Systematic change of focal mechanism with depth in the Western Great Basin

Abstract

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 areas 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; S3 is horizontal and about 260–300 MPa.

Evaluation of the seismicity of the southern Great Basin and its relationship to the tectonic framework of the region

“…An alternative interpretation suggested in the past-that the rate of increase in the vertical principal stress with depth is greater than the rate of increase of the greatest horizontal principal stress Zoback, 1980a, andVetter andRyall, 1983)-does not fit our observations. To satisfy this alternative, the focal mechanism types should display a depth dependence, such that strike-slip events would be restricted to shallow 66 depths and normal fault events would occur at greater depth.…”

“…The Vetter and Ryall (1983) model requires a moderate amount of horizontal compressional tectonic stress in the direction parallel to the intermediate principal stress, such that av < OH at depths less than about 10-15 km. Although this model permits both normal and strike-slip faulting, each mode is confined to certain sections of the crust as noted above.…”

Intraplate earthquakes and the state of stress in oceanic lithosphere

“…[ Vetter and Ryall, 1983]. Using independent information (from in situ stress measurements) on the increase in shear stress with depth and assuming that an increasing overburden pressure accounts for the change in mechanism, Vetter and Ryall were able to constrain the magnitudes of the principal stresses of the regional stress field.…”

State of stress and modern deformation of the Northern Basin and Range Province

“…Focal mechanisms for m = 2.0 to 5.0 events along the Walker lane belt-Sierra Nevada boundary zone indicate a combination of normal dip-slip, oblique-slip, and nearly pure strike-slip deformation [Vetter and Ryall, 1983]. There is a general pattern of primarily strike-slip deformation at depths less than about 9 km and predominantly normal or obliquenormal slip at greater depths [Vetter and Ryall, 1983].…”