William B. Bull 1973 <p>In the west-central San Joaquin Valley, California, pumping of ground water has changed water levels, thereby increasing the stresses that tend to compact alluvium by as much as 50 percent and creating a large area of intense land subsidence.</p><p>The estimated 1943–1960 specific unit compaction (compaction during a time period, per unit thickness, per unit applied-stress increase) of the deposits in a northern subarea is four times that of a southern subarea, which suggests marked differences in the compressibility of the deposits. A third of the compressibility difference is real and is due to less prior total applied stress in the northern than in the southern subarea.</p><p>The other two-thirds of the compressibility difference is only apparent and is attributed to different water-expulsion rates in the clayey beds of different depositional environments. In the northern subarea, the deposits are mainly flood-plain sediments that contain extensive sand beds and thin clayey beds that are dewatered relatively quickly under increasing effective stress. In the southern subarea, the sediments are mainly alluvial-fan deposits that contain thick clayey sequences adjacent to lensing sandy beds. These deposits are de-watered more slowly than those in the northern subarea.</p><p>Variations of mean lithology in the chief compacting zone could not be correlated with variations in the specific unit compaction of the deposits.</p> application/pdf 10.1130/0016-7606(1973)84<3783:GFACOD>2.0.CO;2 en Geological Society of America Geologic factors affecting compaction of deposits in a land-subsidence area article