A mechanism of apparent inter-sonic propagation of shear fractures

Inter-sonic (faster than the shear wave velocity) propagation of zones of shear over faults are observed both in the Earth’s crust and in specially designed laboratory experiments. This is usually interpreted as propagation of shear fractures caused by postulated special fracture mechanisms. This interpretation is however at variance with experimental facts that shear fractures in solids do not propagate in their own planes, kinking instead. Extensive (and fast) in-plane shear fracture propagation seems to only be possible over pre-existing planes considerably weaker than the surrounding material. A limiting case of fracture propagation over such a weak plane is the propagation of a sliding zone resisted by friction only. Another limiting case is shearing over a narrow elastic layer (shear Winkler layer) without rupture. The shear Winkler layer models both traditional elastic connections (positive stiffness) and rotation of non-spherical particles of the fault gouge (negative stiffness), e.g. [1, 2].

In both cases propagation of sliding/shear zone also involve longitudinal deformation in the surrounding material. Using a configuration different from [3, 4] we demonstrate that the presence of the longitudinal deformation makes the sliding/shear zone propagate with p-wave velocity. Propagation of such zones create seismic signals with power spectra resembling those observed in earthquakes.

Acknowledgement.   AVD and EP acknowledge support from the Australian Research Council through project DP190103260.