Some geologic factors determining land subsidence

Abstract

Exogenic subsidence caused by overdraft of ground water is a rapidly spreading, worldwide geologic hazard. It is a surficial expression of the reduction of porosity at depth, due to an increase of actual (unconfined aquifers), or effective (confined aquifers) loading of overburden. Some geologic parameters controlling such subsidence are discussed below. Theoretically, at a certain load the original porosity of sediments will be reduced to its minimum and no further compaction will take place under “non-metamorphic” conditions. The depth at which the weight of overburden creates such a loading is designated here as the “stable depth.” Compaction should be expected only in the “unstable field” above this depth and potential amounts of compaction should decrease toward the stable depth. Hence, subsidence is more probable in depositional areas in topographic basins, than in uplifts where erosion removes the uppermost, most compressible portions of the “unstable field.” In confined aquifer systems the position of the stable point may not correspond to the loading of overburden if the piezometric pressure were increased by downfolding occurring contemporaneously with deposition. In such a case, the newly developed piezometric pressure partially compensates for the loading of new overburden. For example, on the western side of the structure trough of the San Joaquin Valley, California, U.S.A., the main confining aquifer, Corcoran Clay, was secondarily folded during deposition of the overburden valley fill. Along the present San Luis Canal, a more or less uniform historic piezometric decline of 167 m has resulted in 0.75 to over 7 m of subsidence. Maximum subsidence has occurred in areas underlain by small, buried synclines in the Corcoran Clay, and minimum subsidence has occurred over anticlines. For project planning and design purposes, the concept indicates that subsidence could be anticipated in basins showing evidence of late Cenozoic folding.