Ultradeep melting rocks drive invisible quakes beneath San Andreas Fault – Live Science

Along the San Andreas fault
near the town of Parkfield, California, an incredibly sensitive array of seismic and GPS instruments has for years been detecting something strange: deep earthquakes
that occur well below where most seismic movement occurs in California. 
These quakes can’t be felt at the surface, but they’re intriguing because this same segment of the San Andreas near the town of Parkfield also shudders with magnitude-6 or so earthquakes every 20 or 40 years strong enough to dam…

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