In this thesis, three recent earthquake datasets are analyzed in which local recording-site geology strongly modifies near-source ground motion in the 1-30 Hz band. it also discusses estimates of the impact of these geotechnical factors on the environment in the Imperial Valley, if geothermal development proceeds. This report discusses geology, geophysics, hydrogeology, seismicity and subsidence in terms of the availability of data, state-of-the-art analytical techniques, historical and technical background and interpretation of current data. The objective of this part of the study was to investigate the geotechnical aspects of geothermal power generation and their relationship to environmental impacts in the Imperial Valley of California. The objective of the project was to assess the technical, geotechnical, environmental and economic more » feasibility of producing electricity from hydrothermal resources like those known to exist in the US. At the same time, the experience and data bases in these areas are very limited. The impact of power generation from hydrothermal resources on subsurface water flow, seismicity and subsidence are of acute interest in the determination of the environmental acceptance of geothermal energy. This report presents a portion of the results from a one-year feasibility study sponsored by the Electric Power Research Institute (EPRI) to assess the feasibility of constructing a 25-50 MWe geothermal power plant using low salinity hydrothermal fluid as the energy source. These apparent velocity anomalies could be related to crustal thinning, but because of the very mild Bouguer gravity anomalies in these areas, it seems more likely that the velocity anomalies occur entirely within the crust. These areas are nearly aseismic, and overlie crust with apparent seismic velocities only mildly higher than the regional average. Known geothermal resource areas at Heber, Dunes, and Glamis, lie away from the projected trace of the Brawley fault. Neither local earthquake nor teleseismic arrival times can discriminate between these hypotheses, but the issue might be resolved by combining both types of data. Known geothermal resource areas at Salton Sea (or Obsidian Buttes), Brawley, and East Mesa, lie on the axis of a zone of thin crust, and they may be intimately related to the Brawley fault. Residual delays between the teleseismic observations and predicted crustal delays imply crustal thinning of 3 or 4 more » km along the axis of the valley. The agreement was excellent, suggesting that the teleseismic delays are caused primarily by crustal velocity variations. The authors compared results with those of teleseismic studies by Savino et al (1977). This zone corresponds to the region of thickest sediments. The authors found a zone of relatively high-velocity trending southeast from the Salton Sea. Their model consisted of near-surface station corrections and local perturbations to a standard crustal velocity model. The authors inverted local earthquake arrival times to estimate spatial variations of seismic velocity. The purposes of this study are to: (1) examine crustal structure using recently available data on P-wave arrival times of local earthquakes (2) examine the leveling data for evidence of tectonic subsidence or uplift and (3) study correlations between seismicity, seismic velocity, geodetic motion, geothermal activity, and local geology to provide a more consistent picture of the tectonics of the Imperial = , In recent years, the United States Geological Survey and Caltech have added new seismic stations into a dense network in the Imperial Valley to study in detail the relationship between geothermal areas and earthquakes, and to understand the tectonic processes taking place there. The Imperial Valley exhibits perhaps the most active current tectonism in the United States patterns of gravitational and thermal anomalies, along with geodetic measurements, strike-slip faulting, and recent volcanism suggest that the continental crust may still be spreading (Elders et al., 1972).
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