Subsidence in Cerro Prieto Geothermal Field, Baja California, Mexico

Abstract

During the period 1977 - 1987 the subsidence rate at the center of the Cerro Prieto Geothermal Field (CPGF) increased after every large, sustained production increase. Maps of subsidence rate for 1994 - 1997 show that the area with a subsidence rate °› 8 cm/y has an elliptical shape with a NESW major axis, which coincides with a thermal anomaly and with the orientation of a pull-apart basin located between the Cerro Prieto and Imperial faults. The area of maximum subsidence rate, around 12 cm/y, coincides with the area of extracting wells, and the subsidence velocity is much higher than the expected tectonic one. The area east of the Imperial Fault has a subsidence rate less then 2 cm/y, suggesting that this fault is an eastern boundary for recharging water. Continuous measurements of vertical deformation made during the last few years at the Imperial Fault show that vertical movement on the fault is not constant, but is concentrated during creep events, separated by months of quiescence. The high value of 11 cm/y for the mean vertical movement, and a pattern of rate changes similar to that of the CPGF, suggest that the movement in the Imperial Fault is influenced by water extraction in the CPGF. The occurrence of creep events, however, seems to have no apparent relation with the almost constant extraction in the field for 1996 - 1998. It seems that the displacement along the fault is influenced in the long-term by the deformation process (by the subsidence induced by fluid extraction), and in the shortterm by local elastic phenomena such as friction, triggering and viscoelasticity. The relation between vertical displacement on the fault and seismicity is still not clear.