Stress field

About: Stress field is a research topic. Over the lifetime, 11926 publications have been published within this topic receiving 226417 citations.

Origin of regional, rooted low-angle normal faults: A mechanical model and its tectonic implications

Abstract: Rooted listric low-angle normal faults (< 20°) of regional extent have been recognized widely in the past few years in the North American Cordillera and elsewhere. The low-angle geometry of these crustal-scale normal faults conflicts with Anderson's [1942] classic theory of faulting. In that theory the orientations of principal stresses are assumed to be vertical and horizontal; the predicted dip angle of normal faults is about 60° rather than 20° or less. Recent geological and geophysical studies in the mid-Tertiary extensional terrane of southeastern California and western Arizona suggest that thick mylonitic gneisses in the lower plates of low-angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust. Directed ductile flow, possibly related to the gravitational spreading of thickened lower crust, may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust. Thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface. A simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust. This model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered. In particular, appropriate stress conditions for the formation of regional low-angle normal faults (< 20°) can be produced by the superposition of two stress fields: a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant. This superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust. These results may explain both puzzling parallel belts of extension and compression and the occurrence of major low-angle normal faults in some orogenic systems.

Statistical Size Effect in Quasi‐Brittle Structures: II. Nonlocal Theory

Abstract: The failure probability of structures must be calculated from the stress field that exists just before failure, rather than the initial elastic field. Accordingly, fracture‐mechanics stress solutions are utilized to obtain the failure probabilities. This leads to an amalgamated theory that combines the size effect due to fracture energy release with the effect of random variability of strength having Weibull distribution. For the singular stress field of linear elastic fracture mechanics, the failure‐probability integral diverges. Convergent solution, however, can be obtained with the nonlocal‐continuum concept. This leads to nonlocal statistical theory of size effect. According to this theory, the asymptotic size‐effect law for very small structure sizes agrees with the classical‐power law based on Weibull theory. For very large structures, the asymptotic size‐effect law coincides with that of linear elastic fracture mechanics of bodies with similar cracks, and the failure probability is dominated by the...

Observations of Earthquake Source Parameters at 2 km Depth in the Long Valley Caldera, Eastern California

Abstract: To investigate seismic source parameter scaling and seismic efficiency in the Long Valley caldera, California, we measured source parameters for 41 earthquakes ( M 0.5 to M 5) recorded at 2 km depth in the Long Valley Exploratory Well. Borehole recordings provide a wide frequency bandwidth, typically 1 to 200–300 Hz, and greatly reduce seismic noise and path effects compared to surface recordings. We calculated source parameters in both the time and frequency domains for P and S waves. At frequencies above the corner frequency, spectra decay faster than ω3, indicating that attenuation plays an important role in shaping the spectra (path averaged Q p = 100–400, Q s = 200–800). Source parameters are corrected for attenuation and radiation pattern. Both static stress drops and apparent stresses range from approximately 0.01 to 30 MPa. Although static stress drops do not vary with seismic moment for these data, our analyses are consistent with apparent stress increasing with increasing moment. To estimate tectonic driving stress and seismic efficiencies in the region, we combined source parameter measurements with knowledge of the stress field and a Coulomb failure criterion to infer a driving stress of 40–70 MPa. Subsequent seismic efficiencies are consistent with McGarr's (1999) hypothesis of a maximum seismic efficiency of 6%.