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Sigsbee Escarpment, which marks the seaward edge of the salt deformation province (fig. 7). The extensively deformed continental slope is an important frontier for oil and gas exploration. In the central Gulf, the Mississippi Canyon and fan system is the dominant morphologic feature (fig. 8). Understanding the processes contributing to the development of the modern Mississippi Fan is important in developing exploration models for ancient fan systems. The GLORIA data provide a unique regional perspective of a deep-water-frontier region important for its oil and gas resource potential.

The first prototype digital map in the Continental Margin Map (CONMAP) Series, a digital compilation of information on the EEZ (fig. 5), has been completed. The first sheet (topography/bathymetry) for the Baltimore Canyon Trough was prepared by combining the coastline topography in digital format from the National Oceanic and Atmospheric Administration with onshore topography data from the National Mapping Division of the USGS and digitized bathymetry data. The first thematic map (tectonic) for the Baltimore Canyon Trough area is in preparation. The first thematic map for the Georges Bank area, which depicts information on the sediments in the area, is in review, and a topography/bathymetry map for the Georges Bank area is presently being prepared. The digital maps, of both the GLORIA Sonar imagery and the CONMAP thematic maps, provide a regional perspective of our knowledge of the EEZ–the Nation's underwater frontier.

Hawaiian Volcano Observatory 75th Anniversary

By Thomas L. Wright and Robert W. Decker

The 75th anniversary of the founding of the U.S. Geological Survey Hawaiian Volcano Observatory (HVO) was celebrated in January 1987. The festivities began on January 9 with the opening in

Hilo of a major exhibit at the Wailoa Center on the current work of HVO, its history, and its special relationship to the Hawaii Volcanoes National Park. In addition to spectacular photographs of volcanic activity and HVO fieldwork, the exhibit featured continuous showings of underwater footage taken during the recent eruptive activity (December 1986) and a computer monitor that displayed the island's earthquake activity in real time. A new HVO facility and an adjoining Thomas A. Jaggar Museum (fig. 9) were dedicated by the U.S. Geological Survey and the National Park Service the following week in an impressive ceremony marked by a Hawaiian oli (blessing) in honor of Pele, the Hawaiian volcano goddess. Dedication remarks were made by Wayne Marchant (Acting Assistant Secretary for Water and Science), Dallas Peck (USGS Director), William Penn Mott, Jr. (National Park Service Director), Congressman Daniel Akaka (Hawaii), Dante Carpenter (Mayor, County of Hawaii), and Gudmundur Sigvaldason (head of the World Organization of Volcano Observatories; University of Iceland, Reykjavik). Additional remarks were made by Howard Mimaki, who represented Governor John Waihee of Hawaii, and by representatives of other members of Hawaii's congressional delegation (Senators Daniel Inouye and Spark Matsunaga and Representative Pat Saiki) and the Hawaii County Council. Gordon Eaton, former Scientist-in-Charge at HVO (now president of Iowa State University) delivered an inspiring keynote address, relating HVO and its work to the origins of the Hawaii-Emperor Volcano Chain. Thomas Wright, current HVO Scientist-in-Charge, presided. All of the former Scientists-in-Charge who are still living were in attendance (with the exception of Howard Powers, who was ill). The three deceased Scientists-in-Charge were all represented by a family member. The dedication was followed by a tour of HVO and the Jaggar Museum. The following week (January 19–25, 1987), 420 scientists from 21 countries met in Hilo to discuss problems and the remarkable progress in the discipline of volcanology at the Hawaii Symposium on “How Volcanoes Work.” The convenors of the symposium were Robert Decker, Thomas Wright, and Reginald Okamura (all of the USGS) and Joseph Halbig and Richard Hazlett (University of Hawaii at Hilo). Sponsors of the meetings were the U.S. Geological Survey, University of Hawaii at Hilo, International Association of Volcanology and Chemistry of the Earth's Interior, Circum-Pacific Council for Energy and Mineral Resources, Hawaii Institute of Geophysics, American Geophysical Union, Geological Society of America, and World Organization of Volcano Observatories. The meeting began with a summary of Kilauea's current eruption; three other keynote speakers then presented their thoughts on subduction volcanoes, rift volcanoes, and hot-spot volcanoes.

