Expanding the Shallow Surface Velocity at the Apollo 17 Landing Site to Determine Thermal Moonquake Locations

Abstract:

Many previous studies have been performed to determine the shallow surface velocity model at the Apollo 17 landing site. The Lunar Seismic Profiling Experiment (LSPE) had both an active component with eight explosive packages (EPs) and a passive experiment collecting data at various time intervals. The active data were used to determine initial shallow surface velocity models with variations in subsequent studies. Our recent efforts to re-analyze the passive LSPE data found three different thermal moonquake event types occurring at preferred times within the lunar day. This abstract describes our efforts to co-locate these thermal moonquakes with physical surface features (primarily rocks) to determine whether thermal failure contributes to the cyclic breakdown of rocks over the course of a lunar day.

Our relocations of the EPs using previous velocity models did not produce locations within acceptable parameters, but these previous models all used single arrival time methods without including uncertainty estimations. We incorporate uncertainty into our model by propagating seismic pick errors through the model formulation. We formed the velocity model by plotting distance versus time. Through fitting straight lines to various segments, we found the velocity of the layer through the inverse slope and the depth via the intercept.

Using a combination of new arrival times and updated distances, we found a new shallow surface velocity model for the Apollo 17 landing site. We picked new arrival times for all eight EPs using several filters, including a bandpass filter, an average magnitude filter, a sliding window polarization filter and a short term-long term average (STA/LTA) ratio. Choosing independent arrivals using these filters also produced an uncertainty range for the travel times. This uncertainty range was used to find bounds on a velocity model in a statistical approach. The shallow surface velocity model was applied to a location grid to locate thermal moonquakes.