Investigations into Fractured Granite Rock by Detailed Geologic and Geophysical Methods

Fractured granitic rock in an area of the northern Sierra Nevada, California, is investigated by detailed field geologic methods and by seismic methods. The study area is 180 m by 110 m and is located in a glaciated valley with relative flat topography. The detailed field geologic methods entailed mapping all the fractures and discontinuties over 1 m in length on a scale of 1:100, coring the rock and fractures for bulk mineralogy and petrographic features of joint and microfault structures, and development of a possible joint and microfault formation history. All of the joints formed by dilation displacement only. There is no evidence suggesting a conjugate shear joint system. The joint system striking N82-87E formed by dilation and appears to be older than the joint system striking N25-30W; the evidence for this is the relative amount of secondary mineralization within the fracture planes. Within the study area, some of the joints have been reactivated as left and right-lateral microfaults with average displacements between 0.2 and 20 cm. The geophysical research of the thesis was aimed at utilizing SH-waves (shear horizontal) to investigate the solid-earth tide effect on jointed rock: tidal strain may be accommodated by opening of the joints. The research suggests the following: (1) The continued firing of the shear-wave generator altered the rigidity characteristics of the soil resulting in a change in frequency and amplitude of the emitted shear-wave; (2) Diurnal temperature change in the upper 1 m of the rock may have effected the shear-wave propagation by way of thermoelastic tilt and strain; (3) The frequency and amplitude began to stabilize after 19 hours, whereupon, a correlation in shear-wave travel-time and theoretical tidal strain is observed for the remaining 21 hours of the test