many repeating events are likely caused by thermal expansion of the Lunar Module or of enclosed volatiles, while most isolated events may be caused by thermal fracturing of rock or sliding of regolith along crater slopes. In particular, we pinpoint several boulders at the Apollo 17 site whose thermally induced fractures may be the source of 45 moonquakes. Future lunar missions, such as deployments of seismic instruments or gravitational wave detectors, should be aware of thermally induced, repeating seismic signals caused by nearby boulders and structures.