Deformation Around the Cerro Prieto Geothermal Field Recorded by the Geotechnical Instruments Network REDECVAM During 1996-2009

Abstract

Since 1996, CICESE has been operating a network of geotechnical instruments, REDECVAM (Red de Monitoreo de Deformación de la Corteza en el Valle de Mexicali), for continuous recording of deformation related to tectonic (seismic, aseismic and interseismic) phenomena, as well as anthropogenic deformation caused by deep fluid extraction at the Cerro Prieto Geothermal Field (CPGF). The instruments are installed along the faults that limit the Cerro Prieto Pull-Apart Basin (CPB). Currently, the network includes 4 extensometers and 9 tiltmeters (two in 5 meters deep boreholes). All instruments have sampling intervals in the 1 to 20 minutes range. Instrumental records typically show continuous creep, episodic slip events, sometimes triggered slip events and coseismic slip jumps. The continuous slip rate observed on the Saltillo fault has increased with time, from 5cm/year to 8cm/year, while the location of the larger subsidence rates migrated to the NE. The slip rate at the Cerro Prieto fault slip is about 1 to 4cm/year, with the larger rate closer to the CPGF. In previous works Glowacka et al. (1999, 2005, 2009) suggested that the slip rates observed on both faults are related mainly to subsidence caused by fluid extraction at the CPGF. Since the network installation, dozens of aseismic slip events on the Saltillo fault and some on the Cerro Prieto fault have been recorded. The episodic fault slip in the Saltillo fault appears mainly as slip-predictable, normal, aseismic slip. The occurrence of slip events in the Saltillo fault appears to have no relation with the small or moderate local earthquakes; except for the stepwise, discontinuous, and permanent deformation caused by the M=5.8, December 2009 earthquake. However, it seems that slip can be triggered in the Saltillo fault by distant earthquakes with magnitude around 7, if these occur close to the end of a slip event cycle. Some of the instruments installed on other faults, however, recorded permanent deformation caused by moderate local earthquakes. Our results are compared with published evaluation of anthropogenic subsidence in the CPGF.