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Executive Summary More than 44,000 km2 of lanai in 45 states in the United States has been lowered by the types of subsidence considered in this report. Underground mining of coal, groundwater withdrawal, and drainage of organic soils are the principal causes of subsidence. Approximately 8,000, 26,000, ant! 9,400 km2 of lanct has subsided from each of these causes, respectively. In addition, about 18 percent of the conterminous United States is underlain by cavernous limestone, gypsum, salt, or marble and is locally susceptible to catastrophic collapse into sinkholes. Annual costs from resulting flooding and structural damage exceed $125 million. Although these costs are small relative to those of many other ear~-science hazards, their geographic distribution is not uniform. Thus, localized areas bear disproportionate shares of these costs. In addition to this uneven cost distribution, parties damaged by subsidence associates! win resource removal commonly are stymied from reimbursement by legal recovery systems that are in conflict with doctrines that establish rights to resource removal. Many examples are available of successful efforts at federal, state, and local levels to mitigate specific subsidence problems. The efforts include public information programs, mapping programs, regulation of resource development, land-use management and building codes, market-based methods, and insurance programs. Despite these successes, continued mitigation of subsidence requires action in three areas. First, basic earth-science data and information on the magnitude and distribution of subsidence are needled to recognize and to assess future problems. Such data include geodetic, geologic, hy~irogeologic, hydrologic, soils, and land-use information. These data, in bow map and tabular formats, help not only to address local subsidence problems but to identify national problems. Collection of these data in general should be overseen by earth-science agencies, particularly state geological surveys and the U.S. Geological Survey. Channels of communication should be developed to designate levels of government and interest groups advising them of the availability of this information. Second, research on subsidence processes an(l engineering methods for (leafing with subsidence is needed for cost-effective damage prevention or control. Although general understanding of subsidence processes is well developed, prediction of subsidence magnitudes, rates, and location is commonly hampered by incomplete understanding of specific details 1

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2 of He relevant processes and the inability to determine adequately subsurface conditions and physical properties of the deforming materials. Even when a specific subsidence occurrence is well understood, however, U.S. experience with engineering designs to accommodate ground deformation, and methods to control it, is modest. New funding is needed to support research on subsidence processes by the U.S. Geological Survey, Bureau of Mines, Bureau of Reclamation, and Agricultural Research Service, and on engineering methods by the Federal Highway Administration, Corps of Engineers, Bureau of Reclamation, Federal Housing Administration, and Soil Conservation Service. And third, although many types of mitigation methods are in use in the United States, stud- ies of their cost-effectiveness would facilitate choices by decisionmakers. Such studies should be funded by the Federal Emergency Management Agency, National Science Foundation, and industrial and professional organizations.