HOW DO WE KNOW? How do we know?

WHAT CAUSES ? BY ANDREW ROBINSON They’ve devastated cities throughout human history, but have proved to be one of the most difficult of natural phenomena to understand

OT LONG AFTER midnight Every year in Britain, some 200 philosophers proposed natural on an ordinary evening in minor tremors are recorded by explanations. Aristotle, during the 2008, I had an uncanny seismographs. A magnitude-4 4th Century BC, believed in a ‘central seismic experience. I had just occurs every two or three fire’ inside caverns in the Earth. As finished drafting a review years on average; a magnitude-5 quake the subterranean fires burned away of a book called Apocalypse: every 10 years. In 1931, there was a the rocks, the underground caverns Earthquakes, Archaeology, quake of magnitude 6.1 – the largest collapsed, generating earthquakes. And The Wrath Of God British earthquake measured to date. Aristotle even classified earthquakes when I felt the floor of my As a rule, 90 per cent of these into types according to whether they Nupstairs flat in London shift almost tremors go undetected by the public. shook structures and people in mainly imperceptibly for a second or two. Those that are noticed – like the a vertical or a diagonal direction, and Perhaps the vibration had been caused magnitude-5.2 earthquake on 27 whether or not they were associated by a London Underground train, so I February 2008 – are nevertheless with escaping vapours. Much later, the forgot about it and went to bed. rapidly forgotten. To most people, Roman philosopher Seneca, inspired The following morning, however, earthquakes and England would in part by an Italian earthquake in the BBC radio news bulletin announced appear to have little connection with AD 62 or 63 that devastated Pompeii, that there had been an earthquake, at the almost apocalyptic earthquakes proposed that the movement of air – 12.56am. The British Geological Survey that have shaped society in countries rather than smoky vapours – trapped had monitored the event as having like China, Japan, Iran and Pakistan, and compressed within the Earth, was occurred at a depth of 5km, with its where violent shaking of the earth has responsible for both violent storms > IN A NUTSHELL epicentre in Lincolnshire, roughly killed millions of people in modern and destructive rock movements. north of London around 200km away, times. Yet, it was in Britain that and with a potentially destructive emerged as a science. Once thought to be the work of magnitude of 5.2. One serious injury The first attempts to account for MADE IN CHINA callous gods, we’ve gradually come was reported and many houses earthquakes in other than divine The first measurement of an to understand the mechanics of close to the epicentre – where the terms come from ancient Greece earthquake comes from ancient earthquakes to the point where intensity of an earthquake is greatest – and Rome. Rather than imagining China, however. The earliest known we can accurately measure them Y and even predict where the next suffered damage. This was the biggest the god Poseidon striking his trident seismometer was invented in AD A road is decimated by a magnitude-6.8 ETT earthquake that struck Ojiya, Japan, in devastating blow might strike. : G earthquake in the UK since 1984. on the ground in anger, some Greek 132 by Zhang Heng. It consisted 2004, killing 68 people OTO PH

96 / FOCUS / OCTOBER 2013 How do we know? How do we know?

