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variations in the amplitude and frequency content of ground shaking that can be used to study the characteristics of damaging levels of strong ground shaking.
Aftershock recordings obtained in the town of Coalinga, which is located in an alluvium-filled valley, and at nearby sites located on bedrock, suggest that certain frequencies of shaking were considerably larger in Coalinga than at bedrock sites less than 2.5 miles away (fig. 3). This increased shaking due to the alluvium beneath Coalinga may have contributed to the significant damage caused by the mainshock.
Additional scientific investigations have provided a preliminary estimate of the rate
of offset along the Anticline Ridge fault. Scientists have found an area where the ground surface is upwarped and incised by small creeks. Determination of the age of the upwarped ground surface suggests that there is a 0.2 inch per year uplift rate for the area over about the last 2,500 years. The repeat time for similar earthquakes along the Anticline Ridge fault has been estimated at about one event every 350 years. Similar geologic and tectonic settings for other regions along the west side of San Joaquin Valley suggest that similar earthquakes could also occur in these regions.
Earthquakes in the Eastern United States
Earthquakes are about 10 times more common in southern California than they are in the Eastern United States. However, damaging earthquakes do constitute a significant hazard in the Eastern United States. Historic examples of violent eastern earthquakes occurred in Charleston, South Carolina, on August 31, 1886, and in the New Madrid, Missouri, area in the winter of 1811-12. Extensive damage and 60 deaths were caused by the South Carolina event, and widespread dislocation of the ground surface occurred in the Mississippi River Valley area. These earthquakes were felt over much of the Eastern United States as is characteristic of major eastern events. Intensities registering VI and higher on the Modified Mercalli Intensity Scale (MM) were observed across several Eastern States in 1811 and 1886.
For the past decade, the U.S. Geological Survey has focused research on understanding the origins of eastern seismicity. One natural target for these investigations has been the Charleston area which experienced the 1886 event (MM X) and which continues to experience seismicity. A major multidisciplinary research program has significantly increased the understanding of Charleston seismicity and its regional tectonic setting. Results of this study are described in Professional Paper 1313, Studies
Related to the Charleston, South Carolina, Earthquake of 1886. Because several competing hypotheses exist to explain the specific tectonic origin of Charleston seismicity, resolution of these hypotheses into a workable seismotectonic model remains an important research goal. Although it is recognized that the Charleston area remains a likely area for future large earthquakes, the probability of such events elsewhere along the east coast has also been addressed. Although there is no recent or historic evidence for strong earthquakes elsewhere in the East, the fact that most tectonic elements of the Charleston area are similar to those in other parts of the East allows for the possibility that major earthquakes with long recurrence intervals may occur elsewhere in the region. A full understanding of the cause of 1886 Charleston earthquake within the regional tectonic framework is important to understanding the likelihood of strong earthquakes throughout the East.
In May 1983, a conference, The 1886 Charleston Earthquake and Its Implications for Today, gave scientists and individuals having diverse backgrounds in government, academia, and the private sector an opportunity to discuss what is known and what still needs to be known about the historic Charleston earthquake. The conference participants also discussed ex
Strategic and Critical Minerals—Potential Supply Problems and New Research Directions
In 1 979, civil war in Zaire spurred industry fears of a supply interruption and resulted in an eightfold explosion to as much as $50 per pound in the spot market price of cobalt, a metal that imparts hightemperature strength to ferroalloys and is used extensively in jet engines and for super magnets. The cobalt crisis was short lived, but it indicates how sensitive the world metal market is to threats of supply disruption. Many industrialized nations lack not only cobalt but a wide variety of minerals. Figure 1 shows that the United States is strongly dependent on imports for a large proportion of its mineral needs and that the sources of some of these materials may be vulnerable to political, economic, or even military disruption. Those minerals which are essential to our Nation's economic well being and military security and for which domestic or secure foreign sources are not assured are termed critical and strategic. The United States has not always been an import-dependent nation; indeed, within the last century, the United States was, for a brief time, a world leader in the production of such now-strategic commodities as chromium,
nickel, cobalt, and aluminum. However, as industrial demands have multiplied, as domestic deposits have been depleted, and as richer deposits in foreign lands have become developed, the United States steadily moved toward an importer status. Since the Second World War, a number of major domestic mining districts have closed, and the steady trend of increasing dependence on foreign suppliers shows that the slack has not been taken up by new domestic producers. Among those districts closed are Bunker Hill, Idaho (lead, zinc, silver), Butte, Montana (copper, silver), Keewenaw, Michigan (copper), and Ducktown, Tennessee (sulfur).
