Underground : Shakin’ All Over

The “Fault Activity Map of California and Adjacent Areas” looks like what might happen if you left a lovely glass etching of our state to a little boy with a hammer and lots of Twinkies. In red, green, brown, black and violet, the 20-square-foot map shows normal faults and reverse faults and strike-slip faults, faults of unknown affinity and faults with oblique slip, faults onshore and off- and, for good measure, faults in parts of Nevada and Oregon as well as Baja. All told, the map, published by the California Division of Mines and Geology, indicates thousands of faults, plus a few hundred volcanoes. The oldest faults are rendered in bold black lines; active faults--like the San Andreas and the San Jacinto--slash across the state in unnerving red swaths.

With the exception of Japan, no other region in the world has a fault map covering such a large area in such detail (and at $20 through any of the division’s offices, it’s also a bargain). “This pretty much represents what we know,” says Charles W. Jennings, the retired senior geologist for the state’s mines and geology division who authored the map. An update and an elaboration of a fault map he produced in 1975, this version took Jennings and an assistant four years to complete. One reason: It was drafted entirely by hand.