Abstract: Biot poroelasticity has been widely influential in earth science. Here that and related poromechanical formulations are applied to phenomena of fault inception, earthquake nucleation, and shear rupture dynamics in fluid-infiltrated crustal materials. These phenomena include the mechanics of intense shear localization in rapidly deforming granular materials, pore fluid effects on earthquake nucleation, thermal pressurization of fluids as a primary fault weakening process during large seismic slip, and additional pore pressure alterations induced by propagating mode II slip between poroelastically dissimilar media.