Have you ever climbed a staircase two or three steps at a time? You stretch one leg out, drop your knee over the foot that's about to kick you upward and suddenly, right about where your legs begin their upside-down 'V', your pants make the noise of an over-stressed rope ladder . Well, imagine your left leg is the Eastern Pacific Plate and your right leg is the North American Plate and your trousers is the surface crust of the Earth and you might get an idea how Baja was created when this part of the planet tried to trundle the Stairway to Heaven.
Take that 'V' and flipped it over. The left leg is now the Baja and the apex is where all that stress is happening as your legs slowly spread. There's a certain amount of chiropratic activity going on there, a lot of subcutaneous slipping, heaving and throwing, micro-fissures, slabs rubbing, scaping, popping and deflecting. It's a lot like the lower back of an 80 year old man.
Volcanoes are a sure sign of tectonic activity. Active volcanoes advertise tremendous ongoing pressures and frictions at the boundaries between plates, as the renown Pacific Ring of Fire, with its 452 volcanoes, demonstrates. Our own local quakes and dormant volcanoes derive from the transfrom fault known as the San Andreas, which begins off Cape Mendocino in northern California and terminates somewhere under the Imperial Valley. It is through the activity along this faultline that the Cerro Prieto was born between 10,000 and 100,000 years ago. There is no record of it ever having erupted. So benign is its aspect that its thirty acre lava dome is now covered with ernormous scrolls of boulder-scribe graffiti, a sure sign of dormancy, both in the volcano and the graffiti artist.
This area of the Baja is where the Cerro Prieto Geothermal Field (CPGF) lives, one of the largest producing geothermal fields in the world. It is also one of the few places where a spreading fault performs on land. They normally occur in ocean floor ridges. Because of this rarity and because of the manner in which the field is exploited to produce electricity, the geothermic field is closely monitored.
The CPGF, along with a network of spreading centers linked through a series of transform faults, is seismically quite active. Last year (2008) in February, a swarm of over 500 quakes and aftershocks, two of them measuring 5.1, were recorded in the Cerro Prieto location. Earthquake swarms generally precede volcanic eruptions.
A continuous monitoring of the Cerro Prieto geothermal field was carried out from August 1994 to December 1995 to investigate the seismicity of tectonic origin and the seismicity triggered by exploitation activities. Hypocenters for 148 events were located mainly around the northern end of the Cerro Prieto fault and within the geothermal zone. The estimated focal depths range mostly from 1 to 6 km, for earthquakes with magnitudes from 0.5 to 4.6. It was difficult to distinguish between natural and triggered seismicity.
Evidence exists that an on-land spreading episode occurred at Cerro Prieto between 1979 and 1981. There was the Imperial Valley earthquake of 15 October 1979 and the Victoria earthquake of 9 June 1980, both of which produced slip near Cerro Prieto. There was also extension in the expected NW–SE spreading direction, both from direct measurements of areal strain and from the combined slip on the two faults. A subsidence of up to 25cm near the Cerro Prieto geothermal field was recorded using levelling and gravity data. And there was an abrupt rise in temperature and HCO3 concentration in the Cerro Prieto geothermal reservoir, both of which indicate the presence of magma at depth.
The geothermal system is a large high-temperature (280-350ºC), liquid dominated field, contained in sedimentary rocks. The Cerro Prieto field began producing electricity in 1973. In 1976 Toshiba supplied two 37.5 megawatt (MW) Geothermal Power Plants. Presently, there are 13 power units in
operation, grouped into four powerhouses (CPI through IV) with a
total installed capacity of 720 MW. The power units have distinct
capacities, ranging between 25 and 110 MW. The last four 25
MW units at CP IV were commissioned in July 2000.
field has more than 120 km of steam pipe; 40 km of pipe and
60 kilometers of channels to channel brine, and 10 km of pipe to
conduct non-separated fluids (mixing).
Three power plants at Cerro Prieto, operating at over 90% effieciency, produce a total of 620 megawatts, enough to power 80% of Baja California (Norte) with ample reserves left to export to the San Diego/ Imperial Valley region.
Excess steam from hydrothermal power production is condensed and released to form the Cerro Prieto Lake. During the last three decades, more than 870 million metric
tonnes of steam and 1,300 million metric tonnes of brine have been
extracted from Cerro Prieto geothermal system, totaling approximately
2.5 km3 of fluids. More than 90,000 GWh of electricity had been generated at
the Cerro Prieto Geothermal Field through the end of 2002.