The town of Parkfield, located on the San Andreas Fault in central California, is one of the world's most seismically active areas. It has been the site of an intensive earthquake study since the late 1970's and a team of scientists recently reached the fault's active zone through a 2-mile deep borehole. This project, named SAFOD, is a major step in learning more about earthquakes and maybe predicting them. Population was estimated as 37 in 2004.

Parkfield, The Earthquake Study Capital of the World

The town of Parkfield, located on the San Andreas Fault in central California and 200 miles from San Francisco, is one of the world’s most seismically active areas. It has been the site of an intensive earthquake study since the late 1970's.

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The Parkfield Earthquake Experiment, led by the United States Geological Survey (USGS) and the State of California, is a long-term earthquake research project on the San Andreas Fault. Its goal is to observe the fault and better understand the physics of earthquakes - what actually happens on the fault and in the surrounding region before, during and after an earthquake.

Ultimately, scientists hope to better understand the earthquake process and, if possible, to provide a scientific basis for earthquake prediction.

The Earth's crust is fractured into a series of "plates" that have been moving very slowly over the Earth's surface for millions of years. Two of these moving plates meet in western California and the boundary between them is the San Andreas Fault. The Pacific Plate (on the west) moves northwestward relative to the North American Plate (on the east), causing earthquakes along the fault.

The San Andreas Fault is approximately 1300 km (800 miles) long. If a person stood on one side of the fault and looked across it, the block on the opposite side would appear to have moved to the right. Surveying shows a drift at the rate of as much as 2 inches per year
Sudden offset that initiates a great earthquake occurs on only one section of the fault at a time. Total offset accumulates through time in an uneven fashion, primarily by movement on a first section of the fault and then on another one. Great earthquakes are produced by sections that remain "locked" and quiet over a hundred or more years while strain builds up then, in great lurches, released.
During the 1906 earthquake in the San Francisco region, roads, fences, and rows of trees and bushes that crossed the fault were offset several meters or yards. Earthquakes are the most costly natural hazard faced by the United States.

The town of Parkfield, population 37, was chosen for the experiment for several reasons. Moderate earthquakes (magnitude about 6) have occurred on the Parkfield section of the San Andreas Fault at fairly regular intervals- one approximately every 22 years, except for the last one in 2004. All these Parkfield earthquakes have struck in the same area and historical seismograms show that at least the 1934 and 1966 shocks initiated at the same point on the fault. These observations suggest that there may be some predictability in the occurrence of earthquakes.

Building on more than 15 years of experience from the Parkfield Earthquake Experiment, the National Science Foundation (NSF) and the USGS started in June 2004 to drill a deep hole in order to install instruments directly within the San Andreas Fault Zone near the initiation point of previous magnitude 6. They reached the fault’s active zone, 2 miles deep, early August 2005.

These instruments, set 2 to 3 kms beneath the Earth's surface, will form a San Andreas Fault Observatory at Depth (SAFOD). This project is part of the global EarthScope Project and will directly reveal, for the first time, the physical and chemical processes controlling earthquake generation within a seismically active fault. As Dr Mark Zobak, one of SAFOD’s principal investigators, told the BBC: ”It’s like using a stethoscope and listening very, very carefully”.

Fault-zone rocks and fluids will be retrieved for laboratory analyses, and geophysical measurements will be made within the active fault zone. SAFOD's long-term monitoring activities will include detailed seismological observations of small to moderate earthquakes and continuous measurements of rock deformation and other parameters during the earthquake cycle.
SAFOD will provide direct information on the composition and mechanical properties of rocks in the fault zone, the nature of stresses responsible for earthquakes, the role of fluids in controlling faulting and earthquake recurrence, and the physics of earthquake initiation and rupture. By observing quakes "up close," SAFOD will mark a major advance in the pursuit of a rigorous scientific basis for assessing earthquake hazards and predicting earthquakes.

Sources and Resources: United States Geological Survey (USGS), EarthScope, SAFOD
USGS’s website: http://www.usgs.gov
Safod’s website: http://earthscope.org/safod/
Parkfield Experiment http://quake.wr.usgs.gov/research/parkfield

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Schematic diagram of a Global Positioning System (GPS) station. Covering North America and Alaska , EarthScope's network of GPS stations will measure deformation across the plate boundary between the Pacific and North American plates.  Courtesy of EarthScope.
Title:
Parkfield, The Earthquake...
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July 22, 2005
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Schematic of SAFOD and the SAFOD pilot hole, superimposed on electrical resistivity structure inferred from surface surveys. Approximate locations of small earthquakes are shown as white dots, with the location of the SAFOD target earthquakes in red. The SAFOD pilot hole was drilled in the summer of 2002 to a depth of 2.2km. Drilling of the main SAFOD hole began in June 2004, and an extensive program of downhole measurements, spot coring and fluid sampling will be conducted during drilling. Following a two-year period of downhole monitoring, four 250-m-long core holes (shown in black) will be drilled off the main hole to obtain samples from the most active portions of the fault zone. SAFOD will then be instrumented with downhole sensors (red ovals) for long-term monitoring of seismicity, deformation, fluid pressure and temperature. The surface locations of the pilot hole and SAFOD are within 10 m of each other, and are separated here only for clarity. Courtesy of EarthScope.
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Parkfield, The Earthquake...
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Schematic cross section of the San Andreas Fault Zone at Parkfield, showing the drill hole for the San Andreas Fault Observatory at Depth (SAFOD) and the pilot hole drilled in 2002. Red dots in drill holes show sites of monitoring instruments. White dots represent area of persistent minor seismicity at depths of 2.5 to more than 10 km. The colors in the subsurface show electrical resistivity of the rocks as determined from surface surveys; the lowest-resistivity rocks (red) above the area of minor earthquakes may represent a fluid-rich zone.  The SAFOD is a deep 2-mile borehole observatory that has reached, for the first time, the fault's active zone early August 2005. The SAFOD will directly measure the physical conditions under which plate boundary earthquakes occur. It is a major step in learning more about earthquakes and maybe predicting them.  Courtesy of EarthScope.
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Parkfield, The Earthquake...
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July 22, 2005
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Shaded relief map of California showing the location of EarthScope's San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, CA. Portions of the San Andreas Fault that ruptured in major historical earthquakes are shown in red, with the creeping and micro seismically active segment of the fault in blue. The SAFOD is a deep 2-mile borehole observatory that has reached, for the first time, the fault's active zone early August 2005. The SAFOD will directly measure the physical conditions under which plate boundary earthquakes occur. It is a major step in learning more about earthquakes and maybe predicting them.  Courtesy of EarthScope.
Title:
Parkfield, The Earthquake...
Date:
July 22, 2005
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