Stress field

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

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

Abstract: We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line) Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area The minimum principal compression stress (σ3), with its near north–south trend, is dominant throughout the entire region However, the σ 3 axes around the shear zone are rotated normal to the zone This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data Conversely, the stress field of the normal fault is dominant in the Beppu–Shimabara area, which is located in the central part of the island This result and the direction of σ3 are consistent with the formation of a graben structure in the area

On tidal triggering of earthquakes at Campi Flegrei, Italy

Abstract: SUMMARY The caldera at Campi Flegrei underwent an inflationary episode during 1982–84 that produced a maximum uplift of 1.6 m at Pozzuoli, Italy. The seismicity at Pozzuoli increased enormously during the time of the uplift, but was delayed by several months. Ground deformation during inflation has been previously well modelled with a finite element model of a pressurized magma chamber in an elastic medium that takes into account the effects of increasing pressure and temperature with depth on elasticity. We used the output from this model to estimate the temporal change in the stress field that presumably controls the seismicity during inflation. The result is that the solid-earth tidal stress should modulate heavily the seismogenic inflationary stress, which in turn should result in some tidally triggered earthquakes. This expectation is based on the assumptions that: (a) the inflationary model is valid; (b) tidal and inflationary stresses can be superimposed; (c) the inflation is smooth on the time-scale of periodic tidal stress variations; and, most importantly (d) earthquakes occur when a critical level of stress has been reached. We checked the Pozzuoli catalogue for evidence of tidal triggering with the Schuster test and found none. The Schuster test is sensitive enough to easily detect a diurnal variation of reported seismicity caused by day-to-night changes in noise levels. The lack of tidal triggering suggests that one (or more) of the above assumptions is wrong. After evaluating each assumption, we conclude that the most likely explanation is that the failure threshold for seismicity is time dependent at Pozzuoli. In other words, earthquakes do not necessarily occur when the stress exceeds the yield strength of a fault for a short time only.

Elastic moduli of a cracked body

Abstract: Statistical solutions for the internal stress and displacement fields are constructed for a body in an external applied stress field, and containing a prescribed distribution of penny shaped cracks. From the fields, a stress-strain relationship is calculated for the body, and effective elastic moduli are derived. The fields are constructed as expansions in the parameter $$\overline {Na^3 }$$ wich is assumed small (N is the crack density, anda is the crack radius). Angular integrals over the distribution function are performed for two special cases that represent a bedded crack distribution, and an isotropic crack distribution. A criterion for the validity of the use of these quasistatic results in dynamical calculations is presented.

Assessing the Permeability Characteristics of Fractured Rock

Abstract: The geometric and hydraulic characteristics of fractures and fractured rock masses are reviewed to assess the current state and future direction of fracture hydrology research. Laboratory data suggest that the parallel-plate analogy for flow through a single fracture is valid. Fracture-flux is a function of the cube of the fracture aperture. Flow through fractures is a function of normal stress, shear stress, and fracture surface characteristics such as roughness. Flow through fractured rock masses is determined by fracture orientation, spacing, fracture interconnection, and the stress field. These factors must be considered in assessing the directional permeabilities of fractured rocks. Consideration of these factors and the structural characteristics of fractured rock leads to the formulation of a conceptual framework for flow in fractured rock masses that is a form of coupled discrete-fractured porous media model. In this conceptual framework, the shear zones and fracture zones are described as discrete hydrogeologic features; individual fractures (joints) are defined as discrete features near zones of interest, such as underground excavations; and the fractured rock mass in general is described as a form of equivalent fractured porous medium. It is proposed that the properties of the equivalent continuous porous medium must be developed from discretemore » fracture properties, reflecting the dependence of fracture permeability on the stress tensor, fracture geometry, distribution of fracture apertures, and degree of fracture interconnection.« less