Determination of stress condition and friction coefficient from orientation distribution of outcrop-scale faults

  Friction coefficients along faults control the brittle strength of the earth's upper crust, although it is difficult to estimate them especially of ancient geological faults. This study proposes to estimate the friction coefficient of faults with stress condition which activated them by the following procedure. Stress tensor inversion using fault-slip data can calculate principal stress axes and a stress ratio, which allows us to draw a normalized Mohr’s circle. Assuming that faulting occurs when the ratio of shear stress to normal stress on the fault (the slip tendency) exceeds the friction coefficient, a linear boundary of distribution of points corresponding to the observed population of faults should be found on the Mohr diagram. The slope of the boundary (friction envelope) provides the friction coefficient. Since this method has a difficulty in the graphical recognition of the linear boundary, this study automated it by considering the fluctuations of fluid pressure and differential stress. The fluctuations yield a density distribution of points representing faults on the Mohr diagram according to the friction coefficient. Then we can find the optimal value of friction coefficient so as to explain the density distribution.

  The method was applied to some examples of natural outcrop-scale faults. The first example is from the Pleistocene Kazusa Group, central Japan, which filled a forearc basin of the Sagami Trough. Stress inversion analysis showed WNW-ENE trending tensional stress with a low stress ratio. The friction coefficient was calculated to be about 0.7, which is typical value for sandstone.

  Another example is from an underplated tectonic mélange in the Cretaceous to Paleogene Shimanto accretionary complex in southwest Japan along the Nankai Trough. The stress condition was determined to be an axial compression perpendicular to the foliation of shale matrix. The friction coefficient ranges from 0.1 to 0.3, which is extremely low indicating a weak plate boundary under the accretionary wedge.