Variation of the regional stress tensor at the western Great Basin Boundary from the inversion of earthquake focal mechanisms

Abstract

The orientation of the principal axes of the regional stress tensor in and near the western Great Basin boundary is calculated from the inversion of earthquake focal mechanism data. Data were taken from three regions along the western Great Basin‐Sierra Nevada boundary: Mammoth Lakes, Round Valley, and Chalfant Valley, and at two regions within the Great Basin interior: Mono‐Walker Lake and west central Nevada. The depth range is similar for all data, earthquakes are essentially restricted to the uppermost 16 km of the crust. The stress tensor changes from the Great Basin boundary to the Basin interior, for example, the extension direction changes by about 50° from roughly E‐W at the boundary in the west to roughly NW‐SE inside of the basin. In the boundary region the mean T axis inferred from focal mechanisms rotates clockwise by 27° from N64°E in the Mammoth Lakes area to N91°E in the Chalfant Valley, about 50 km southeast of Mammoth Lakes. However, the components of the best fitting stress tensors in the two regions vary only by 12° so that a common stress tensor with a greatest principal stress azimuth of N10°W and a least principal stress azimuth of N80°E, both components nearly horizontal (favoring strike slip), describes the seismic deformation of the boundary region quite well. The stress tensors for the Mono region and west central Nevada have similar greatest and least principal stress azimuths (northeasterly and northwesterly, respectively), however, the plunges of the greatest principal stress are different: nearly horizontal for the Mono area and about 45° for central Nevada, reflecting the strong contribution of normal faulting in the central Nevada earthquake data. The resulting Φ values, which describe the relative sizes of the three stress components, vary from 0.5 to 0.65, which indicates that the magnitudes of the three principal stresses are of similar size.