Nontectonic subsidence

Abstract Geologists have investigated many different types of subsidence (Table 1) in North America during the past 100 years. Their principal contribution has been a better understanding of subsidence processes associated with the sudden formation of sinkholes, volcanic activity (Schuster and Mullineaux, this volume), tectonism (Bonilla, this volume), and sediment compaction induced by withdrawal or natural expulsion of underground fluids. Although major advancements in the understanding of other types of subsidence processes have been made primarily by engineers and soil scientists, geologists have outlined the geologic framework within which these subsidence processes are active. Allen (1969) provides an overview of the geologic processes that contribute to subsidence and the geologic setting of subsidence. This chapter traces the evolution during the past 100 years of the conceptual understanding of land subsidence in North America caused by compaction of unconsolidated sediment induced primarily by the withdrawal of underground fluids. It reviews the ways these concepts have been applied, both to development of the theory of fluid flow through porous media and to gain insight into natural geologic processes. Four important case histories are examined, and the chapter concludes with discussions of the outlook for future investigations of subsidence in North America and a summary of research needs. Terminology used in the chapter follows Poland and others (1972). Subsidence associated with man-induced compaction is one of man’s major inadvertent engineering feats. At least 34 areas in Mexico and the United States have subsided (Fig. 1); an aggregate area of about 22,000 km2, approximately equal to the area of New Jersey, has been lowered more than 30 cm.