Page 350 - Laboratory Manual in Physical Geology

EXPLANATION

San Francisco Bay
Alluvium and bay deposits
Niles
Consolidated rocks

Fault
Hayward Fault
Dashed where approximate,
dotted where concealed.
0.1
Line of equal subsidence
Interval 2, 0.5, and 0.1 foot; dashed
Menlo
Park where poorly controlled.

0 4 8 km
0 2 4 6 mi
Agnew
Sunnyvale
Santa
Clara
8 San Jose
6
4
2
0.5
0.1
This
map
San Andreas Fault

FIGURE 12.9 Land subsidence, 1934–1967, in the Santa Clara Valley, California. (Courtesy of U.S. Geological Survey)

Initially, wells as far south as Santa Clara were arte- the subsidence had been stopped due to a reversal of the
sian, because the water-pressure surface was above the land water-level decline.
surface. However, pumping them for irrigation lowered Most wells tapping the artesian system are 150–300
the water-pressure surface 40–60 m (150–200 ft) by 1965. m (500–1000 ft) deep, although a few reach 365 m
This decline was not continuous. Natural recharge of the (1200 ft). Well yields in the valley are 500–1500 gallons
aquifer occurred between 1938 and 1947. As of 1971, per minute (gpm), which is very high.

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