Propagation velocity of joints: A debate over stable vs. unstable growth of cracks in the earth

One of the hotly debated questions in the geoscience literature as of 2006 concerns the velocity of joint (i.e., a mesoscopic crack in rock) propagation in the crust of the Earth. Earthquake rupture loads rocks at stress rates of many MPa/sec, whereas plate tectonic deformation yields long-term loading rates as much as ten orders magnitude slower. This large range in loading rates leaves open the possibility of joint propagation in rocks anywhere from subcritical (i.e., stable growth) to critical and post-critical (i.e., unstable growth). Several facts are relevant during adjudication of the propagation-velocity debate: 1.) joints are inherently planar but may propagate as gradually curving single surfaces on scales > 10 m, 2.) hackle fringes are relatively rare, and thus, the exception to the planar surface (i.e., ≪1% of all joint surface area in the crust of the earth consists of hackle fringe), 3.) fixed-grips loading is the normal configuration for propagation of joints in the brittle crust of the Earth if there is a fatigue limit, 4.) plume morphology on planar surfaces of joints is consistent with a velocity = f(K₁) relationship, 5.) fracture under fixed-grips loading is inherently stable for several reasons, mainly, 5A.) developing elastic properties of a bulk rock with fractures favors stable propagation, 5B.) fluid-drive mechanisms in the earth favor stable propagation. The fractography of rock can demonstrate facts 1, 2, and 4 directly, and facts 3, 5A, and 5B indirectly. In summary, these facts point to a large role for subcritical propagation (i.e., stable growth) in the crust of the Earth whereas critical and post-critical propagation (i.e., unstable growth) in natural rock is exceptionally rare.