Chapter 8: Earthquakes

On 25 April 2015 the ground under Nepal moved. The tremor measured 7.8 on the Richter scale, the scale scientists use to describe how much energy an earthquake releases. Within seconds, entire villages were flattened across many districts. More than 8,000 people were killed and over 21,000 were injured. Buildings that had stood in the Kathmandu Valley for centuries, among them sites listed by UNESCO as World Heritage, came down in dust, and thousands of families were left homeless in a single morning. Then the ground refused to settle. Aftershocks kept arriving throughout Nepal at intervals of 15 to 20 minutes, so that people who had run into the open were afraid to go back indoors. On 12 May, more than a fortnight later, a major aftershock of magnitude 7.3 struck. It killed more than 200 people and injured 2,500. Nothing was added to the earth on those days and nothing was taken away. Rock that had been bending under strain for a long time simply broke, and the energy stored in it travelled outward as waves. This chapter is about that strain, that break, and the waves that reach us.

Diagram

Fig. 8.A1 Caption: The break happens deep below the ground, and the shaking we feel is only the far end of it. Brief: Cut-away view of the ground with a break along a fault at depth, the seismic focus marked on it and the epicentre marked directly above on the surface; curved arrows show waves spreading outward and upward from the focus to the surface, where buildings are shown shaking; label the depth range 8 to 35 km.

Curiosity Corner

Q1. Why do most of the world’s earthquakes happen inside a few narrow belts instead of being spread evenly over the earth? Q2. How can a single instrument work out how far away an earthquake was, without anyone going to look? Q3. How can a shock deep under the sea drown a coast that lies far away from it? Q4. Earthquakes destroy so much. Can they build anything at all? This chapter will let you answer every one of these questions for yourself.

8.1 The Meaning Of An Earthquake

An earthquake is a tremor below the surface of the earth which causes shaking of the crust. The definition points to two things at once, and they are not the same thing. There is a break somewhere below the ground, and there is the movement of the ground that reaches us at the surface.

The crust is the outer shell of solid rock on which we live, and it is thin compared with everything that lies beneath it. These devastating tremors are caused by the stresses that break the earth’s crust. Rock is strong, but it is not endlessly strong. When the force pressing on a mass of rock grows greater than the rock can bear, the rock gives way, and it gives way all at once rather than slowly.

How Often The Ground Breaks

About 20,000 earthquakes occur every year across the world, according to scientists. It is worth turning that figure into something you can picture:

\[\frac{20{,}000 \text{ earthquakes}}{365 \text{ days}} \approx 55 \text{ earthquakes per day}\]

Here 20,000 is the number of earthquakes recorded in a year and 365 is the number of days in a year, so the answer is the number of earthquakes on an average day. More than fifty tremors somewhere in the world, every day of the year, is not a rare accident. It tells us that the crust is breaking somewhere almost all of the time, and that the few earthquakes we hear about are the largest members of a very crowded family.

8.2 Causes Of Earthquakes

Earthquakes are caused by the strain in the earth’s crust, which in turn is the result of a number of factors. Rock can bend and adjust a little when it is pushed, and for years it may do exactly that. When the earth’s crust is unable to accommodate itself to the strain any longer, the result is a sudden release of tremendous energy in the form of a sudden violent shock.

A series of shocks that result from sudden earth movement, for example along a fault, are known as earthquake waves or seismic waves, the moving vibrations that carry the energy of the break outward through the rock. A fault is a fracture in the earth’s crust along which the blocks of crust on either side have moved relative to one another. The waves spread out from the seismic focus of the earthquake, the point below the ground at which the movement begins, and they can cause widespread destruction.

Where those breaks happen is not random. In areas of the world that lie along the edges of lithospheric plates, for example down the western edge of the Pacific Ocean, earthquakes are common. Lithospheric plates are the great slabs of the earth’s rigid outer layer, and their edges are the weakest lines in the crust. The severity of earthquakes is measured on the Richter scale, a numbered scale on which a higher reading means a more powerful shock.

The main factors that cause earthquakes are four, and it is worth seeing them together before taking them one at a time.

