Spatial heterogeneity of tectonic stress and friction in the crust

Abstract

The complex geometry of faults, seismicity, and diversity of earthquake mechanisms suggest that the stress and strength in Earth's crust are spatially heterogeneous. We investigated the degree of heterogeneity using the following two end-member models. In one end-member model, we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies. In this model, the variability of earthquake mechanisms means that friction during faulting must vary for each event. We computed friction μ from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure. The values of μ vary over a large range from 0 to 1.5. In the other extreme model we assumed optimally oriented slip and a constant μ = 0.6, as is suggested by Byerlee's law, for all the earthquakes, and determined the local stress orientation for each earthquake. The orientation of the stress changes drastically from one earthquake to another, and the assumption of uniform stress field commonly used in stress inversion is not warranted. An important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive. The actual situation in the crust is most likely to be intermediate between these two end-member models. From the existing data alone, we cannot determine the degree of heterogeneity uniquely, but both μ and the local stress field near earthquake faults are likely to vary substantially, and studies on earthquake rupture dynamics must take these heterogeneities into consideration.

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