The closing of these mines has encouraged a widespread perception among persons outside of the minerals community that the United States is thoroughly explored and that no new mineral deposits remain to be discovered, but recent experience demonstrates that this is far from correct, as a few examples demonstrate: • Within the last 30 years, the world's largest lead deposit (Viburnum, Missouri) has been discovered and put into intensive operation; it now yields
about 90 percent of the Nation's lead production. Continued exploration is locating more and more mineralization in this area.
• A dozen or more substantial gold
deposits have been discovered in the Great Basin (principally Nevada). These are an unusual, low-grade, large-tonnage type only recently recognized as having economic potential.
• Large deposits of copper and zinc have
been discovered at Crandon, Wisconsin, and Bald Mountain, Maine.
• A recently discovered extensive zone of
platinum metals in the Stillwater complex of Montana is probably the world's second largest deposit (after the Merensky reef deposit in the Bushveld complex of the Republic of South Africa).
• The Brooks Range in Alaska is the site
of two large new, and as yet undeveloped, deposits, Arctic Camp (copper and zinc) and Red Dog (zinc). Although there has been a continuing flow of new discoveries, the levels of exploration, discovery and development have lessened, reflecting a decline in the health of the industry and a gradual reduction of good exploration targets. Furthermore, despite some success in finding additional ore deposits of the types already known to be present, the platinum find in the Stillwater, Montana, and the nickel find at Brady Glacier, Alaska, are the only major discoveries of minerals high on the strategic and critical list. Chromium, manganese, cobalt, tin, aluminum, tantalum, and many others still are predominantly imported, and some of the supplies may not be dependable in the future. To assure that the Nation is in the best possible position with respect to minerals, the Geological Survey's program of basic mineral research is now focused on strategic and critical minerals. Three main elements comprise this program: systematic local and regional mineral surveys, investigations of the geologic controls on mineralization, and establishment and maintenance of the data base concerning the geologic availability of minerals at home and abroad.
Three general components of regional mineral mapping are included within this program: Conterminous U.S. Mineral Assessment Program (cusmap), in which
large areas are mapped at a scale of 1 inch equals 4 miles in blocks of about 8,000 square miles each (1 latitude by 2 longitude) (fig. 2). The 48 conterminous States contain about 150 such quadrangles which are favorable for the occurrence of strategic and critical minerals. Eleven of these have been assessed since the program began in 1977, and 12 more quadrangles are in progress. Studies include geologic mapping; geochemical sampling of stream sediments, soils and rocks; and geophysical investigations. This work provides a basis to delineate certain areas as having potential for mineral deposits; because different geologic settings possess different probabilities for the occurrence of different minerals (for example, lead and zinc are often found in dolomitized carbonate rocks, whereas uranium occurs in sandstones), the resulting mineral assessment map may be highly complex with several kinds of potential indicated. Priorities for the study of Cusmap quadrangles are assigned on the basis of the amount of Federal land to be assessed, the level of previously known mineral potential, the probability that the study will identify significant mineral potential, and interest from States and from other Federal agencies.
In Alaska, a similar program called Alaska Mineral Resource Assessment Program (amrap) is systematically covering the State (fig. 3). Difficulty of access is a severe problem in many parts of Alaska, but, nevertheless, about 41 percent of the quadrangles containing significant mineralization in the State have been mapped and assessed thus far.
Under the provisions of the Wilderness Act of 1964, Public Law 88-577, and the Federal Land Policy and Management Act of 1976, Public Law 94-579, the Geological Survey, in cooperation with the Bureau of Mines, conducts mineral surveys of certain lands designated and considered for incorporation into the National Wilderness Preservation System. Since the passage of the Wilderness Act in 1964, about 800 tracts of Forest Service lands totalling more than 42 million acres have been assessed, and, since 1978, 36 tracts under Bureau of Land Management jurisdiction totalling about 3 million acres have been studied, with emphasis on strategic and critical minerals. Where practical, such tracts have been integrated into the Cusmap and Amrap programs, but, in