Flowchart

Fig. 8.A2 Caption: Four different factors, one shared ending — strain that the crust can no longer hold. Nodes: strain builds in the earth’s crust → crust cannot accommodate the strain → sudden release of energy → violent shock, felt as an earthquake; the strain is built by four factors → volcanoes → hot gases push the crust surface upward; → plate tectonics → plates slip past or collide → faults form along lines of weakness; → folding and faulting → compressional and tensional forces displace rocks in the crust; → man-made causes → dams, nuclear explosions, blasting and mining near fault zones

8.2.1 Volcanoes

Volcanic eruptions are one of the main causes of earthquakes. During volcanic eruptions, hot gases are pushed upward, and they push violently against the earth’s crust surface from below, forcing it upward and causing earthquakes. The shock is the crust’s answer to being shoved from underneath.

Volcanic activities and seismic occurrences are interrelated, and the link runs in both directions. An earthquake follows a volcanic eruption, and a volcanic eruption follows an earthquake.

The violent eruption of Krakatoa volcano, which stood between the islands of Sumatra and Java in Indonesia, caused a severe earthquake around a vast region. That earthquake generated a tsunami, a huge wave that races across the open ocean, and the tidal waves killed about 30,000 people in Java and Sumatra. The eruption itself was on one island; the dying happened along coastlines far from it.

8.2.2 Plate Tectonics

According to the plate tectonic theory, the crust of the earth is made of a series of moveable plates. These plates move and slide over each other, and most earthquakes occur on account of plate movements. When two plates slip past each other, or collide against each other, their edges produce faults along the lines of weakness.

The tectonic earthquakes are the most frequent of all, and they are immensely powerful. They occur in high risk regions of the Pacific Ring of South East Asia, which includes the Philippines and Indonesia, and in northern India, in each case along plate boundaries.

Curiosity Corner

Q1. Why do most of the world’s earthquakes happen inside a few narrow belts instead of being spread evenly over the earth?

A. Because the crust is made of moveable plates, and their edges are the weakest lines in it. Plates slip past or collide there, faults form, and the shocks stay concentrated along those boundaries.

In India, a severe earthquake occurred at Bhuj, in the Kachchh region of Gujarat in the west of the country, in 2001. Its cause was the lowering of the Indian plate below the Asiatic plate. India is not sitting still on the map; the plate carrying it is pressing northward, and Bhuj is what that pressure occasionally feels like.

8.2.3 Folding And Faulting

Internal horizontal and vertical movements of the crust are driven by two kinds of force. Compressional forces squeeze rock from opposite sides, and tensional forces pull it apart. Together they cause folding and faulting in the earth’s crust. A fold is a bend in layered rocks in the crust, most commonly found in sedimentary rocks, though folds can also occur in igneous and metamorphic rocks.

Folding and faulting cause displacement of rocks in the crust, which means blocks of rock end up out of the position they held before. Such an imbalance causes earthquakes.

Worked Case

WC 8.A1 — The San Andreas Slip Of 1906 The situation: In 1906 the San Andreas Fault of California, on the western coast of the USA, slipped. The figures below are the ones this chapter gives for that event. The working: The fault did not give way at a single point. It slipped along a segment about 430 km long. That slip gave rise to a major earthquake of magnitude 7.8 on the Richter scale. The shaking was followed by a devastating fire. Together, shaking and fire left a zone of destruction 400 miles long and 25 miles wide. Taking the zone as a rectangle, \[\text{Area} = 400 \text{ miles} \times 25 \text{ miles} = 10{,}000 \text{ square miles}\] so the damage covered a long, narrow strip of country rather than a circle around one town.

The shape of that number is the lesson. A fault is a line, so the destruction it causes is a strip, and the strip lies along the fault. This is why a road, a railway line or a canal that crosses a known fault is at greater risk than one that runs beside it, and why maps of past damage are drawn as bands and not as dots. It also shows that the shaking is rarely the whole story: the fire that followed in 1906 did much of the harm the earthquake began.

8.2.4 Man-Made Causes

Human activity near fault zones can also set the ground shaking. The activities the chapter lists have one thing in common: each of them adds a load, a shock or a hole to rock that was already under strain.

The construction of huge dams places a large new structure and a large new body of stored water into a river valley.
Nuclear explosions send a violent shock directly into the ground.
The blasting of rocks, done to open quarries and cut roads, breaks rock apart deliberately.
Mining removes rock from below the surface, leaving the ground above it differently supported.

The Koyna earthquake in Maharashtra, in western India, was caused by the construction of the Koyna dam in a seismically active region. The region was already one where the crust moved; the dam was built there anyway.

