Monitoring crustal strain, Cerro Prieto geothermal field, Baja California, Mexico
Significant ground deformation may be occurring in the Cerro Prieto geothermal area, Mexicali Valley, Mexico, as a direct result of extensive withdrawals of geothermal fluid and of continuing tectonism. In order to measure the magnitude and rate of this deformation, three networks of precise geodetic control were established in 1977-78, as part of an international agreement between Mexico and the United States. This report gives the background data for one of these survey nets--a local network of precise horizontal control designed to monitor the horizontal component of ground movement in the area of geothermal production. It represents the first year's effort of a 3-year investigation by the U.S. Geological Survey. It is anticipated that the network of horizontal control will become part of the monitoring program of the Mexican Comision Federal de Electricidad (C.F.E.) for periodic surveys. Fifty-nine lines of precise horizontal control comprising the Cerro Prieto horizontal-control net were established and measured for the first time during January-March 1978. The net encompasses the area of geothermal interest, and spans many of the active local and regional faults. By repeated measurement of the lines in the network, magnitudes and rates of relative ground movement can be calculated. Surveyed control lines of the net tie stations in the geothermal area to bedrock reference stations on Cerro Prieto volcano and four peaks of the Cucapas Mountains. Thus, measured ground movement in the geothermal area can eventually be correlated with regional changes in western Mexicali Valley. To measure accurately the deformation caused by geothermal production and to differentiate this deformation from changes due to other causes are among the challenges of this research program.