Earthquakes are triggered by stress — tectonic plates grinding, faults accumulating strain, rock failing under pressure. The Moon also exerts stress on the crust — tidal forces that stretch and squeeze the Earth twice daily. The tidal stress is tiny compared to tectonic stress. But it is periodic, predictable, and externally controlled.
Tidal forcing can trigger earthquakes when faults are critically stressed — poised near failure, needing only a small push. The response to tidal stress therefore diagnoses the state of the fault. A fault that responds to tides is close to failure. A fault that doesn't is far from it. The Moon is not causing earthquakes. It is revealing which faults are ready to fail on their own.
This diagnostic principle has been debated since 1902. The evidence has been mixed because the tidal signal is small and the statistical tests are difficult. But the underlying logic is robust: if a system's response to a weak periodic perturbation changes over time, the change reveals something about the system's internal state, not about the perturbation.
The Earth is the instrument reading itself, with the Moon providing the test signal. No equipment is needed beyond the earthquake catalog and the tide tables. The measurement is passive — the Moon applies the perturbation whether anyone is listening or not. The question is whether the crust's response to that constant perturbation changes as stress accumulates.
The general principle: weak periodic forcing applied to a system near criticality produces a response that diagnoses proximity to failure. The perturbation is the probe. The response is the measurement. The closer the system is to its threshold, the more it responds to the same small push. Sensitivity to weak signals is itself a signal — of proximity to a transition.