Cause and Effect

A Dam In A Seismically Active Region Koyna dam built in a seismically active part of Maharashtra → a large new load placed on crust already under strain → the strain released as a shock → the Koyna earthquake, now named after the dam that helped trigger it

Human activity of another kind produced the same result on another continent. Chemical wastes dumped into the ground and underground explosions in the USA caused several earthquakes in Colorado in the 1960s, in a place where nobody had gone looking for an earthquake.

Whatever sets the crust off, the shock that arrives at the surface is the same, and it meets the same brick, concrete and steel. Fig. 8.2 shows what that arrival can do to a building.

Diagram

Fig. 8.2 Caption: The damage due to an earthquake. Brief: Source a photograph of a multi-storey residential building brought down by earthquake shaking, with the lower floors crushed and rubble spilling into the street; no labels, but keep the surviving upper storeys visible so the reader can see how the building failed from below.

8.3 Anatomy Of An Earthquake

The tension caused by the movement of the earth’s plates builds up until it is released in the form of powerful vibrations. Those vibrations do not all travel in the same way. They travel in three types of waves: P or Primary waves, S or Secondary waves, which are also called Shear waves, and L or Long waves, which are also called Surface waves.

The three are told apart by how the rock particles move, by what materials the wave can cross, and by the order in which they arrive. Each type is described below.

P waves, also called primary or push waves, are the first earthquake waves to be recorded on a seismogram of an earthquake, the paper record traced out by an instrument that senses ground movement. They are compressional, which means the particles vibrate in the direction of movement of the wave, much as a sound wave behaves, and this allows them to pass through solids, liquids and gases.
S waves, also called secondary or shear waves, are recorded on a seismogram after the P waves. They are distortional waves, in which the particles vibrate at right angles to the direction of movement of the wave, so S waves travel through the earth’s interior but cannot be transmitted by liquids.
L waves, also called surface or long waves, travel along the surface of the earth and are recorded after the P and S waves. Two types of L waves are identified, Love waves and Rayleigh waves.

The Order In Which They Arrive

The P waves and the S waves travel through the interior of the earth, taking a route below the ground from the place where the break happened. The surface waves, or L waves, are the last to arrive, because they keep to the surface instead of cutting through. The S waves are slower than the P waves and do not pass through liquids at all.

It is the waves that run along the surface that make the inhabitants of the earth feel the ground motion. This is worth holding on to: the first wave to reach a place is not the one that does the shaking people remember.

Flowchart

Fig. 8.A3 Caption: One break, three waves, and a fixed order of arrival at every recording station. Nodes: rock breaks at the seismic focus → energy released as seismic waves → P waves arrive first, travelling through the interior, through solids, liquids and gases → S waves arrive next, travelling through the interior but stopped by liquids → L waves arrive last, travelling along the surface and felt as ground motion

Where An Earthquake Begins

Two terms fix an earthquake in space, one below the ground and one on it. The seismic focus is the point of origin of the seismic waves, the place inside the crust where the rock actually gave way. The epicentre is the point on the earth’s surface directly above the seismic focus, so it is not where the earthquake happened but the spot on the map nearest to where it happened.

Most of the earthquakes have their seismic focus between 8 to 35 km in depth. That is a distance you could cover in a short journey along a road, except that here it runs straight down, through solid rock, and it is why the shaking arrives at the surface spread over an area rather than at a single point.

Fig. 8.3 sets all of this out together, showing the focus, the epicentre above it, and the separate paths the three kinds of wave take through the body of the earth.

Diagram

Fig. 8.3 Caption: Earthquake waves. Brief: Cut-away circle of the earth showing crust, mantle and core, with the seismic focus marked inside and the epicentre directly above it on the surface; draw the P, S and L wave paths in three distinct line styles keyed in a legend, with S waves stopping at the core and other waves deflected there, and mark the S wave shadow zone and the P and S shadow zone as printed.

Read It

Fig. 8.3 Match each line style to the key first: L waves along the surface, P and S waves through the interior. Trace each path outward from the focus and watch where the S waves stop and where paths bend. The diagram gives the routes the waves take, not how long each one takes to arrive; only a seismogram can tell you that.

8.4 Measurements

An earthquake is over in seconds, and nobody stands at the seismic focus watching it happen. Everything we know about a particular earthquake therefore comes from instruments and from careful observation afterwards. The seismograph, the Richter scale and the Mercalli scale are used for measuring the direction and intensity of earthquakes.