of eight dragon-heads facing must have comprised a pendulum as But scientific understanding of flawed, geological paper, ‘Conjectures the eight principal directions of the primary sensing element, somehow how earthquakes form had to await concerning the cause and observations the compass. They were mounted on connected to levers that caused the the destruction of Lisbon in 1755 by upon the phaenomena of earthquakes’, CAST OF Masters of destruction: the minds who the outside of an ornamented vessel bronze balls to drop. an extremely powerful earthquake in the Royal Society’s Philosophical CHARACTERS unlocked the mystery of earthquakes said to resemble a wine jar with an According to Chinese history, in AD followed by a tsunami and fires. In Transactions for 1760. approximate diameter of 2m. Around 138 the seismometer enabled Zhang Britain, data on the effects of the He correctly concluded that the base, directly beneath the dragon- Heng to announce the occurrence of a Portuguese earthquake were collected earthquakes were ‘waves set up by Zhang Heng heads, were eight squatting toads major earthquake at Rosei, 650km to from all over the country and abroad shifting masses of rock miles below (AD 78-139) designed with open mouths. In the event of the northwest of the Chinese capital by the Royal Society, supplementing the surface’. However, his explanation the earliest recorded an earthquake, a bronze ball would Loyang – two or three days before that collected after a series of British for this shifting relied wrongly on seismometer. He was drop from a dragon-head into a news of the devastation arrived by earthquakes in 1750. John Michell, an explosions of steam when underground a Chinese astronomer toad’s mouth with a resonant clang. messengers. This prediction restored astronomer at Cambridge University, water encountered underground fires. and mathematician. In The direction of the earthquake was the faith of court officials in the took up the challenge of analysing the When the shifting occurred beneath 138, the seismometer probably indicated by which dragon- seismometer, and led the imperial eyewitness reports, and accounting the seabed, Michell also rightly measured a major head dropped its ball, unless more government to appoint a secretary for earthquake motions in terms concluded that it would produce a earthquake far from than one ball dropped, indicating a to monitor the instrument, which of Newtonian mechanics. Michell sea wave (a tsunami), as well as an the Chinese capital, more complex shaking. The device remained in existence for four centuries. eventually published an important, if earthquake. There were two types of two or three days before news of the earthquake wave, he said, once again Robert Mallet (1810-81) earthquake’s damage coming close to the truth: the first was an Irish civil arrived by messenger. was a ‘tremulous’ vibration within engineer and inventor the Earth, followed shortly by an THE KEY The movement of roads, fences and streams crossing the who became interested undulation of the Earth’s surface. in earthquakes DISCOVERY enabled Harry Fielding Reid to develop a theory of how earthquakes are triggered From this he argued that the during the 1830s. He speed of an earthquake wave could created experimental be determined by its arrival times at earthquakes with IN 1906, THE San Francisco earthquake the fault had been deformed by its The lower the friction, the weaker the fault different points on the surface. Such dynamite, investigated produced a surface rupture 435km (270 movement, and how after the earthquake and the more easily it would slip, suggested times were approximately known from the 1857 Neapolitan John Milne (1850-1913), miles) in length, and wide enough to swallow they were displaced or o!set by up to 6.4m. Reid. In places where the friction was of eyewitness reports for far-flung places earthquake through a British geologist and a cow, according to a famous folk tale of the He proposed that before the earthquake, medium size, the fault would slip frequently, affected by the Lisbon earthquake, detailed analysis of its mining engineer, was a time. The rupture happened in an area friction between the two sides of the fault producing many small earthquakes. But which enabled Michell to calculate damage to buildings, professor in Japan from named the San Andreas Fault. To explain it, had locked part of it, deforming it as the where friction was high and the fault strong, a speed for its wave of 1,930km/h. and compiled world 1876-95. His teaching, geophysicist Harry Fielding Reid published sides moved past each other – until finally it would slip only occasionally: there would He was the first scientist to attempt maps that showed the seismographs his mechanism of ‘elastic rebound’. the fault snapped. The sides sprang away be few, but large, earthquakes. While Reid’s such a calculation – unaware though earthquakes clustering he designed and his Reid had noticed that in the years before from each other and they elastically model su!ers from serious di"culties, it’s he was that the speed of seismic waves in mysterious belts. 1906, roads, fences and streams crossing rebounded, creating the surface rupture. still the best that seismologists have. varies with the types of rock through investigation of the 1891 which they pass. He then went further, Mino-Owari earthquake by theorising that the surface origin established Japanese Around 3,000 people were of an earthquake, what we now call seismology, while the killed and 80 per cent of the the epicentre, could be located by international earthquake city destroyed as a result of the combining the same data on arrival bulletin he issued after 1906 San Francisco earthquake times. Although he curiously chose his return to Britain a different – and inaccurate – way to Harry Fielding Reid led to the International SS calculate the epicentre of the Lisbon (1859-1944) served Seismological Summary RE on California’s started in 1918. NG earthquake (relying instead on reports O of the direction of the tsunami), his state commission theoretical principle is the basis of to investigate the Y OF C

R 1906 San Francisco A today’s method for locating an epicentre.

BR earthquake. An I Charles Richter L

S American geophysicist, (1900-85), like Reid,

Y, U FINDING THE EPICENTRE he closely examined was an American ET I The next major development came land movements due physicist-turned- OC in the mid-19th Century from a to earthquakes over seismologist. Working brilliant Irish civil engineer, Robert the course of the with Beno Gutenberg at AND S previous half-century. CE Mallet, who had spent two decades the California Institute N

E He then proposed that I collecting data about historical quakes. of Technology, Richter C His catalogue of world seismicity earthquakes were the devised a magnitude contained 6,831 listings, giving the result of the ‘elastic scale for Californian Y X3, S Y X3,

R rebound’ of geological A date, location, number of shocks, earthquakes in 1935. BR probable direction and duration of faults such as the San Until the 1980s, ‘Richter LI the seismic waves, along with notes Andreas Fault. magnitude’ was used OTO

H on related effects. In 1858, he internationally, but it has P

CE travelled to Naples to investigate now been replaced by a N E I the destruction wrought by more accurate ‘moment C

