EARTHQUAKE PREDICTION FOR A SEISMIC GAP IN ALASKA During the past decade, the U.S. Geological Survey has acquired more responsibility and developed a greater capability for earthquake prediction and warning and hazard reduction. The Geological Survey's responsibility in these areas was established by the Disaster Relief Act of 1974 and Executive orders in 1975 and 1976 which required the Survey to provide warnings of geologic-related hazards-volcanic eruptions, landslides, subsidence, glacier surges, and earthquakes. To meet this responsibility, the Survey established a program to: • Increase awareness of geologic- • Identify geologic hazards, evaluate • Assist public officials in evaluating While no formal earthquake prediction has been issued in the United States, the Nation's first earthquake Hazard Watch was issued for the area of Cape Yakataga, Alaska. YAKATAGA SEISMIC On February 29, 1979, a strong earthquake shook a sparsely inhabited region of Alaska near Cape Yakataga, halfway between Juneau and Anchorage. The event, which had a magnitude of 7.7 on the Richter scale, was not as large as some other recent earthquakes in Alaska nor was there significant damage. Nevertheless, the earthquake commanded much attention because data gathered by a Geological Survey seismometer network showed that the movement was at the southeastern edge of a large seismic gap-an area that has not had a major earthquake in recent years but which lies between areas where great earthquakes have recently occurred. Seismic gaps are often subject to future great earthquakes. The existence of seismic gaps was pointed out in 1965, and, since then, much work has been devoted to estimating the earthquake potential of seismic gaps and to understanding what causes gaps. Gaps may be explained by the theory of plate tectonics, which states that the Earth's crust is composed of large areas, called plates, that move as independent units. As the plates slowly slide against or over each other, a tremendous amount of stress builds up. This stress must be released, either by slow movement or by the sudden slippage of one plate past the other, thereby producing an earthquake. A seismic gap may then be interpreted as a segment of a plate margin that has not slipped recently while its neighboring segments have. Under these circumstances, the gap is thought to have a higher potential for an earthquake than the adjacent segments. The plate margin segment near Cape Yakataga was identified by scientists from the Lamont-Doherty Geological Observatory at Columbia University as a seismic gap a decade ago. Movement in the gap occurred last during two great earthquakes that occurred in 1899. Movement occurred in the region southeast of the gap in 1958 and to the northwest in 1964. Stimulated by the February 29 earthquake, Lamont researchers restudied seismic data. They noted that seven large events, with magnitudes between 5.9 and 7.7, had occurred adjacent to the Yakataga gap since 1958, while no earthquakes of comparable magnitude occurred in those areas between 1933 and 1958. The interior of the gap has been free from comparable earthquakes since 1908. Similar situations have been observed elsewhere and have been followed by major earthquakes centered within the seismic gaps within a few decades. The amount of fault slip expected during an earthquake in the Yakataga area could be as much as 10 feet. If this amount of movement were to occur along the entire Yakataga seismic gap today, an earthquake larger than magnitude 8 on the Richter scale would be generated. Such an earthquake could produce damaging ground motion over an area as large as 30,000 square miles. Based on these observations and interpretations, the Lamont group, working as part of the Survey's Earthquake Hazard Reduction Program, proposed that the Yakataga gap was likely to be the site of a great earthquake in the future and should be monitored intensively to detect any warning signs. In response to these suggestions, the Survey convened a committee to evaluate the probability of an impending earthquake in the Yakataga gap and to make recommendations for any necessary additions to the Survey program in Alaska. Although alternative interpretations of the available data on the gap seemed plausible, the committee generally agreed that a large earthquake was likely to occur within 100 years, that it might occur very soon, and that it probably would occur within a few decades. The committee also agreed that the area should be intensively monitored. INTERPRETIVE REPORTS TO AID REGIONAL PLANNERS Since 1970, experimental multidisciplinary urban-area studies designed to aid local and regional planners have been completed in five areas of the country; projects are currently active in four additional areas. These studies were made to promote the use of earth science information in regional planning and decisionmaking. The first study program, the San Francisco Bay Region Environment and Resources Planning Study, was cosponsored by the Earth Sciences Applications (ESA) Program of the Geological Survey and the Office of Policy Development and Research of the Department of Housing and Urban Development. It produced 71 basic data maps and reports and several technical reports that have been widely used by local and regional planners. A final series of interpretive reports for nonscientists is now being produced. The other urban area studies will result in similar reports. Five interpretive reports have been published to date: • Professional Paper 941-A, Studies for Seismic Zonation of the San Francisco Bay Region, 102 p. • Professional Paper 942, Flood- • Professional Paper 943, Flatland • Professional Paper 944, Relative • Professional Paper 945, Quantitative Land-Capability Analysis, 115 p. The last three publications were published during 1979 and are described briefly below. FLATLAND DEPOSITS Professional Paper 943 and its accompanying maps describe 13 geologic units, including estuarine muds and marsh, alluvial-fan, channel, floodbasin, dune, and beach deposits. The units have different ages and engineering characteristics; decisions on the use of these lands must be based on the hazard and Potential seismic hazards on the alluvial plain and near the bay in the San Francisco, Calif., area. resource potential of each individual unit. In the report, each geologic unit is described in detail, and the potentials for geologic hazards such as subsidence, flooding, and liquefaction are evaluated, and resources-possibly including sand and gravel, peat, shells, and salts-are identified. In addition, each unit is rated on its relative capability to support agriculture, urban residential development, ground-water recharge, and sand and gravel extraction. SLOPE STABILITY of the maps in the regional land use planning process. The relative slopestability maps have a variety of uses for planning the location of nuclearreactor sites, transportation and communication networks, and open