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Subsidence of the land surface in the Santa Clara Valley, Santa Clara County, Calif., has been observed since 1933. A network covering the area of subsidence was first surveyed in the spring of 1934. That network was releveled seven times between November 1934 and April 1940, again in 1948 and 1954, and in part in 1956 and 1959, by the U.S. Coast and Geodetic Survey. The releveling of the network in 1954 indicated that the land surface had subsided 7.75 feet at bench mark P 7 at San Jose. Subsidence near Sunnyvale from 1934 to 1954 was greater than at San Jose. The maximum subsidence from 1948 to 1954 occurred at San Jose and was on the order of 2.4 feet. By December 1959, total subsidence at bench mark P 7 since 1912 was 9.04 feet. The area of land-surface subsidence extends southward about 25 miles from Redwood City to and beyond San Jose, has a maximum width of about 13 miles, and includes at least 230 square miles. Plots of subsidence at individual bench marks compared to fluctuation in artesian pressure indicate that decline in artesian pressure is a major cause of subsidence. The rate of subsidence is generally in accord with the rate of fluctuation of artesian pressure. Subsidence virtually ceased during a period of increasing artesian pressure from 1937 to 1940, but resumed at an accelerated rate during a period of rapidly decreasing artesian pressure from 1944 to 1950. INTRODUCTION The Santa Clara Valley is a large structural trough that extends from Hollister to San Francisco, Calif. It is bounded on the southwest by the Santa Cruz Mountains and on the northeast by the Mount Hamilton Range (inset map, fig. 4). The San Andreas fault system is a few miles southwest of the valley plain and the Hayward fault borders the valley on the northeast. Land subsidence occurs in the central reach of the valley in an area of intensive ground-water development. Discussion in this report is limited to this central reach of the valley, which extends southeastward about 30 miles from Eedwood City and Niles to Coyote (fig. 4). The purpose of this report is to present a brief summary of the C-l C-2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATESsubsidence that lias occurred in the central part of the Santa Clara Valley, as defined by repeated leveling by the U.S. Coast and Geodetic Survey; to compare the subsidence with change in artesian pressure observed at the two principal centers of subsidence; and to draw preliminary conclusions concerning the principal cause of the subsidence.

Subsidence in the Santa Clara Valley, California: A progress report