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 e arthquake focal mechanisms because it requires alignment of principal stres s axes with vertical and horizontal orientations. A survey of earthquake fo cal mechanisms in northern California shows that their principal axes are a lso closely aligned with the vertical and the horizontal in the upper few k ilometers of Earth's crust. Thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well. The f ocal mechanism alignment can also be characterized by the relative magnitud e 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 mo ment tensor element" m(s), which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principa l 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 d epths, there is a gradual decrease, which suggests decreasing horizontal sh ear traction toward the base of the seismogenic zone. We interpret this ten dency 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 me chanics of lithospheric deformation.