HR: 0800h
AN: T41F-1275    [Abstracts]
TI: The New Geophysics in a Crack Critical-System
AU: Gao, Y
EM: gaoyuan@seis.ac.cn
AF: Institute of Earthquake Science, China Earthquake Administration, P.O. Box 166, 63 Fuxing Road, Beijing, 100036 China
AU: * Crampin, S
EM: scrampin@ed.ac.uk
AF: School of GeoSciences, University of Edinburgh, Grant Inst. West Mains Road, Edinburgh, EH9 3JW United Kingdom
AB: Seismic shear-wave splitting indicates stress-aligned fluid-saturated cracks, in almost all in situ rocks, that are so closely spaced they are critical systems. Evolution of fluid-saturated cracks in changing conditions can be calculated by anisotropic poro-elasticity (APE): the deformation mechanism is fluid-movement along pressure-gradients between neighboring grain-boundary cracks, flat pores, and pore throats at different orientations to the stress field. Difficult to confirm directly, because of the inaccessibility of deep-rocks, there are at least 20 different phenomena that are approximately matched by APE-modeling. Using earthquake swarms as shear-wave source, systematic changes in shear-wave splitting are seen, with hindsight, before about a dozen earthquakes, and the time and magnitude of a M5 event in SW Iceland was successfully stress-forecast. In the most direct confirmation, modeling fluid-injections into hydrocarbon reservoirs, the response of high-pressure and low-pressure CO2-injections were matched almost exactly by inserting the actual injection pressures into APE. This sensitivity is confirmed at the first borehole Stress-Monitoring-Site (SMS) in a transform zone in Iceland. With spectacular 0.02ms sensitivity, 10% variations in shear-wave splitting are observed, in shear-wave propagation over 300m at 500m-depth, correlating with small-scale seismicity at 70km with a total energy equivalent to less than one M$\sim$3.5 event. This high resolution is achieved by using the highly-repeatable Downhole Orbital Vibrator (DOV) as a source of shear waves. The physical implications are that microcracks are so closely spaced that they verge on fracture criticality and failure. Hence rock is so weak to shear-stress that stress accumulation occurs over enormous volumes: 100s of thousands to billions of cubic km before the largest earthquakes. This means that the approach to criticality and failure occurs very widely (witness the large distances that changes in shear-wave splitting are observed). The eventual failure typically occurs along pre-existing faults where there is some form of stress-relaxation before the impending earthquake. The implications are that SMSs can routinely monitor stress-accumulation before earthquakes.
UR: http://www.glg.ed.ac.uk/$\sim$scrampin/opinion/
DE: 8123 Dynamics, seismotectonics
DE: 8164 Stresses--crust and lithosphere
DE: 7203 Body wave propagation
DE: 3902 Creep and deformation
DE: 1645 Solid Earth
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting