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
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
- 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
- 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!
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