8.4.1 Seismograph

The direction of movement of waves, and their passage at a particular point, is recorded by an instrument called a seismograph. It has a pen attached to it. The pen vibrates with the earthquake waves and records their movements on a travelling strip of paper, so that a shock lasting a few seconds leaves behind a permanent line that can be studied for years.

The instrument does more than draw. It also calculates the difference in the arrival of the P and S waves at that station. That interval is then used to calculate the distance between the recording station and the epicentre.

Curiosity Corner

Q2. How can a single instrument work out how far away an earthquake was, without anyone going to look?

A. P waves arrive before S waves, and the further away the break, the wider that gap grows. A seismograph measures the gap, and the gap gives the distance from the station to the epicentre.

Fig. 8.5 shows how the instrument manages to stay still while the ground beneath it moves.

Diagram

Fig. 8.5 Caption: Seismograph. Brief: Draw the instrument on a block of ground, with a heavy mass hanging from a rigid frame and labelled as resisting motion, a pen fixed to the mass touching a paper strip below, the paper labelled as moving in the direction of the arrow, and a double-headed arrow across the block labelled earth motion due to earthquake; show the trace already drawn on the paper.

The idea behind the drawing is worth noticing. When the earthquake arrives, the frame and the paper move with the ground, but the heavy mass resists motion and stays nearly still. The pen attached to that mass therefore traces the difference between a moving ground and a still point, and that difference is the record.

8.4.2 Richter Scale And The Mercalli Scale

Both these scales measure the intensity of an earthquake, that is, the amount of energy released during it. They go about it in completely different ways, and it is the difference that matters.

The Richter scale measures the power of an earthquake on a scale of 1 to 9. It measures the absolute intensity with mathematical precision, because it works from what the instrument recorded rather than from what people saw. The reading given for an earthquake on this scale is called its magnitude. Fig. 8.4 approximately shows the effect of earthquakes according to the Richter Scale reading.

Diagram

Fig. 8.4 Caption: Scale showing damage due to Earthquake. Brief: Semicircular gauge with a needle, graded from 1.0 at the left to 10 at the right, with the bands labelled Micro, Minor, Light, Moderate, Strong, Major and Great and the values 2.0 to 9.0 marked along the arc; place a small picture of a building at each band showing progressively heavier damage, from an untouched house to a collapsed one.

Read It

Fig. 8.4 Read the arc from left to right: rising magnitude, with a picture at each band showing the damage typical of that reading. Match a number to its band before you judge it. The gauge shows only what such a shock usually does, not what it will do in one particular town, where the ground and the buildings decide the outcome.

The Mercalli scale works the other way round. It grades the earthquakes on the basis of observed effects, on a 12-point scale, so its readings come from what happened to walls, windows, furniture and people rather than from a pen on paper. The Richter scale is the one most commonly used.

One feature of the Richter scale is easy to misread. Each point upward from 1 represents a 10-fold increase in the amplitude of shock waves, the size of the swing the waves make. Thus an earthquake wave of magnitude 6 will be 10 times stronger than magnitude 5.

Worked Case

WC 8.A2 — What One Point On The Richter Scale Is Worth The situation: The chapter states that each point upward on the Richter scale is a 10-fold increase in the amplitude of shock waves, and gives magnitude 6 against magnitude 5 as its example. The working: Going up by one point multiplies the amplitude by 10, so from magnitude 5 to magnitude 6 the amplitude becomes \[10 \times 1 = 10 \text{ times as large}\] Going up by two points multiplies by 10 twice over, so from magnitude 5 to magnitude 7 the amplitude becomes \[10 \times 10 = 100 \text{ times as large}\] Each further point multiplies by 10 again.

This is why two magnitudes that look close on paper are not close on the ground. The scale compresses very large differences into small-looking numbers, which is exactly what makes it useful for comparing earthquakes and exactly what makes it easy to misjudge. A reading that is one point higher is not a little worse; it is ten times the swing.

Reading The Numbers Against The Damage

India’s own records show what the numbers can and cannot settle. The Assam earthquake in India of 1897 was of 8.7 magnitude. The Gujarat earthquake in 2001 had a magnitude of only 7.7 on the Richter Scale, yet it caused the greatest damage.

