C Jones

Temporal variations in seismicity during quasi-static and dynamic rock failure

Abstract A comprehensive model is presented which can explain temporal fluctuations in seismic b-values in the period leading to mechanical failure in terms of the underlying physical processes of time-varying applied stress and stress corrosion-enhanced crack growth under conditions of constant strain rate. The form of the b-value anomaly in the period leading to failure depends on the form of the stress/time relationship. For the case where dynamic failure occurs at peak stress after a period of strain hardening, the model predicts a single cusp-like b-value anomaly, reaching a critically low value of 0.5 at failure. For the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening, the model predicts two minima in the b-value. separated by a temporary maximum or inflection point. These fluctuations in the b-value are consistent with reported “intermediate-term” and “short-term” earthquake precursors separated by a period of seismic quiescence. For the case of quasi-static cataclastic flow, the b-value mirrors the stress and never falls to the critical value because there is no critical rupture. New results from a series of controlled laboratory experiments are presented in which microseismic event rates and b-value were monitored contemporaneously with stress/time data for all variants of the stress/time relationship. Recent field observations of temporal changes in seismicity rates and b-value preceding major earthquakes are also reported. Both data sets exhibit b-value anomalies which are consistent with the model predictions.

INFLUENCE OF FRACTAL FLAW DISTRIBUTIONS ON ROCK DEFORMATION IN THE BRITTLE FIELD

Abstract The geometrical distribution of flaws plays a crucial role in the physical behaviour of geological materials under stress. Flaws are present in the earth on all scales, from microcracks to plate-rupturing faults. They may be distributed on one characteristic length scale (e.g. joints, ‘characteristic’ earthquakes), or more commonly exhibit scale-invariance over a specified range of sizes. Scale-invariance implies that the discrete length distribution in a finite range is a power law of negative exponent D, where 1 ≤ D < 3. Fault systems where motion is concentrated on a dominant fault (e.g. San Andreas) have D ≈ 1, but more diffuse fault systems have D near 2. D is one of the fractal dimensions of the fracture system. The length distribution of faults or microcracks may be inferred from the slope b of the log-linear frequency—magnitude distribution of earthquakes, or laboratory-scale acoustic emissions, since it can be shown that D = 3b/c. The scaling factor c depends on the relative time constants of the seismic event and the recording instrument, and is usually equal to 3/2. b is found experimentally to be negatively correlated with the stress intensity on the dominant flaw, which depends in turn on the applied stress and the flaw length. Thus a fracture mechanics model of rock failure which includes a range of flaw sizes can be tested by seismic monitoring. We describe a fracture mechanics model of rock failure for a variety of styles of deformation, ranging from elastic failure to quasi-static cataclastic flow, and predict the time-dependence of D and the seismic b-value at different times up to and including failure. Critical coalescence of microcracks during dynamic failure (e.g. earthquake foreshocks) occurs when D = 1 (b = 0.5); random processes (e.g. cataclastic flow, background seismicity) are associated with D = 2 (b = 1); positive feedback in the concentration of stress on the dominant flaw (e.g. during strain softening and shear localisation) occurs when D < 2 (b < 1); negative feedback in stress concentration (e.g. during the early stages of dilatancy), and where a highly diffuse fracture system is produced, occurs at low stress intensities and is associated with D > 2 (b > 1). It has long been a goal of structural geologists to measure stress on rocks, since most geometrical signatures of deformation are strain-related. We show that stress is not usually as significant in rock fracture as stress intensity, and furthermore that the geometric signature of the length distribution of microcracks is well-correlated with the stress intensity.

Acoustic emission and fluid permeability measurements on thermally cracked rocks

Abstract Thermal cracking experiments have been conducted to investigate the role of anisotropic thermal expansion of the constituent minerals of a basalt, a microgabbro and a microgranite in changing the physical properties of the rocks. Samples of an Icelandic basalt, a microgabbro from Sweden, and a fine-grained microgranite from Ailsa Craig, Scotland, have been selected for these experiments. Samples were heated to a range of peak temperatures up to 800°C at ambient pressure. Thermal cracking was monitored through acoustic emission and elastic wave velocity measurements. Fluid permeability of the basalt was measured under hydrostatic conditions at elevated confining pressure using the steady state flow technique.

Ocular findings in HIV-1 positive and HIV-1 negative participants in a rural population-based cohort in Uganda

Purpose: To report the ocular findings in HIV-1 seropositive individuals and HIV-negative controls in a population-based cohort in rural Uganda. Methods: Participants were examined by an ophthalmologist, who was unaware of their HIV-status. Results: 238 participants were examined, 43 were HIV-1 prevalent cases (infected prior to 1990); 62 were HIV-1 incident cases and 133 were HIV-negative controls. Eleven of the HIV-positive participants had AIDS, and a further 8 had a CD4 count of less than 200 cells/mm3 Bilateral low vision was recorded in 9 participants and cataracts were the main cause of bilateral low vision. The mean intra-ocular pressure (IOP) was 12.6 mm Hg, and no association was found between IOP and CD4 lymphocyte count. Although significantly more HIV positives had some sign of intraocular inflammation compared to the negatives (p = 0.02) there was no significant linear trend in the HIV positives with decreasing CD4 count. Overall, 11 (26%) prevalent, 6 (10%) incident cases and 8 (6%) HIV-negatives controls had some evidence of intraocular inflammation, however, only one person had reduced visual acuity attributable to these lesions. Punctate epithelial erosions and keratic precipitates in the anterior chamber were seen significantly more frequently in HIV-positives compared to the HIV-negative controls. Cotton wool spots were not seen. Conclusions: A substantial proportion of HIV-infected persons had ocular findings which may have been attributable to their infection, however in only one case did this result in reduced visual acuity. Although the ocular complications of AIDS seem to comprise a large extra element in the work load of tertiary care hospitals dealing with eye problems, on a population basis such cases are infrequent.

Experimental And Theoretical Study of Sedimentary Rock Properties

Introduction Rocks can be considered in general as microinhomogeneous, macroanisotropic composites. The components of the composite are mineral grains, pores, and cracks of various shape and orientation. Pores and cracks can be filled in by different types of fluid (brines, oil, gas, and melt). If wavelengths are much greater than the size of inhomogeneity (the long wavelength approximation), the effective physical properties (elastic and transport) of such a medium can be calculated, these relate to the physical characteristics averaged over the volume of the medium.