Shallow moonquake mechanisms illuminated by rheologic characteristics of basaltic gouges

The projected evolutionary history of the Moon and observed occurrence of moonquakes suggest that brittle faulting is present in the shallow lunar crust. Chang'e 5 samples of lunar regolith show a mineral composition almost identical to basaltic bedrock. We measure the friction-stability characteristics of dry synthetic gouges representative of basaltic faults assumed to be present in the lunar crust. Frictional strengths are ~0.7 and exhibit overall velocity-strengthening response but transition to velocity-weakening at intermediate temperatures (100-300℃) and stresses (10-100 MPa). Bounding temperature profiles representative of the lunar crust suggest that moonquakes are feasible throughout the shallowest ~40-60 km of the crust. However, observations of unusually high stress drops (up to 210 MPa) are inconsistent with the measured frictional strengths – suggesting that high degrees of healing on preexisting faults or intact crust are present – to augment stress accumulation mechanisms due to tidal forcing or differential thermal cooling.