Depth-dependent earthquake focal mechanism orientation: Evidence for a weak zone in the lower crust

The traction free boundary condition across the Earth's surface provides an opportunity for studying the relationship between stress orientation and earthquake focal mechanisms because it requires alignment of principal stress axes with vertical and horizontal orientations. A survey of earthquake focal mechanisms in northern California shows that their principal axes are also closely aligned with the vertical and the horizontal in the upper few kilometers of Earth's crust. Thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well. The focal mechanism alignment can also be characterized by the relative magnitude of the off-diagonal elements, M-xz and M-yz, of the seismic moment tensor. We find significant and systematic depth variations in the "horizontal moment tensor element" m(s), which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principal stress and focal mechanism axes. Values of m(s) near Earth's surface are small but increase with depth to a maximum between 5 and 8 km. At greater depths, there is a gradual decrease, which suggests decreasing horizontal shear traction toward the base of the seismogenic zone. We interpret this tendency of axes to become oriented near the base of the seismogenic zone (and its expression in m(s)) as the signature of a weak zone in the lower crust. If correct, this observation would have important implications for the mechanics of lithospheric deformation.