Shallow Moonquake Mechanisms Illuminated by Rheologic Characteristics of Basaltic Gouges

Abstract

The projected evolutionary history of the Moon and observed occurrence of moonquakes suggest that brittle faulting is present in the shallow lunar crust. The main component of the lunar crust, plagioclase, shows velocity-strengthening behavior in the range of crustal temperatures. Chang'e 5 samples of lunar regolith show a mineral composition almost identical to basaltic bedrock. We measured 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 an overall velocity-strengthening response but transition to velocity-weakening at intermediate temperatures (∼200–300°C) and stresses (∼25–100 MPa). Bounding temperature profiles representative of the lunar crust suggest that moonquakes are feasible in the lunar crust. The rheological heterogeneity of mineral fragments in basalt is a potential cause of unstable sliding on faults with the related steady-state stress drop close to the minimum of the estimated dynamic stress drop. This suggests that some events with small stress drops are associated with the instability of mature basalt faults. However, observations of shallow moonquakes with high stress drop but merely moderate magnitude suggest that high degrees of healing on immature faults, small seismic nucleation lengths, or the failure of intact crust are present. We emphasize that moonquakes may arise from stress transfer and accumulation due to processes such as cooling contraction.