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SUMMARY The mechanical heterogeneity of Earth's lithosphere leads to significant amplification of stresses across spatial scales ranging from mineral grains to tectonic plates. These stress amplifications play a key role in mechanical and chemical processes within the rock that affect bulk rock strength. Identifying the most effective causes of stress amplification is critical for understanding processes such as strain localization and fluid transport at scales ranging from microshear zones to tectonic plate boundaries. However, studies quantifying and predicting stress heterogeneities and amplifications are limited. We used numerical modelling of two-phase isotropic viscous systems to explore the factors influencing and controlling stress amplification and the potential magnitude of stress amplification in viscous regimes. We found the most geologically relevant amplification factors to be weak-phase spacing, rheological contrast and loading type. Our results indicate that stress amplification can reach a factor of ∼9 under specific conditions, but most of our experiments suggest amplifications at or below a factor of 2. Pressure differences across the model domains generally do not exceed ∼55 MPa, but some are as high as ∼110 MPa. The stress and pressure amplifications resulting from our analyses are large enough to drive a variety of geologically important processes such as failure and strain localization, as well as transient permeability and fluid migration. 

Abstract Mature faults with large cumulative slip often separate rocks with dissimilar elastic properties and show asymmetric damage distribution. Elastic contrast across such bimaterial faults can significantly modify various aspects of earthquake rupture dynamics, including normal stress variations, rupture propagation direction, distribution of ground motions, and evolution of off‐fault damage. Thus, analyzing elastic contrasts of bimaterial faults is important for understanding earthquake physics and related hazard potential. The effect of elastic contrast between isotropic materials on rupture dynamics is relatively well studied. However, most fault rocks are elastically anisotropic, and little is known about how the anisotropy affects rupture dynamics. We examine microstructures of the Sandhill Corner shear zone, which separates quartzofeldspathic rock and micaceous schist with wider and narrower damage zones, respectively. This shear zone is part of the Norumbega fault system, a Paleozoic, large‐displacement, seismogenic, strike‐slip fault system exhumed from middle crustal depths. We calculate elastic properties and seismic wave speeds of elastically anisotropic rocks from each unit having different proportions of mica grains aligned sub‐parallel to the fault. Our findings show that the horizontally polarized shear wave propagating parallel to the bimaterial fault (with fault‐normal particle motion) is the slowest owing to the fault‐normal compliance and therefore may be important in determining the elastic contrast that affects rupture dynamics in anisotropic media. Following results from subshear rupture propagation models in isotropic media, our results are consistent with ruptures preferentially propagated in the slip direction of the schist, which has the slower horizontal shear wave and larger fault‐normal compliance.