Comparison between the tectonic stress measured in situ and stress parameters from induced seismicity at Monticelo Reservoir, South Carolina
During May and early June 1979, five three-component digital seismographs were deployed near the Monticello Reservoir, South Carolina, to determine the stress drops of reservoir-induced earthquakes and compare them with earlier hydrofracture stress measurements of Zoback and Hickman (this issue). Records for 327 events were obtained; 10 of these events, each recorded on four or more stations, were analyzed to determine stress drops. Depths for the 10 events ranged from 0.5 to 1.4 km. Fault plane orientations were calculated from an inversion of the body wave displacements; individual radiation pattern corrections were applied in calculating the moments at each station. Final estimates of seismic moment were obtained from the individual estimates of the source radius, and stress drop was then calculated once from the final estimates of seismic moment and source radius for each event. Moments of the 10 digitally recorded earthquakes ranged from 2.5 x 10/sup 17/ to 2.7 x 10/sup 18/ dyne cm for the P wave data and from 4.6 x 10/sup 17/ to 4.6 x 10/sup 18/ dyne cm for the S wave data. The stress drops (S wave data only) generally ranged from about 0.2 to 4 bars. Four earthquakes were recorded by a strong motion instrument at the dam site. Magnitudes (M) for these events ranged from 2.8 to 3.0. Moments taken as averages of the values from each component ranged from 1.7 to 3.4 x 10/sup 20/ dyne cm and the stress drops from 13 to 92 bars. The variation in shear stress inferred from in situ measurements at the deeper levels indicates an inconsistency between the thrust-faulting mechanisms of the earthquakes and the state of stress which is more consistent with thrust faulting at 0.2-km depth and strike slip or normal faulting below. Thus the estimates of shear stress from in situ measurements of the same order as the stress drops of the Mapprox. =2.8-3.0 events but about 1 order of magnitude larger than the stress drops obtained for the smaller events.
- Research Organization:
- U.S. Geological Survey, Menlo Park, California 94025
- OSTI ID:
- 6082852
- Journal Information:
- J. Geophys. Res.; (United States), Vol. 87:B8
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
13 HYDRO ENERGY
SOUTH CAROLINA
WATER RESERVOIRS
STRESS ANALYSIS
DAMS
EARTHQUAKE MAGNITUDE
EARTHQUAKES
SEISMIC SOURCES
SEISMIC WAVES
SHEAR
FEDERAL REGION IV
NORTH AMERICA
SEISMIC EVENTS
SURFACE WATERS
USA
580201* - Geophysics- Seismology & Tectonics- (1980-1989)
130200 - Hydro Energy- Site Geology & Meteorology