CONSTRAINING FAULT ZONE MECHANICS USING MULTIPHYSICS MODELING OF PSEUDOTACHYLYTE INJECTION VEIN FORMATION

Pseudotachylyte, a fault rock formed through coseismic frictional melting, provides an important record of coseismic mechanics. In particular, injection veins formed at a high angle to the fault surface have been used to estimate rupture directivity, velocity, pulse length, stress and strength drop, as well as slip weakening distance and wall rock stiffness. These studies, however, have generally treated injection vein formation as a purely elastic process and have assumed that processes of melt generation, transport, and solidification have little influence on the final vein geometry. We have developed a finite-element model of the injection vein forming process using COMSOL Multiphysics simulation software that couples an elastic model describing wall rock deformation with a fluid model simulating melt flow and a thermal model that controls the temperature-dependent viscosity of the melt. This model provides insight into the relationship between these processes as well as an indication of the conditions under which a given process might have a significant impact on the final vein geometry. Under conditions of relatively low melt viscosity, vein formation is elastic-limited, producing an elliptic opening profile extending the length of the host crack, similar to the results of previous studies. High melt viscosity, a result of low initial melt or ambient temperature, produces a flow-limited geometry with maximum length independent of pre-existing crack length and a more steeply tapering opening profile. By integrating these results with field measurement and X-ray microtomographic imaging of injection vein geometry we hope to provide new constraints on the mechanics of injection vein formation and the earthquake source zone.