Parkfield, California

Parkfield (formerly Russelsville) at an elevation of . Google satellite images show that the bridge, which Simon Winchester described as "bent" and having cracked and patched pavement, has been replaced since Winchester's book was published, and a sliding joint installed; however, the west piers of the bridge remain displaced to the north. The bridge sports signage announcing to drivers that they are crossing the plate boundary.

Since at least 1857, Parkfield has experienced a magnitude 6 or greater earthquake about every 22 years. In 1985, the US Geological Survey predicted that there would be a comparably-sized earthquake in this community by 1993, but no such event came until September 28, 2004, when a 6.0 Mw earthquake struck at 10:15 am Pacific Daylight Time.

Parkfield is the most closely observed earthquake zone in the world. Scientists constantly measure the strain in rocks, heat flow, microseismicity, and geomagnetism around Parkfield. The observations of the San Andreas fault in Parkfield are hoped to help scientists better understand the physics of earthquakes and faulting; information gathered from Parkfield may still be used someday to issue predictions for major earthquakes along the San Andreas fault and around the world.

Since 1985, the United States Geological Survey has been working on a project known as "The Parkfield Experiment", a long-term research project on the San Andreas Fault. "The experiment's purpose is to better understand the physics of earthquakes — what actually happens on the fault and in the surrounding region before, during and after an earthquake."

In 2004, work began just north of Parkfield on the San Andreas Fault Observatory at Depth (SAFOD). The goal of SAFOD is to drill a hole nearly 2.5 miles (4 kilometers) into the Earth's crust, across the San Andreas Fault. The specific target of the probe was a patch of fault known to generate sequences of repeating microearthquakes of around magnitude 1.0. The drilling was completed in the midyear of 2005, and an array of sensors was installed to capture and record earthquakes that happen near this area. It is hoped that SAFOD observations will provide insight into the source mechanisms of these small earthquakes, which can be scaled up in an effort to understand larger events. The drilling discovered a mineral called serpentinite at the site of origin of the 2004 earthquake. Serpentinite can transform into talc, a very soft mineral, thus facilitating easy slippage of plates. It has been hypothesized that aseismic creep at Hollister is also due to the presence of serpentinite. No drilling has been done at Hollister to confirm this.

Parkfield, California

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