Four types of land subsidence have been observed in the southern San Joaquin Valley. These have distinct causes, and generally occur in different areas.

Subsidence caused by the decline of ground-water levels affects 400 sq mi on the valley floor at rates to 0.5 ft/yr. It results from the compaction of the water-bearing deposits as effective stresses are increased by pumping, and will continue as long as ground-water levels continue their downward trend. Subsidence due to the hydrocompaction of surficial deposits is occurring on the southern and western perimeter of the valley. Hydrocompaction results when moisture-deficient, susceptible deposits are wetted for the first time. Subsidence associated with the production of oil and gas has been observed in several fields. Also, tectonic adjustment results in the continuing northward tilt of the mountain block at the southern end of the valley.

The areal extent and rate of subsidence are determined principally by periodic releveling of surface bench marks. At selected locations in the valley, specially designed installations furnish a continuous record of compaction of the aquifer system due to artesian-head decline.

Importation and extensive use of surface water of irrigation should greatly modify the subsidence pattern in the next decade.

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