Effects of Rock Properties on the Hayward Fault
In this calculation we try to determine how the heterogeneous rock properties may be affecting the Hayward fault.
For this calculation, we import the rock properties from the geologic models into the finite element mesh, as shown in the overview. Then we apply a right-lateral shear distortion to the side and bottom boundaries of the model. There is no fault slip in this calculation.
But there is a problem. The geologic models include both the rock properties and the fault geometry. In our first attempt to perform this calculation, the results included effects from both rock properties and fault geometry, and it was impossible to separate the two.
So we adopted the following procedure. We "morph" the entire geologic model to change the fault into a vertical plane, while preserving as closely as possible the relative sizes and positions of all the rock units. Then we perform the finite element calculation on this "morphed" model. This gives us pictures that show very clearly the effects of rock properties, with no contribution from fault geometry. The pictures below are the results of applying this technique.
DISCLAIMER: This information is excerpted from work in progress. It has not been reviewed for accuracy or completeness, and should not be relied upon for hazard assessment or any other purpose.
Fig. 1. Normal stress on the fault plane. Central part of the fault plane, viewed from the west, colored according to calculated normal stress. North is left. Blue denotes areas of compression (clamping) and red denotes areas of tension (unclamping).
Because this is a "morphed" model, all stresses are due to the heterogeneous rock properties.
White squares lie along the fault at intervals of 10 km, with the leftmost (northernmost) square located at Point Pinole. Purple dots are relocated seismicity, and white polygons are proposed locked patches. The smallest cells are 625 meters on a side.
Some of the red and blue areas are associated with a boundary between two rock units on one side of the fault.
Fig. 2. Displacement perpendicular to the fault plane. Central part of the fault plane, viewed from the west, colored according to calculated displacement perpendicular to the fault plane. North is left. Blue denotes westward deflection (toward the viewer), and red denotes eastward deflection (away from the viewer).
Because this is a "morphed" model, all deflections are due to the heterogeneous rock properties.
White squares lie along the fault at intervals of 10 km, with the leftmost (northernmost) square located at Point Pinole. Purple dots are relocated seismicity, and white polygons are proposed locked patches. The smallest cells are 625 meters on a side.
Fig. 3. Shear stress on the fault plane. Central part of the fault plane, viewed from the west, colored according to calculated shear stress. North is left. Blue denotes areas of low shear stress and red denotes areas of high shear stress.
Because this is a "morphed" model, all stresses are due to the heterogeneous rock properties.
White squares lie along the fault at intervals of 10 km, with the leftmost (northernmost) square located at Point Pinole. Purple dots are relocated seismicity, and white polygons are proposed locked patches. The smallest cells are 625 meters on a side.
The shear stress pattern is dominated by the increasing rigidity of the rock with depth.
