Photo: Next Great Quake: Drilling the San Andreas Fault for Answers

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A cross section of the San Andreas Fault zone at Parkfield, California, shows the drill hole for the San Andreas Fault Observatory at Depth (SAFOD) and the pilot hole drilled in 2002.

Red dots in the drill holes show sites of monitoring instruments. White dots represent areas of persistent minor seismic activity.

The colors underground show the 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.



Illustration courtesy USGS

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