The Hayward Fault: Exposed!

Some photos from Opening Day (April 29. 2006) for The Hayward Fault Exposed!, a trench across the Hayward fault that's open to the public on Saturdays and Sundays through June 30th, 2006 (and weekday tours by reservation).

I'm a docent, which means I'm one of the people in light-blue shirts who answer questions, try to explain to people what they're seeing, and make sure nobody hurts themselves or messes things up.

Here's a list of Useful Web Sites related to Faults and Earthquakes.

Fun Hayward Fault facts:

• The fault is 8 miles deep and 40 miles long (stretching from Milpitas to San Pablo bay).
• The top two miles are creeping at about 4-5mm/year; the bottom six miles are locked.
• Strain accumulating on the fault is estimated to be about 9mm/year (so don't feel too complacent even about that creeping part -- it isn't enough to relieve all the strain).
• The east side of the trench is 9,000 year old silt from ancient Alameda creek; the west side is layers of silt, gravel, and sand from the same creek, but only 4,000 years old.
• Why 4,000 and 9,000 year old layers on opposite sides of a strike-slip fault? This page explains why.
• Those dates are from radiocarbon dating of charcoal. No fossils were found in this trench (but nearby trenches have exposed horse teeth, mastodon bones, and even a 4,000 year old human bone).
• Total motion represented in the trench is 46 meters, all lateral motion. The uplift represented by the difference between the older sediments and the younger ones represents a basin on the east side, not any vertical motion along the fault.
• The last earthquake on the Hayward was in 1868, roughly magnitude 6.9. The fault jumped about three feet in that earthquake.
• Paleoseismology studies say that this fault experiences a large earthquake on average every 130 to 150 years. Do the math!