YEAR IN REVIEW 1996: EARTH-AND-SPACE-SCIENCES

ASTRONOMY The year 1995 presented astronomers with another set of exciting discoveries. As insights into cometary dynamics and gas-planet atmospheric physics continued to emerge from the spectacular crash of Comet Shoemaker-Levy 9 into the planet Jupiter in 1994, a new comet was detected that could turn out to be even more spectacular. Perhaps the biggest newsmaker in astronomy was the announcement of the discovery of a planet outside the solar system orbiting a star much like the Sun. Other noteworthy reports ranged from the discovery of new satellites of the planet Saturn to a better understanding of the nature of intergalactic matter at the most distant reaches of the universe. (For information on eclipses and other standard astronomical events due to take place in 1996, see Table.) GEOPHYSICS The most deadly earthquake of 1995, having a magnitude of 7.2, struck January 17 in the vicinity of Kobe, Japan. Named the Great Hanshin Earthquake, it killed some 6,000 persons and injured more than 30,000. Nearly 200,000 buildings were destroyed or seriously damaged, and more than 300,000 people had to be housed in temporary shelters. Ground effects included liquefaction of the surface in the vicinity of the epicentre and a nine-kilometre surface fracture, with horizontal displacements reaching 1.5 m. (One kilometre is about 0.62 mi; one metre is about 3.3 ft.) Another high-fatality earthquake, having a magnitude of 7.5, occurred May 28 in and around the town of Neftegorsk, Sakhalin Island, in the Sea of Okhotsk off eastern Russia; nearly 2,000 people lost their lives. Scientists from Oregon State University mapped a blind thrust fault in Ventura county, Calif. The structure, named the Oak Ridge Fault, was designated as blind because it does not reach the surface but is overlaid by the Santa Susana thrust fault. It is the site of the Jan. 17, 1994, Northridge earthquake, which caused more than 60 deaths and major destruction throughout the stricken area. During the Northridge quake both sides of the Santa Susana Fault were displaced owing to the movement on the fault hidden beneath it. It was postulated that if a fault runs through the mountains, rather than along the edge of a valley, as is the case with the Santa Susana Fault, then it is probable that a blind fault lies beneath it. The physical mechanism by which energy is suddenly released in deep-focus earthquakes--i.e., those that occur below about 400 km depth--has long been a puzzle to seismologists. At such depths high temperature and pressure should cause rock under stress to flow smoothly rather than rupture suddenly, as it does in earthquakes near the surface. Recent studies by researchers at the University of California, Santa Cruz, showed that on average the deeper the focus, the more symmetrical the pattern of energy release over time. As recorded on a seismograph, the disturbances caused by a deep-focus earthquake tend to begin abruptly, build to a maximum, and then end relatively quickly and smoothly. The researchers believed that such a pattern is due to the uniformity of the material at the focus but could not determine whether it is the result of a rupture or a geochemical transformation that releases a burst of energy. A strong impetus to the search for an acceptable theory for deep-focus earthquakes resulted from the occurrence of the great Bolivian earthquake of June 9, 1994. At magnitude 8.2 it was the largest shock on record to have had a focus more than 600 km below the surface, at the base of the upper mantle. Upon analysis by investigators of the Carnegie Institution of Washington, D.C., and the University of Arizona, the rupture zone was found to be many times too large--it covered a horizontal area 30 50 km--to fit the currently accepted olivine-spinel transformation theory. According to that explanation, transformation under pressure of the mineral olivine into a more stable mineral, spinel, causes microfissures, which permit an earthquake to occur. Because deep-focus earthquakes generally take place beneath areas of active subduction, where the edge of one of a pair of colliding crustal plates is descending beneath the edge of the other plate, it was thought that such quakes have their origin in subducted crustal slabs that have survived the descent to deep-focus depths. Because the slab supposedly erodes and thins as it descends, however, at 600 km or deeper it would be much thinner than the size of the fracture zone calculated for the Bolivian earthquake. Several studies were under way to test various alternative theories. One speculative idea was that under the extremes of temperature and pressure at depth, some kind of nuclear reaction occurs that releases energy directly, with little or no physical deformation. As was happening in other spheres of science, geophysics was benefiting greatly from high technology. Developments in computers and instrumentation were increasing accuracies and resolution manyfold. Two techniques for exploring beneath the Earth's surface recently gained recognition. One, called cross-borehole seismology, was first used by scientists at the French Petroleum Institute in the early 1970s but did not attain wider acceptance until advances in instrumentation made it feasible. Seismic studies on the surface collect data on wavelengths of 20-100 m, while well logs (records made during well drilling) register wavelengths of 0.3-1 m and measure the environment immediately around the borehole. In contrast, cross-borehole seismology covers the range of wavelengths from two to five metres. Instruments are set up in an array, with receivers vertically spaced in one borehole and signal generators placed in surrounding boreholes at distances of 100-300 m. The generated signals are tailored so as not to damage the borehole but still be strong enough for reception. By means of multiple receivers and multistation receiver cables, it is possible to record as many as 25,000 seismograms in a few days. The analysis of the data is quite complex, combining the techniques of