For most areas in the western Great Basin, focal mechanisms show a consistent pattern of primarily strike-slip motion for shallow events and oblique or normal slip for deeper events. However, orientation of the axis of least principal stress (T axis) is different for different areas: NW-SE for western Nevada and the Mono Lake region, and NE-SW for the Mammoth Lakes area. Along the remainder of the Sierra Nevada frontal fault zone, T axes show both orientations. In general the change in mechanism with depth is interpreted as being caused by increasing overburden pressure, resulting in rotation of the maximum compressive stress (P axis) from horizontal at depths less than about 6 km to vertical at depths greater than about 9 km. The absence of normal-slip events at depths greater than 10 km in one area (Mono-Excelsior-Luning zone) may be explained by a larger horizontal compressive stress than exists in areas that do have normal faulting at such depths. In some area conjugate right- and left-lateral shear on nearly vertical fractures may be associated with the formation of clusters of magma-filled dikes at shallow depths. Assuming strike-slip faulting to be characteristic of earthquakes with depth less than 6 km and normal faulting for events deeper than 10 km, extrapolation of measured shear stress in the upper few kilometers of the crust provides a rough estimate of the maximum and minimum principal stress as a function of depth. At about 3 km depth we find that S1 and S3 are both horizontal, with values of about 104 and 55 MPa, respectively. At depth of 20 km, S1 is vertical and equal to the overburden pressure, about 530 MPa: S1 is horizontal and about 260-300 MPa.