: S a recent major earthquake. magnitude’ scale. OTO H P

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Assessing every crack with a trained eye, Mallet compiled NEED TO KNOW The key discoveries that have enabled us to understand isoseismal maps: that is, maps with Key terms that will help you TIMELINE the mechanism of earthquakes and detect them contours of equal earthquake damage/ understand earthquakes intensity. It’s a method employed today, albeit with refinements, to The world’s first map seismic hazard. Mallet placed seismometer (pictured), EPICENTRE invented by Zhang too much reliance on the direction Heng, establishes of fallen objects and the type of 1 The area of origin of an earthquake the principle that an cracks in buildings as indicators of underground is its hypocentre or earthquake can be earthquake motion (cracking is in focus. The point on the Earth’s surface scientifically measured AD 132 fact mainly a function of the type of immediately above the hypocentre is the with an instrument building construction). But his maps epicentre, where there is often visible located far from movement and cracking of the Earth, as the epicentre. did allow him to estimate the centre of the earthquake and its size relative to surface waves radiate from the epicentre. other earthquakes. A catastrophic earthquake in Lisbon Using the new technique of leads John Michell, photography, he documented the FAULT a British clergyman damage. He then reported to the 2 At its simplest, a geological fault is and astronomer, to Royal Society in a two-volume study, a joint between two rock planes. The conclude, in the Royal Great Neapolitan Earthquake Of 1857: fault is usually not exactly vertical and 1755 Society’s Philosophical The First Principles Of Observational so one plane of the fault overhangs the Transactions in 1760, other. Fault movements and earthquakes that earthquakes are Seismology, published in 1862. ‘waves set up by shifting Elsewhere, he published maps of world are intimately connected, but their masses of rock’. seismic intensity, providing the first precise relationship is controversial. indication that earthquakes cluster in certain belts around the Earth. Robert Mallet publishes his An explanation of this fact would INTENSITY two-volume study, Great 3 The intensity of an earthquake Neapolitan Earthquake, have to wait another century, but in and maps of world seismic the meantime Mallet’s map focused measures its e!ects on objects, intensity demonstrating that 1862 scientific attention on these patterns. humans and animals. Intensity generally earthquakes cluster in certain increases the closer the observer is belts around the Earth. to the epicentre. It’s also higher for a poorly constructed building than for a Charles Richter studies earthquake GLOBAL NETWORK tremors in his laboratory in well-built one, given the same shaking. An earthquake in Over the next half-century seismology Pasadena, California – he developed San Francisco and a became a truly international science, the Richter scale in 1935 subsequent fire destroy as measuring instruments improved the city but establish the in sensitivity to the point where they MAGNITUDE discipline of seismology in were able to monitor, and record, 4 Unlike intensity, magnitude is was driving them? Not until the 1960s, achievable during the last few decades California. Harry Fielding N independent of the observer’s distance

O earthquakes all over the planet from with the advent of plate tectonic of the 20th Century, all seismologists I Reid proposes the ‘elastic T from the epicentre. It is, so to speak, IA rebound’ theory of a single location. A British geologist theory, did seismologists appreciate now admit it’s currently impossible. OC 1906 earthquakes, which is still and mining engineer, John Milne, the amount of explosive in a bomb, that the San Andreas Fault was the The scale of the task was outlined influential, if flawed. having designed more than one such as opposed to the bomb’s e!ects. boundary between two tectonic by Charles Richter, who devised

SS ASS SS seismograph while living for two An earthquake can have only one plates, the Pacific plate and the North the ‘Richter’ magnitude scale for magnitude, fundamentally; but it PRE decades in Japan, returned to Britain American plate, which were moving in measuring earthquakes in the 1930s. always has many intensities.

SS, SS, in 1895 and established a central opposite directions. Some other plate The seismologist said: “One may

RE earthquake observatory at his house boundaries, for example near Japan, compare it to the situation of a man NG Charles Richter, following the on the Isle of Wight. It had inputs from were also seismically active. Hence who is bending a board across his CO

F 1933 Long Beach earthquake a worldwide network of seismographic Instead, the tectonic movement of the fact, first noted by Mallet, that knee and attempts to determine in O

Y near Los Angeles, creates

R stations. Although Milne’s theoretical geological faults came to be seen as earthquakes clustered in bands, which advance just where and when the A a magnitude scale, which contributions were small, he has a the chief origin of earthquakes. Known were now understood to coincide with cracks will appear.” „ BR enables seismologists to allot 1935 considerable claim to be considered as the ‘elastic rebound’ model, abrupt plate boundaries. S LI a size to each Californian U earthquake, regardless of its the founder of seismology. fault movement was first proposed Yet, despite progress, seismologists varying intensities. With the vast increase in seismic by the American geophysicist Harry remain far from fully understanding Andrew Robinson is the author of Earthquake: Y X3, Y X3, data, theoretical understanding Fielding Reid in 1906 to account for earthquakes. Plate tectonics do not Nature And Culture and Earthshock ETT advanced in the first decade of the the surface rupture of the San Andreas really explain British earthquakes, far The theory of plate tectonics 20th Century. The theory that volcanic Fault in the San Francisco earthquake from the mid-Atlantic plate boundary. Y X2, G Y X2,

R explains why the vast majority A action might be related to earthquakes (see ‘The Key Discovery’, p98). What’s more, geological faults such of earthquakes cluster in belts, Find out more BR – believed by Aristotle and lent But while the model was plausible as the San Andreas have turned out LI at plate boundaries such as the ‘Ring of Fire’ around the Pacific credence by the contiguous volcanoes enough, it offered no explanation as to be very much weaker than would Forecasting Earthquakes OTO

H S and earthquakes of southern Italy and to why the sides of certain geological be expected from the elastic rebound P 1960 Ocean; but it fails to explain reveals the past, present and intra-plate earthquakes, for CE Japan – was largely abandoned when faults should be grinding past each model for large earthquakes. future of earthquake detection. N

E example in Missouri. I it became clear that active volcanoes other in a regular fashion, causing As for earthquake prediction, which www.bbc.co.uk/programmes/p01c4qgt C

: S were often free from earthquakes. periodic earthquakes. What force was touted by many seismologists as OTO PH

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