medical X-ray computed tomography and more conventional wave-tracing techniques of exploration seismology with enhancement from standard reflection imaging. The dramatic enhancement of rock-structure definition gained by the technique was expected to increase the detection of high-porosity zones and permeability barriers and thus help identify oil reservoirs and their dimensions. The second technique, geophysical diffraction tomography, is similarly derived from medical tomography. First developed in the early 1980s, it involves the mathematical combination of many individual signals from a specifically designed array of instruments to produce a three-dimensional image of the region traversed by the signals. As of 1995 it had been used to detect underground tunnels across the demilitarized zone between North Korea and South Korea; to trace the outline of the still unexcavated fossil bones of Seismosaurus, an enormous dinosaur discovered in the southwestern U.S.; and to map the remains of ancient underground settlements in the Negev region of Israel. Using data collected by satellites of the Global Positioning System (GPS), researchers from the University of Colorado and Stanford University found that Australia is moving north-northeast with respect to Antarctica at a rate of five to eight centimetres (two to three inches) per year. The detection of that heretofore unknown movement was made possible by means of weekly measurements of the relative positions of points all over Antarctica, Australia, Hawaii, New Zealand, Tahiti, and Tasmania carried out by GPS satellites and disseminated on the Internet. The GPS system was capable of measuring positional variations of less than 2 mm (0.08 in). Work carried out on Legs 152 through 158 of the International Ocean Drilling Program (ODP), which studied the crust beneath the world's oceans by means of the coring and extraction of rock samples from below the seafloor, was confined to the Atlantic Ocean. Exploration proceeded from sites on or near the continental shelf southeast of Greenland (Leg 152) to the Mid-Atlantic Ridge south of the Kane Fracture Zone (Leg 153), to a transept across the Ceara Rise in the western equatorial Atlantic (Leg 154), to the Amazon River deep-sea fan (Leg 155), to the deformation front of the North Barbados Ridge (Leg 156), to the Canary Basin (Leg 157), and finally to the Mid-Atlantic Ridge at latitude 26 N (Leg 158). The ODP expeditions collected data relevant to paleoceanography (study of the ocean in past ages), seafloor spreading, and the evolution of the Mid-Atlantic Ridge at those critical sites. (RUTLAGE J. BRAZEE) This updates the articles earthquake; plate tectonics. HYDROLOGY Floods and drought again played a large role in global hydrology during the year. Although flooding in the U.S. Midwest was less severe than that experienced in 1993, it continued to raise questions about the need for flood-management policy in the major river basins. California pursued its recovery from the multiyear drought of the late 1980s and early '90s with a vengeance as storms and floods hit throughout the state early in the year. In northwestern Europe flooding of the Rhine, Main, Meuse, Waal, and other major rivers during January and February was as great as it had ever been in the past 40 years. Valley residents evacuated as rivers rose throughout the subcontinent; the Rhine reached the highest level witnessed since the 18th century. Paradoxically some of the same areas later endured a summer that was among the hottest and driest on record. Flooding also plagued Morocco and Egypt, and North Korea was so badly affected that it requested aid from the UN. Drought persisted in the northeastern U.S. and the Caribbean, including Puerto Rico. Scientists speculated that the Caribbean islands were experiencing a Sahel-like dry period that recurred about every 25 years. Desperate farmers in northern Mexico watched their fields wither once again under the onslaught of a third year of drought. Water-management efforts around the globe continued to effect large-scale geologic changes and thus to raise concerns about environmental problems. Dam-building projects in India promised to create large amounts of water-storage capacity and hydroelectric power within three years, but opponents objected on environmental and social grounds since the reservoirs would flood many villages and much farmland and inundate thousands of hectares of riverside habitat. In the face of both local and worldwide criticism over population displacement and environmental damage, construction continued on the nearly 2-km (1 1/4-mi)-wide Three Gorges Dam on the Chang Jiang (Yangtze River) in China, which would form a reservoir 600 km (370 mi) long when completed. In Germany a plan to alter the flow of the Danube River with locks in order to move more commercial traffic met with vehement objections from residents all along the river. A chronically disappearing lake was caught in the act of reappearing. Lake Merzbacher in the Tien Shan Mountains of Kyrgyzstan, in Central Asia, mysteriously drains and refills on an annual, or sometimes biannual, cycle. Aerial photographic studies in 1995 recorded the lake as it returned. Interest also was focused on another hydrologic mystery in Central Asia, the rise in the level of the Caspian Sea, which has persisted since the late 1970s despite the presence of numerous hydroelectric dams and reservoirs on its inflowing rivers. As the world's largest inland sea encroached on towns and industrial sites along its shores, experts debated various explanations, including changing weather patterns, tectonic activity affecting the seafloor, increased influx from the Volga River, and even an underground shift of water from the shrinking Aral Sea, which lies about 500 km (300 mi) to the east. (N. EARL SPANGENBERG) See also Disasters: Natural. This updates the articles hydrosphere; ocean; river. METEOROLOGY AND CLIMATE An abnormally strong and southward-displaced jet stream across the Pacific Ocean, partially fueled by an unprecedentedly prolonged El Nio warming