The linear Gutenberg-Richter relationship is well-established. In any region of the Earth,the logarithm of the number of earthquakes,greater than any magnitude,is proportional to magnitude. This means that the underlying physics is non-linear and not purely elastic. This nonlinear physics has not been resolved. Here we suggest that a new understanding of fluid-rock deformation provides the physics underlying Gutenberg-Richter: where the fluid-saturated microcracks in almost all in situ rocks are so closely-spaced that they verge on failure and fracture,and hence are critical-systems which impose fundamentally-new properties on conventional sub-critical geophysics. The observation of linear Gutenberg-Richter relationship in moonquakes suggests that residual fluids exist at depth in the Moon. The linear Gutenberg-Richter relationship is well-established. In any region of the Earth, the logarithm of the number of earthquakes, greater than any magnitude, is proportional to magnitude. This means that the underlying physics is non-linear and not purely elastic Here the nonlinear physics has not been resolved. Here we suggest that a new understanding of fluid-rock deformation provides the physics underlying Gutenberg-Richter: where the fluid-saturated microcracks in almost all in situ rocks are so closely-spaced that they verge on failure and fracture, and hence are critical-systems which impose fundamentally-new properties on conventional sub-critical geophysics. The observation of linear Gutenberg-Richter relationship in moonquakes suggests that residual fluids exist at depth in the Moon.