Theassumption that the strength of the Earth’s upper crust is bounded by itsfrictional limit (i.e., the frictional equilibrium) is plausibly supported byin situ stress characterizations already made worldwide. In this context, the classicCoulomb frictional failure theory, employing the laboratory-derived frictionalcoefficient values, has enabled the estimation of in situ stress and faultcriticality, albeit with some degree of uncertainty. Despite doubts, this talkexplores several independent lines of field evidence that support the applicabilityof the classic Coulomb theory, and presents an innovative methodology to invertthe in situ stress, via data analytics. Both aspects corroborate the interconnectednessbetween in situ stress, crustal strength and fault stability.