of the eastern tropical Pacific (see Sidebar), steered strong storms into the western United States that produced excessive precipitation and severe flooding across California in January and again in March. In stark contrast, a relatively mild, dry winter prevailed over the eastern United States, while a severe drought, also influenced by the El Nio, afflicted Hawaii from October 1994 to March 1995. As spring progressed, the displaced jet stream pushed strong storms into the Midwest, bringing precipitation more than twice normal to many areas between mid-April and mid-June. Water levels along the middle and upper Mississippi River, the lower and middle Missouri, and their tributaries approached but did not exceed those reached during the 1993 floods. In contrast, the aforementioned atmospheric pattern kept much of the Atlantic Seaboard unusually dry, and during the summer subnormal rainfall persisted across the Northeast and Middle Atlantic states. In July a short-lived but intense heat wave enveloped the central and eastern U.S., accounting for nearly 1,000 heat-related deaths from the High Plains to the Atlantic Seaboard, including more than 700 in the Chicago area alone. One of the most active Atlantic hurricane seasons in history, featuring 17 storms of at least tropical-storm strength through mid-October, abetted wetness across parts of the Caribbean islands, Florida, and the southern U.S. Allison, the first June hurricane in 10 years, tracked through western Florida and the south Atlantic states. In August remnants of Tropical Storm Dean inundated southeastern Texas and parts of the Great Plains, while Hurricane Erin pushed through The Bahamas before striking Florida twice, once along the central Atlantic coast and again along the western Panhandle. Subsequently, Hurricane Felix buffeted Bermuda with strong winds and heavy rain and then stalled in the western Atlantic, which resulted in prolonged high winds, rough surf, and beach erosion along the U.S. East Coast. In late August and September Hurricanes Iris, Luis, and Marilyn all battered parts of the eastern Caribbean islands. The latter two storms hit the northeastern Leeward Islands head on, causing widespread damage. All three storms stayed away from the eastern U.S., but the coastline again took a prolonged beating from rough surf and very high tides. In October yet another hurricane, Opal, struck the western Florida Panhandle with winds gusting to 232 km/h (144 mph). Opal's remnants spawned locally heavy rains and tornadoes in the East but brought much-needed rainfall to the Northeast. The storm also took 10 lives on Mexico's Yucatn Peninsula, which then was hit by Hurricane Roxanne a week later. During January and February heavy rains caused localized flooding and crop damage in south-central Brazil. In contrast, almost eight months of exceptionally dry weather were reported across east-central Brazil. April brought beneficial rains to those regions, but heavy rains farther south soaked northeastern Argentina and produced brief but severe flash flooding near Buenos Aires. Between 100 and 250 mm (4 and 10 in) of precipitation fell on saturated ground across much of central and western Europe during the last two weeks of January, pushing several rivers to levels rivaling those observed during the December 1993 "Flood of the Century." In June hot, dry weather enveloped the British Isles, eastern Europe, and western Asia. The conditions expanded across most of Europe and northwestern Africa through July and August and were particularly extreme in the British Isles. Dryness dominated many areas of southern Africa in late January and February, and above-normal temperatures further stressed crops. Late-March rains finally brought relief to most locations, although heavy rains evaded Zambia and northern Zimbabwe, where soil-moisture shortages persisted. The African Sahel wet season (May-September) was rather uneventful, with most areas receiving near-normal rains. A heat wave overspread Pakistan and northern India during June. Temperatures reached 50 C (122 F) at some locations, causing hundreds of deaths. By month's end, however, monsoonal showers had begun advancing through the region, and torrential rains fell on many locations throughout July and early August, causing sporadic river flooding. For the Indian subcontinent as a whole, the summer of 1995 was the seventh wettest since 1934. Conditions varied markedly with time and location across the Far East. Between mid-April and mid-July, heavy rains doused parts of northern Hunan and Jiangxi provinces in China, leading to severe flooding that claimed more than 1,000 lives. In addition, heavy rains during an 11-week period that ended in early September spawned severe flooding across lower northeastern China and North Korea. Beginning in April unusually wet weather also dominated southeastern China (through August) and western Japan (into late July), punctuated by Typhoon Faye, which lashed southern South Korea and western Japan in mid-July. From late July through early October, eight tropical storms or typhoons pummeled parts of the Philippines, Taiwan, southern China, and northern Vietnam. By contrast, much of central and east-central China endured abnormally dry summer conditions. Summer dryness also plagued south-central and eastern Japan before Typhoon Oscar soaked the region, including Tokyo, in September. After a rather dry start to Australia's 1994-95 wet season, the year commenced with subnormal January rains along the northwestern and eastern coastlines, but at least twice the normal January rain pelted areas from central Queensland southward through eastern Tasmania, resulting in localized flooding. Farther west, Cyclone Bobby brought rare heavy rains and locally severe flash flooding to much of Western Australia. Subnormal precipitation during March and April adversely affected agriculture in Queensland, but widespread beneficial rains fell on the eastern half of Australia during May. (ELBERT W. FRIDAY, JR.) See also Disasters: Natural. This updates the articles atmosphere; climate.