Universe a grand tour of modern science Phần 4 ppsx

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.’

E 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.

S 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,

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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.

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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.’

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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.

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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.

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Especially valuable is the real-time warning of aftershocks, which endanger

rescue and repair teams. A complication is that, in a very large earthquake, the

idea of an epicentre is scarcely valid, because the great crack can run for a

hundred or a thousand kilometres.

I Squeezing out the water

There is much to learn about what happens underground at the sites of

earthquakes. Simple theories about the sliding of one rock mass past another,

and the radiation of shock waves, have now to take more complex processes

into account. Especially enigmatic are very deep earthquakes, like one of

magnitude 8 in Bolivia in 1994. It was located 600 kilometres below the surface

and Kanamori figured out that nearly all of the energy released in the event was

in the form of heat rather than seismic waves. It caused frictional melting of the

rocks along the fault and absorbed energy.

In a way, it is surprising that deep earthquakes should occur at all, seeing that

rocks are usually plastic rather than brittle under high temperatures and

pressures. But the earthquakes are associated with pieces of tectonic plates that

are descending at plate boundaries. Their diving is a crucial part of the process

by which old oceanic basins are destroyed, while new ones grow, to operate the

entire geological cycle of plate tectonics.

A possible explanation for deep earthquakes is that the descending rocks are

made more rigid by changes in composition as temperatures and pressures

increase. Olivine, a major constituent of the Earth, converts into serpentine by

hydration if exposed to water near the surface. When carried back into the

Earth on a descending tectonic plate, the serpentine could revert to olivine by

having the water squeezed out of its crystals. Then it would suddenly become

brittle. Although this behaviour of serpentine might explain earthquakes to a

depth of 200 kilometres, dehydration of other minerals would be needed to

account for others, deeper still.

A giant press at Universita¨t Bayreuth enabled scientists from University College

London to demonstrate the dehydration of serpentine under enormous pressure.

In the process, they generated miniature earthquakes inside the apparatus. David

Dobson commented, ‘Understanding these deep earthquakes could be the key to

unlocking the remaining secrets of plate tectonics.’

I The changes at a glance

After an earthquake, experts traditionally tour the region to measure ground

movements revealed by miniature scarps or crooked roads. Nowadays they can

use satellites to do the job comprehensively, simply by comparing radar pictures

obtained before and after an earthquake. The information contained within an

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image generated by synthetic-aperture radar is so precise that changes in relative

positions by only a centimetre are detectable.

The technique was put to the test in 1999, when the Izmit earthquake occurred

on Turkey’s equivalent of California’s San Andreas Fault. Along the North

Anatolian Fault, the Anatolian Plate inches westwards relative to the Eurasian

Plate, represented by the southern shoreline of the Black Sea. The quake killed

18,000 people. Europe’s ERS-2 satellite had obtained a radar image of the Izmit

region just a few days before the event, and within a few weeks it grabbed

another.

When scientists at the Delft University of Technology compared the images by

an interference technique, they concluded that the northern shore of Izmit Gulf

had moved at least 1.95 metres away from the satellite, compared with the

southern shore of the Black Sea. Among many other details perceptible was an

ominous absence of change along the fault line west of Izmit.

‘At that location there is no relative motion between the plates,’ said Ramon

Hanssen, who led the analysis. ‘A large part of the strain is still apparent, which

could indicate an increased risk for a future earthquake in the next section of the

fault, which is close to the city of Istanbul.’

E For the driving force of plate motions and the use of earthquake waves as a means of

probing the Earth’s interior, see Plate motions and Hotspots.

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