Perhaps related to the local geothermal activity? This isn't far NE of the Geysers geothermal electric power station, where the microquake activity is beyond abundant.

Seems the specific area around Clear Lake used to/still has borax, sulfur and mercury. Zooming into the aerial map of the exact location of these quakes shows white ground.

http://en.wikipedia.org/wiki/Sulphur_Bank_Mine

Brian