Presentations at the Symposium represented the international character of the delegates and the worldwide concern for and interest in volcanoes and their effects:

Shigeo Aramaki (Earthquake Research Institute, University of Tokyo, Japan), speaking about Japanese volcanoes, noted that dehydration of the descending Pacific plate begins at a depth of about 62 miles. The volatiles and other elements leached from the descending slab contaminate a mantle layer above the slab, which at 80 to 110 miles depth begins to melt and forms diapirs that rise toward the Moho (the boundary that separates the Earth's crust from the underlying mantle). Basaltic liquids separate from the diapirs and accumulate in reservoirs in the lower crust. Magmas evolved from these deeper reservoirs rise periodically through the crust to form shallow magma chambers, from which periodic eruption of lavas and tephra of basaltic andesite, andesite, and dacite occur. Stress fields and subduction rates are the major factors affecting the character of volcanism at individual centers.

Sigvaldason (Iceland) noted that the velocity structure beneath Iceland differs from that at the Mid-Atlantic Ridge. He attributed this difference to diapiric rise of magma generated in the upper mantle into multiple horizontal reservoirs that are located within the thin crust beneath the young volcanic zones of Iceland. Sigvaldason pointed out that eruption rates in Iceland were higher near the beginning of Holocene time (10,000 years ago) and suggested that removal of the ice load from Iceland and (or) rise in sea level around Iceland may have caused this increased eruptive activity.

Robert Christiansen (USGS, United States) proposed that the classification of volcanoes into two categories, those associated with converging or diverging plate margins and those found within plates, might be too simplistic. He suggested that the Earth's crust, upper mantle, and deeper mantle may interact as a dynamic feedback system that includes the generation and transport of fluid phases. Magmatism in the Earth's crust has long-term rates that appear to be coupled to tectonic displacement rates. In addition, original magma composition, degree of partial melting, and changes in composition are all influenced by stress distributions and ascent rates. In this view, plate-margin processes may simply act to focus general magmatic processes of the same types that produce intraplate magmas. Christiansen also warned that the terms “hot spot” and “mantle plume” are becoming too closely associated. The cause of hot spots is still largely unknown.

Nearly 300 papers were presented on the following major topics: • Conceptual models of how volcanoes work • Internal and deep structure of volCanoeS • Physical and chemical dynamics of magma chambers • Physical and chemical dynamics of intrusion and eruption processes • Exploration of submarine volcanoes

Figure 9. (Above) The first Ha

waiian Volcano Observatory

(HVO), located near the site of the present Volcano House Hotel, as it appeared around 1922. (Photographer unknown; cour• Earthquakes and tremors related to volcanism • Monitoring of volcanic activity • Assessment of volcanic hazards • Reduction of volcanic risk.

tesy of the Bishop Museum, Honolulu, Hawaii.) (Below) HVO at its present site, after the addition of a new wing (with observation tower) and

renovation in 1986. (Photograph

by J. D. Griggs.)

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In addition to this formal program, the attendees viewed newly released sonar images of the 200-mile-wide underwater U.S. Exclusive Economic Zone bordering the island of Hawaii. Japanese scientists provided an update on the recent major eruption on Oshima, Japan; and there was a panel discussion on the 1986 gas cloud eruption in Cameroon.

Much progress has taken place in the study of volcanic processes in the past several years, but the symposium also served to emphasize that many problems remain. The symposium helped somewhat in removing the question mark in “How do volcanoes work?”.

