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Depth dependence of source parameters at Monticello, South Carolina

U.S. GEOLOGICAL SURVEY OFFICE OF EARTHQUAKES, VOLCANOES, AND ENGINEERING, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

Three estimates of stress differences, which include Brune stress drop, stress drop from rms of acceleration (a_rms), and the apparent stress, have been determined for 13 earthquakes at Monticello, South Carolina, a site of reservoir-induced seismicity. Data for nine of the events come from digitally recorded three-component seismograms at four or five stations that were deployed around the Monticello Reservoir in May and early June 1979. The data from the other four events come from a strong-motion accelerograph located on the dam abutment at the southwest end of the reservoir. Estimates of the seismic moment (M_o) range from 4.6 x 10¹⁷ to 3.4 x 10²⁰ dyne-cm (S waves) and radiated energy from about 10¹¹ to 3 x 10¹⁶ dyne-cm (S waves). Brune stress drops ranged from 0.5 bars to about 90 bars and show a strong dependence on depth (focal depths range from 0.07 to 1.4 km) and a moderate dependence on M_o. A_rms stress drops from the direct S-wave span a similar range of values and also exhibit a strong dependence on depth. Apparent stresses are usually lower than the other estimates of stress differences by a factor of 2 to 4. Seismic stress differences are highest in the topmost 0.2 to 0.3 km, a depth range for which the in situ measurements of stress and pore pressure suggest that the rock is in a state of incipient failure. In this depth range, where the four largest events occurred, the stress drops are of the same order as the ambient shear stress. These data suggest that at Monticello, where pore fluids have a strong influence on the failure process, the largest stresses released seismically are in regions most conducive to failure and that the seismic efficiencies for events at Monticello are larger than have been reported for other tremors in different tectonic settings.