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The theory and prototype development of a stress-monitoring system

BOX 22ADAMSVILLE, RHODE ISLAND 02801
U. S. GEOLOGICAL SURVEY, BOX 25046, DENVER, COLORADO 80225

Abstract

A new approach has been developed by which changes in rock stress can be measured directly. The measurement of stress change depends on the reaction of a slender, compliant cavity to the transient variations of rock stress about the cavity. A liquid-filled pressurized cell, emplaced in the cavity, provides the means by which changes in pressure can be monitored. The change in pressure in the cell is equivalent to the change in rock stress in the direction normal to the cell if the compliance contrast between cavity and rock is large and the aspect ratio of the cavity is small. Earthquake-related variations in the stress field in a highly fractured quartz-monzonite stock have been observed to occur in a characteristic way prior to a microseismic event. The precursory stress change (decompression followed by compression) lasted about 5 hours and led directly to a stress drop of 15 x 10^–3 bar. Seismic noise monitored during the same period of time suggests that failure in the fractured rock mass resulted from rapid encroachment of meltwater and subsequent increase in fluid pressure.