Assessing the Earthquake Hazards of Urban Areas

By Walter W. Hays, Paula L. Gori, and William J. Kockleman

Major urban areas in widely scattered geographic locations across the United States are at varying degrees of risk from earthquakes. The locations of these urban areas include Charleston, South Carolina; Memphis, Tennessee; St. Louis, Missouri; Salt Lake City, Utah; Seattle-Tacoma, Washington; Portland, Oregon; and Anchorage, Alaska; even Boston, Massachusetts, and Buffalo, New York, have a history of large earthquakes. Cooperative research during the past decade has focused on assessing the nature and degree of the risk or seismic hazard in the broad geographic regions around each urban area. The strategy since the 1970's has been to bring together local, State, and Federal resources to solve the problem of assessing seismic risk. Successful cooperative programs have been launched in the San Francisco Bay and Los Angeles

regions in California and the Wasatch Front region in Utah. In each of these areas the research efforts were directed to finding answers to the following questions: • Where have earthquakes occurred in the past? • Where are they occurring now? • How often do moderate-, large-, and great-magnitude earthquakes occur? • What physical effects (seismic hazards) have been triggered by past earthquakes, and what was the nature and extent of the losses they caused? • What are the most effective options for mitigating losses from future earthquakes, as determined from the knowledge of past events and current technology? The first phase of a 5-year program to assess the seismic hazards of the Wasatch Front, Utah, region, for example, was concluded in fiscal year 1986. This assessment was conducted over a 3-year period and focused on the 10-county Brigham City– Salt Lake City–Ogden urban corridor. This urban corridor, with more than 1 million people and great building wealth, lies adjacent to the 220-mile-long active Wasatch fault zone (fig. 10). Scientists and engineers from the U.S. Geological Survey, the Utah Geological and Mineral Survey, universities, and the private sector joined together to estimate the recurrence rates and maximum size of potential earthquakes, to calculate the nature and severity of the expected ground shaking, and to identify and map areas of potential surface fault rupture, landslides, liquefaction, and other ground failures. Their findings include the following: • The Wasatch fault zone is composed of at least 10 segments (see fig. 10) having surface traces that are 9 to 36 miles in length. Each segment has been the source of repeated large-magnitude earthquakes, of about magnitude 6.5 to 7.5, occurring on the average every several hundred to several thousand years. • Ancient Lake Bonneville may have played an important role in increasing earthquake activity along the Wasatch fault. Recent studies of the slip rates on the Wasatch fault have revealed that the rate of earthquake activities accelerated with the formation of the Pleistocene-age lake and apparently has declined since its disappearance. • Measurements of small-amplitude ground motions, representing a low-strain environment at sites in the Salt Lake City– Ogden-Provo urban corridor underlain by soil and rock, showed that urban areas built in the middle of valleys filled with thick, soft deposits of clays and silts may experience severe levels of ground shaking in moderate- to large-magnitude earthquakes. These effects are 3 to 4 units of intensity (Modified Mercalli Intensity Scale) higher than those at sites on thin gravel and sand deposits on the edges of the valleys. Although these groundshaking effects may not be as serious as those that produced damage in the lake-bed zone of Mexico City in the September 19, 1985, Mexico earthquake, they are significant enough to be incorporated in the earthquake-resistant design of highoccupancy buildings and critical structures. • Potential losses to buildings and life from ground shaking in future earthquakes in the Salt Lake City region may reach $1 billion (1985 dollars) or more, depending on the location of the earthquake relative to the urban area and the time of day the earthquake occurs. • The potential for earthquake-induced liquefaction and landslides is high along the Wasatch Front. These phenomena will significantly increase the overall loss. • Geographic information system (GIS) technology was applied in a pilot study of the Sugar House 7.5-minute quadrangle and proved useful in managing and analyzing multiple geologic and geophysical data bases required for assessing regional seismic hazards. In Utah, the USGS created strong partnerships with the Utah Geological and Mineral Survey; the Utah Division of Comprehensive Emergency Management; the counties of Weber, Davis, Salt Lake, and Utah; universities; the private sector; and the Federal Emergency Management Agency. These partnerships are being used to foster implementation of earthquakepreparedness plans and mitigation measures in Utah. This second phase of the 5-year program will receive high priority in fiscal years 1987 and 1988. A new 5-year effort, identical to that along the Wasatch Front, was initiated in fiscal year 1987 in the Puget Sound, Washington—Willamette Valley, Oregon, region. Although the rate of damaging earthquakes in the Pacific Northwest appears to be low relative to that in California, earthquakes of magnitude 7.1 in

