I will first review of how to relate rock strength determined from lab experiments to geological and geophysical observations - emphasizing apparent paradoxes and successes. I will then introduce why understanding earthquake mechanisms in the oceanic lithosphere can provide new insights into the rheological properties of fault zones, and present an experimental study on fault stability in which analysis of the micromechanics of friction provides justification for the extrapolation of the lab data to natural conditionsl. more...