Universe a grand tour of modern science Phần 4

earthquakes E S Jacques Laskar of the Bureau des Longitudes in Paris was a pioneer in the study of planetary chaos. He found many fascinating effects, including the possibility that Mercury may one day collide with Venus, and he drew special attention to chaotic influences on the orientations of the planets. The giant planets are scarcely affected, but the tilt of Mars for example, which at present is similar to the Earth’s, can vary between 0 and 60 degrees. With a large tilt, summers on Mars would be much warmer than now, but the winters desperately cold. Some high-latitude gullies on that planet have been interpreted as the products of slurries of melt-water similar to those seen on Greenland in summer. ‘All of the inner planets must have known a powerfully chaotic episode in the course of their history,’ Laskar said. ‘In the absence of the Moon, the orientation of the Earth would have been very unstable, which without doubt would have strongly frustrated the evolution of life.’ Also of relevance to the Earth’s origin are Comets and asteroids and Minerals in space. For more on life-threatening events, see Chaos, Impacts, Extinctions and Flood basalts. Geophysical processes figure in Plate motions, Earthquakes and Continents and supercontinents. For surface processes and climate change, see the cross-references in Earth system. hips that leave tokyo bay crammed with exports pass between two peninsulas: Izu to starboard and Boso to port. The cliffs of their headlands are terraced, like giant staircases. The flat part of each terrace is a former beach, carved by the sea when the land was lower. The vertical rise from terrace to terrace tells of an upward jerk of the land during a great earthquake. Sailors wishing for a happy return ought to cross their fingers and hope that the landmarks will be no taller when they get back. On Boso, the first step up from sea level is about four metres, and corresponds with the uplifts in earthquakes afflicting the Tokyo region in 1703 and 1923. The interval between those two was too brief for a beach to form. The second step, 219 earthquakes five metres higher, dates from about 800 bc. Greater rises in the next two steps happened around 2100 bc and 4200 bc. The present elevations understate the rises, because of subsidence between quakes. Only 20 kilometres offshore from Boso, three moving plates of the Earth’s outer shell meet at a triple junction. The Eurasian Plate with Japan standing on it has the ocean floor of both the Pacific Plate and the Philippine Plate diving to destruction under its rim, east and west of Boso, respectively. The latter two have a quarrel of their own, with the Pacific Plate ducking under the Philippine Plate. All of which makes Japan an active zone. Friction of the descending plates creates Mount Fuji and other volcanoes. Small earthquakes are so commonplace that the Japanese may not even pause in their conversations during a jolt that sends tourists rushing for the street. And there, in a nutshell, is why the next big earthquake is unpredictable. I Too many false alarms As a young geophysicist, Hiroo Kanamori was one of the first in Japan to embrace the theory of plate tectonics as an explanation for geological action. He was co-author of the earliest popular book on the subject, Debate about the Earth (1970). For him, the terraces of Izu and Boso were ample proof of an unstoppable process at work, such that the earthquake that devastated Tokyo and Yokohama in 1923, and killed 100,000 people, is certain to be repeated some day. First at Tokyo University and then at Caltech, Kanamori devoted his career to fundamental research on earthquakes, especially the big ones. His special skill lay in extracting the fullest possible information about what happened in an earthquake, from the recordings of ground movements by seismometers lying in different directions from the scene. Kanamori developed the picture of a subducted tectonic plate pushing into the Earth with enormous force, becoming temporarily locked in its descent at its interface with the overriding plate, and then suddenly breaking the lock. Looking back at the records of a big earthquake in Chile in 1960, for example, he figured out that a slab of rock 800 by 200 kilometres suddenly slipped by 21 metres, past the immediately adjacent rock. He could deduce this even though the fault line was hidden deep under the surface. That, by the way, was the largest earthquake that has been recorded since seismometers were invented. Its magnitude was 9.5. When you hear the strength of an earthquake quoted as a figure on the Richter scale, it is really Kanamori’s moment magnitude, which he introduced in 1977. He was careful to match it as closely as possible to the scale pioneered in the 1930s by Charles Richter of Caltech and others, so the old name sticks. The Kanamori scale is more directly related to the release of energy. 220 earthquakes Despite great scientific progress, the human toll of earthquakes continued, aggravated by population growth and urbanization. In Tangshan in China in 1976, a quarter of a million died. Earthquake prediction to save lives therefore became a major goal for the experts. The most concerted efforts were in Japan, and also in California, where the coastal strip slides north-westward on the Pacific Plate, along the San Andreas Fault and a swarm of related faults. Prediction was intended to mean not just a general declaration that a region is earthquake prone, but a practical early warning valid for the coming minutes or hours. For quite a while, it looked as if diligence and patience might give the answers. Scatter seismometers across the land and the seabed to record even the smallest tremors. Watch for foreshocks that may precede big earthquakes. Check especially the portions of fault lines that seem to be ominously locked, without any small, stress-relieving earthquakes. The scientists pore over the seismic charts like investors trying to second-guess the stock markets. Other possible signs of an impending earthquake include electrical changes in the rocks, and motions and tilts of the ground detectable by laser beams or navigational satellites. Alterations in water levels in wells, and leaks of radon and other gases, speak of deep cracks developing. And as a last resort, you can observe animals, which supposedly have a sixth sense about earthquakes. Despite all their hard work, the forecasters failed to give any warning of the Kobe earthquake in Japan in 1995, which caused more than 5000 deaths. That event seemed to many experts to draw a line under 30 years of effort in prediction. Kanamori regretfully pointed out that the task