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1949 and magnitude 6.5 in 1965 caused damage collectively in excess of $200 million (1984 dollars), many injuries, and 15 deaths. Both of these earthquakes occurred at intermediate depths. The potential for two other types of events—shallow, moderate- to large-magnitude earthquakes on inland intraplate faults and a shallow, great-magnitude subduction zone event in the Cascadia subduction zone (fig. 11)—is poorly understood. Three factors may change the current assessment of the earthquake risk in the Seattle-Portland region: • A growing body of scientific data suggests that great earthquakes may be possible in the Cascadia subduction zone. • The population and urban development in the region have grown significantly; approximately 2.6 million people and about $93 billion in property are now vulnerable to the earthquake threat. • The interplay between earthquake hazards and secondary effects such as fires, chemical spills, and technological hazards may have significantly increased the potential for life and property loss.

Figure 10. Map showing the 220

mile-long Wasatch fault zone, a major zone of active normal faulting. The largest urban centers of Utah are located along the fault zone. Geologic and geomorphic evidence shows that earthquakes of magnitude 7 or greater have occurred on the fault in the past 10,000 years. One of the most impor

tant recent research results indi

cates that the fault may be composed of as many as 10 independent segments, each of which has been the source of repeated large-magnitude earthquakes.

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Figure 11. Schematic illustration of the physical processes taking place in a subduction zone where one tectonic plate is slowly being thrust over another. In the Puget Sound area, the North American plate is being thrust over the Juan de Fuca plate at a rate of approximately 1.2 inch/year. Many aspects of this process are still controversial. Technical issues that remain unresolved include (1) the present rate of convergence, (2) the physical features and seismic coupling of the Juan de Fuca and North American plates, (3) the capability of the subduction zone to rupture and produce large to great earthquakes, and (4) the range

of magnitudes, recurrence inter

vals, and physical effects of potential earthquakes.

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To carry out the research and to foster implementation of earthquake preparedness plans and mitigation measures in Washington and Oregon, the USGS has formed partnerships with the State Geological Surveys, the State offices of emergency services, universities, the private sector, and the Federal Emergency Management Agency. Efforts are being made to develop geographic information systems to manage and analyze geological, geophysical, topographical, and land-use data created by the various researchers. These efforts will collectively add to the knowledge base and ultimately lead to enactment of damage- and loss-control measures appropriate for mitigating the seismic hazard of the Pacific Northwest. In fiscal year 1990, another region of the United States will be selected for a comprehensive regional seismic hazard assessment.

Pacific Proving Grounds Revisited

By Thomas W. Henry

Atmospheric nuclear testing was conducted by the United States Government

on isolated atolls in the Pacific Ocean from 1946 through 1958. The United States carried out its only near-surface tests of megaton-range (high-yield), nuclear-fusion devices on Enewetak (fig. 12) and Bikini. Most of these shots were detonated on the small atoll islands, from towers, or from vessels anchored in the shallow lagoons. Most craters formed are now water filled, some of them with as much as 200 feet of Water. The U.S. Geological Survey completed in 1987 a cooperative, multidisciplinary program with the Defense Nuclear Agency (DNA) and other groups within the Department of Defense (DOD). The program was designed to produce geologic, geophysical, and material-properties data and interpretations to resolve a series of critical questions involving the craters formed in the Pacific Proving Grounds (PPG). Known to the DOD by the acronym PEACE (Pacific Enewetak Atoll Crater Exploration), the program was part of a larger research initiative by the DNA to better understand high-yield, strategicscale nuclear bursts and how the PPG craters relate to the basing and targeting of nuclear-weapon systems and related national defense issues. Beyond DOD's strategic concerns, data gathered during the Enewetak Program are helping to address many subjects of broad concern, including the geologic

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