Mechanics of fault junctions

A sharp bend in a fault must be part of a triple junction, and slip on the three fault segments at the junction must have a vector sum of zero, in order to avoid an unphysical 1/r stress singularity. Accompanying the slip, a volume change occurs at the junction; either a void opens or intense local deformation is required to avoid material overlap. The energy absorbed due to the volume change is proportional to the slip increment times the total past slip accumulated at the junction. At a new junction the energy absorbed is a small fraction of the energy released by slip on the fault system, but after a number of earthquakes (proportional to shear modulus over confining stress) the junction becomes a strong barrier to further slip. Although slip occurs more easily on old rupture surfaces than on fresh fractures, the growing barrier strength of junctions requires that there be some fresh fracture in earthquakes. Perhaps a small fraction of the surface that slips in any earthquake is fresh fracture, which could provide the instability needed to explain earthquakes. A numerical model in two-dimensional static plane strain shows, even without accounting for energy absorbed at the junction, that a bend in a fault acts as a barrier if slip is impeded on the associated fault spur. The stress concentration at the bend will tend to induce slip on the spur. Slip occurring on the spur unstably (with a drop in the coefficient of friction) can induce increased slip on the main fault segments with no change in the coefficient of friction. A fault junction provides a natural realization of barrier and asperity models without appealing to arbitrary variations of fault strength. The location of the fresh fracture that occurs after a junction becomes a strong barrier remains an unanswered question. It is likely to be initiated near the stress concentration at the old junction. A model simulating the effect of fresh fracture near old junctions might explain earthquakes without appealing to an unstable friction law anywhere.

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