might be impossible. Micro-earthquakes, where the rock slippage or creep in a fault is measured in millimetres, rank at magnitude 2. They are imperceptible either by people or by distant seismometers. And yet, Kanamori reasoned, many of them may have the potential to grow into a very big one, ranked at magnitude 7–9, with slippages of metres or tens of metres over long distances. The outcome depends on the length of the eventual crack in the rocks. Crack prediction is a notoriously difficult problem in materials science, with the uncertainties of chaos theory coming into play. In most micro-earthquakes the rupture is halted in a short distance, so the scope for false alarms is unlimited. ‘As there are 100,000 times more earthquakes of magnitude 2 than of magnitude 7, a short-term prediction is bound to be very uncertain,’ Kanamori concluded in 1997. ‘It might be useful where false alarms can be tolerated. However, in modern highly industrialized urban areas with complex lifelines, communication systems and financial networks, such uncertain predictions might damage local and global economies.’ 221 earthquakes I Earthquake control? During the Cold War a geophysicist at UC Los Angeles, Gordon MacDonald, speculated about the use of earthquakes as a weapon. It would operate by the explosion of bombs in small faults, intended to trigger movement in a major fault. ‘For example,’ he explained, ‘the San Andreas fault zone, passing near Los Angeles and San Francisco, is part of the great earthquake belt surrounding the Pacific. Good knowledge of the strain within this belt might permit the setting off of the San Andreas zone by timed explosions in the China Sea and the Philippines Sea.’ In 1969, soon after MacDonald wrote those words, Canada and Japan lodged protests against a US series of nuclear weapons tests at Amchitka in the Aleutian Islands, on the grounds that they might trigger a major natural earthquake. They didn’t, and the question of whether a natural earthquake or an explosion, volcanic or man-made, can provoke another earthquake far away is still debated. If there is any such effect it is probably not quick, in the sense envisaged here. MacDonald’s idea nevertheless drew on his knowledge of actual man-made earthquakes that happened by accident. An underground H-bomb test in Nevada in 1968 caused many small earthquakes over a period of three weeks, along an ancient fault nearby. And there was a longer history of earthquakes associated with the creation of lakes behind high dams, in various parts of the world. Most thought provoking was a series of small earthquakes in Denver, from 1963 to 1968, which were traced to an operation at the nearby Rocky Mountain Arsenal. Water contaminated with nerve gas was disposed of by pumping it down a borehole 3 kilometres deep. The first earthquake occurred six weeks after the pumping began, and activity more or less ceased two years after the operation ended. Evidently human beings could switch earthquakes on or off by using water under pressure to reactivate and lubricate faults within reach of a borehole. This was confirmed by experiments in 1970–71 at an oilfield at Rangely, Colorado. They were conducted by scientists from the US National Center for Earthquake Research, where laboratory tests on dry and wet rocks under pressure showed that jerks along fractures become more frequent but much weaker in the presence of water. From this research emerged a formal proposal to save San Francisco from its next big earthquake by stage-managing a lot of small ones. These would gently relieve the strain that had built up since 1906, when the last big one happened. About 500 boreholes 4000 metres deep, distributed along California’s fault lines, would be needed. Everything was to be done in a controlled fashion, by pumping water out of two wells to lock the fault on either side of a third well where the quake-provoking water would be pumped in. 222 earthquakes The idea was politically impossible. Since every earthquake in California would be blamed on the manipulators, whether they were really responsible or not, litigation against the government would continue for centuries. And it was all too credible that a small man-made earthquake might trigger exactly the major event that the scheme was intended to prevent. By the end of the century Kanamori’s conclusion, that the growth of a small earthquake into a big one might be inherently unpredictable, carried the additional message: you’d better not pull the tiger’s tail. I Outpacing the earthquake waves Research efforts switched from prediction and prevention to mitigating the effects when an earthquake occurs. Japan leads the world in this respect, and a large part of the task is preparation, as if for a war. It begins with town planning, the design of earthquake-resistant buildings and bridges, reinforcements of hillsides against landslips, and improvements of sea defences against tsunamis—the great ‘tidal waves’ that often accompany earthquakes. City by city, district by district, experts calculate the risks of damage and casualties from shaking, fire, landslides and tsunamis. The entire Japanese population learns from infancy what to do in the event of an earthquake, and there are nationwide drills every 1 September, the anniversary of the 1923 Tokyo–Yokohama earthquake. Operations rooms like military bunkers stand ready to take charge of search and rescue, firefighting, traffic control and other emergency services, equipped with all the resources of information technology. Rooftops are painted with numbers, so that helicopter pilots will know where they are when streets are filled with rubble. The challenge to earthquake scientists is now to feed real-time information about a big earthquake to societies ready and able to use it. A terrible irony in Kobe in 1995 was that the seismic networks and communications systems were themselves the first victims of the earthquake. The national government in Tokyo was unaware of the scale of the disaster until many hours after the event. The provision for Japan’s bullet trains is the epitome of what is needed. As soon as a strong quake begins to be felt in a region where they are running, the trains slow down or stop. They respond automatically to radio signals generated by a computer that processes data from seismometers near the epicentre. When tracks twist and bridges tumble, the life–death margin is reckoned in seconds. So the system’s designers use the speed of light and radio waves to outpace the earthquake waves. Similar systems in use or under development, in Japan and California, alert the general public and close down power stations, supercomputers and the like. 223 ... - tailieumienphi.vn