Fault growth and acoustic emissions in confined granite

The failure process in a brittle granite was studied by using acoustic emission techniques to obtain three dimensional locations of the microfracturing events. During a creep experiment the nucleation of faulting coincided with the onset of tertiary creep, but the development of the fault could not be followed because the failure occurred catastrophically. A technique has been developed that enables the failure process to be stabilized by controlling the axial stress to maintain a constant acoustic emission rate. As a result the post-failure stress-strain curve has been followed quasi-statically, extending to hours the fault growth process that normally would occur violently in a fraction of a second. The results from the rate-controlled experiments show that the fault plane nucleated at a point on the sample surface after the stress-strain curve reached its peak. Before nucleation, the microcrack growth was distributed throughout the sample. The fault plane then grew outward from the nucleation site and was accompanied by a gradual drop in stress. Acoustic emission locations showed that the fault propagated as a fracture front (process zone) with dimensions of 1 to 3 cm. As the fracture front passed by a given fixed point on the fault plane, the subsequent acoustic emission would drop. When growth was allowed to progress until the fault bisected the sample, the stress dropped to the frictional strength. These observations are in accord with the behavior predicted by Rudnicki and Rice's bifurcation analysis but conflict with experiments used to infer that shear localization would occur in brittle rock while the material is still hardening

The Effect of Microcrack Dilatancy on the Permeability of Westerly Granite

Permeability and volumetric strain were measured under constant confining pressure and pore pressure as a function of increasing and decreasing differential stress. Permeability was found to increase appreciably during dilatancy. Our results have also shown that permeability and dilatant volume changes are not unique functions of differential stress and that permeability changes with differential stress are not uniquely dependent upon dilatant volume changes. Most significant, however, is that if dilatancy-fluid diffusion occurs in situ, our results indicate that microcrack dilatancy is not a reasonable physical mechanismto account for such a phenomenon

The Effect of Cyclic Differential Stress on Dilatancy in Westerly Granite Under Uniaxial and Triaxial Conditions

Laboratory experiments have been performed on samples of Westerly granite in which the differential stress was repeatedly cycled to 85% of the intact sample strength. The experiments have shown that under uniaxial conditions the onset of dilatancy is reduced to fairly low stress; however, under triaxial conditions dilatancy can be an apparently stable process, and the onset of dilatancy is not affected by the repeated cycling. Thus the implication for midcrustal earthquakes is that the onset of dilatancy repeatedly occurs at relatively high stress levels. For example, our results indicate that at typical focal depths of 2.5 and 10 km (corresponding to effective hydrostatic pressures of about 500 and 2000 bars) the onset of dilatancy repeatedly occurs at 1.8 and 3.0 kbar of differential compressive stress, respectively

Integrating seasonal climate prediction and agricultural models for insights into agricultural practice

Interest in integrating crop simulation models with dynamic seasonal climate forecast models is expanding in response to a perceived opportunity to add value to seasonal climate forecasts for agriculture. Integrated modelling may help to address some obstacles to effective agricultural use of climate information. First, modelling can address the mismatch between farmers' needs and available operational forecasts. Probabilistic crop yield forecasts are directly relevant to farmers' livelihood decisions and, at a different scale, to early warning and market applications. Second, credible ex ante evidence of livelihood benefits, using integrated climate–crop–economic modelling in a value-of-information framework, may assist in the challenge of obtaining institutional, financial and political support; and inform targeting for greatest benefit. Third, integrated modelling can reduce the risk and learning time associated with adaptation and adoption, and related uncertainty on the part of advisors and advocates. It can provide insights to advisors, and enhance site-specific interpretation of recommendations when driven by spatial data. Model-based ‘discussion support systems’ contribute to learning and farmer–researcher dialogue. Integrated climate–crop modelling may play a genuine, but limited role in efforts to support climate risk management in agriculture, but only if they are used appropriately, with understanding of their capabilities and limitations, and with cautious evaluation of model predictions and of the insights that arises from model-based decision analysis