space and recreational areas, and the control of urban growth. QUANTITATIVE LAND CAPABILITY ANALYSIS Professional Paper 945 describes a method of evaluating land use proposals by estimating the development costs related to various geologic and hydrologic characteristics and processes when the existing use of the land is converted to housing, commerce, or transportation. Costs can include potential damage from natural hazards such as floods, landslides, or earthquakes; fees for special investigations, designs, or construction practices; and the loss of potentially valuable resources such as sand and gravel. The total costs associated with all geologic problems for a specific use and a given area indicate the relative capability of that land to accomodate that use. Thus, capability maps can be produced for each proposed land use by displaying the sums of these development costs by area on a map. Such landcapability maps are a convenient means of graphically presenting the data needed to evaluate alternatives and to make better decisions on land use. In the San Francisco Bay region, landslides caused damage amounting to more than $25 million during the rainy season of 1968-69 and to more than $10 million during the 1972-73 rainy season. The possibility of damage has been increasing steadily as more development takes place on hillsides. These losses can be greatly reduced by using geologic information to recognize, to evaluate, and to map those areas and slopes that are potentially unstable and by applying this information in planning, designing, and managing the use of hillside areas. Professional Paper 944 presents the first standardized relative slope-stability maps of the entire San Francisco Bay region and discusses the implications and uses Area subject to flooding if dam fails 2 STRONG Bedrock shaking from earthquake of magnitude 6.5 (30-50 inches per second 4-12 inches per second) DECREASING WITH DISTANCE FROM FAULT Salt-water marshes Tidal mudflats S.F. Bay Late Pleistocene alluvium High Late Pleistocene bay mud Pleistocene alluvium SUBSIDENCE DUE TO COMPACTION Recording sites for low-strain seismic amplification 3 A COMPUTER MODEL THAT HELPS PROTECT The Oilspill Trajectory Analysis (OSTA) model is one of the many tools scientists and public decisionmakers use in an effort to ensure that the development of our domestic energy resources does not endanger our environment. The model is a sophisticated computer program that defines both the probability of an oilspill occurring and its potential paths. The OSTA model was developed by a Geological Survey team currently composed of a physical scientist, an oceanographer, a mathematician, and a computer technican. The model became fully operational during fiscal year 1979 and provides information for Outer Continental Shelf (OCS) lease sales and for development and production environmental impact statements. The OSTA model determines the likelihood of an oilspill occurring within a given area and, through oilspill simulation, the possible movement of the oilspill and whether it will contact any environmentally sensitive resources. Because some resources, such as migrating birds, may be present in the study area for only part of the year, the model incorporates monthly vulnerability for each resource. The probability of oilspill occurrence is estimated from the volume of oil expected to be produced, the anticipated method of production, and the distance of transportation to shore. Movement of as many as 500 simulated oilspills per season is modeled based upon historic wind records and monthly ocean currents. Output of the model includes tables of conditional probabilities of oilspills contacting any of 31 categories of vulnerable resources for 100 segments of the coast within 3, 10, and 30 days and the percentage of oilspills contacting a specific environmentally sensitive resource within the same periods. These periods reflect stages in the natural degradation of the oil and resulting decrease in its toxicity. The model calculates oilspill probabilities for each of the places where an oilspill could occur and combines these to determine the overall risks. The model provides the Department of the Interior with a method of realistically assessing oilspill risks associated with OCS development. To date, it has been used for analyzing oilspill risks for 10 OCS environmental impact statements. In addition, the OSTA model will be used to analyze the summer 1979 Ixtoc 1 oilspill in the Gulf of Mexico. PROTECTING THE ENVIRONMENT IN ALASKA The environmental consequences of potential petroleum development in the National Petroleum Reserve in Alaska (NPRA) dictated the preparation of a special environmental "draft" study completed during 1979. Section 105(b) of the Naval Petroleum Reserves Production Act of 1976 directed the President to conduct a study to determine the best procedures for developing, producing, transporting, and distributing petroleum resources of the Reserve. The NPRA study includes analyses of the environmental impacts of development and production of petroleum resources in the Reserve and of transportation of those resources to the conterminous United States. The outstanding environmental concerns stemming from development and production are (1) impacts on wildlife, especially caribou and waterfowl, (2) depletion of gravel and water resources which are in limited supply, and (3) impacts on the culture and lifestyle of the Inupiat Eskimo communities, which rely on the hunting of Bowhead whales. Significant concerns for transportation of the petroleum resources from NPRA are (1) impacts upon the Inupiat culture and lifestyle through increased contact with modern society, (2) increased pressures on government and service organizations, (3) impacts on wildlife, particularly on caribou migration and habitat from pipelines and roads, and (4) potential oilspills along new pipeline or marine transportation routes. In the analysis of impacts, the development and experience at Prudhoe Bay and the construction and operation of the Trans-Alaska Pipeline System (TAPS) were used as general models. Together they represent the current level of technology and experience in the arctic environment. Because wellfield facilities vary widely with petroleum reservoir characteristics, no specific model was used. For the transportation of petroleum from NPRA to the terminals, the following general corridors were assumed for illustrative purposes: (1) from eastern NPRA to TAPS at Prudhoe Bay, (2) from eastern NPRA to TAPS south of Prudhoe Bay, and (3) from southwestern NPRA to the Bering coast at alternate sites north or south of the Bering Strait. A marine terminal at Barrow for submersible tankers was also considered briefly. Environmental studies of this type are a critical element in the decisionmaking process, assuring both public officials and private citizens that environmental issues are addressed during the development of natural resources. |