The earthquake that opened this chapter belongs in the same comparison. In Nepal, an earthquake measuring 7.8 on the Richter scale occurred on April 25, 2015, which killed over 8,000 people and injured more than 21,000. It left many people homeless, with entire villages flattened across many districts of Nepal, and it destroyed centuries-old buildings including UNESCO World Heritage sites in the Kathmandu Valley. Continued aftershocks, shocks that keep arriving in the same region after the main earthquake, occurred throughout Nepal at intervals of 15 to 20 minutes. A major aftershock occurred on May 12, 2015 with a magnitude of 7.3 on the Richter scale, which killed more than 200 people and injured 2500 people.

Set the three side by side and the point becomes plain. The highest magnitude of the three did not produce the heaviest damage, so a magnitude describes the energy the earth released, not the harm it caused.

How Do We Know

How Big An Earthquake Was Evidence: The pen of a seismograph traces the amplitude of the shock waves on a moving strip of paper. Reasoning: Each point up the Richter scale stands for a 10-fold larger swing, so the size of the trace converts directly into a magnitude. Limit: The trace measures energy released, not damage done. Gujarat’s 7.7 in 2001 caused greater damage than Assam’s 8.7 in 1897.

8.5 Effects Of Earthquakes

Whether an earthquake counts as a disaster depends on who is asked. In geological terminology earthquakes are a part of endogenic forces, the forces that act from inside the earth, and they are regarded as constructive movements of the earth’s crust. From the human point of view earthquakes are regarded as destructive, causing loss to property and to life. Both statements are true at once, and the rest of this section takes them in turn.

8.5.1 Constructive Effects

Energy Release

Earthquakes help the earth release its stored up energy. The majority of the earthquakes occur around the plate margins, which is where that energy has been building. This energy release helps to keep the earth in good shape, in the sense that strain which is let go in small quantities does not go on accumulating.

Landforms

On account of both vertical and lateral displacement of the earth’s crust, earthquakes may raise or lower parts of the earth, especially near the seashore. Sagami Bay of Japan, on the Pacific coast of that country, was uplifted 200 m in 1923. Two hundred metres is roughly the height of a tall office block, and it was gained in a single event.

Earthquakes also build indirectly. Landslides triggered by earthquakes cause the formation of lakes, as has happened at many places in the Himalayan region, where the fallen material dams a valley and the water behind it has nowhere to go.

Curiosity Corner

Q4. Earthquakes destroy so much. Can they build anything at all?

A. Yes. They release strain that would otherwise keep building, they raise and lower land — Sagami Bay rose 200 m in 1923 — and the landslides they trigger dam valleys, which is how many Himalayan lakes were formed.

8.5.2 Destructive Effects

The damage an earthquake does is rarely of one kind. The shaking itself is only the beginning, and much of the harm comes from what the shaking sets in motion. The chapter lists eight destructive effects, and the first six are given below.

Collapse of structures: human beings have settlements in active earthquake zones, and often these houses and other structures collapse, causing great loss of property. The destruction caused by a tsunami is one example of such a collapse.
Submergence: the forces of uplift and subsidence also cause submergence of coastal parts, so that land which was dry becomes sea. Dwarka in Gujarat, on the western coast of India, now lies submerged under the sea.
Course of rivers: earthquakes have changed the course of rivers in the past, and a river that has left its old bed does not return to it. They have thus rendered many areas unsuitable for irrigation and agriculture.
Danger to human life: many severe earthquakes in the past have killed hundreds to lakhs of people. They died on account of house collapse, or under the weight of falling trees, poles, bridges and other structures, and the landslides and floods caused by earthquakes also kill people and animals.
Landslides: the shock waves during an earthquake destabilise the loose rocks on a slope, which then give way as landslides. This damages buildings and disturbs the transport system, and the earthquakes in the Himalayas, the Alps and the Andes all cause landslides of this kind.
Fires: during earthquakes, fires break out due to inflammable material thrown out, broken gas pipes and the bursting of gas cylinders. Damaged oil refineries and live electric wires also catch fire, damaging life and property in townships and factory complexes.

A landslide on a mountain road is worth pausing on, because it shows how a single event multiplies. The slope fails, the road is buried, and the villages beyond it are cut off from help at the very hour they need it most. Fig. 8.6 shows a road surface torn apart and carried down in exactly this way.

Diagram

Fig. 8.6 Caption: Landslides caused by Earthquake. Brief: Source a photograph of a hill road broken and displaced by a landslide, with the tarred surface cracked into slabs and slipped downslope, and vegetated hillside above; keep the road visible on both sides of the slip so the reader can see how far the ground has moved.

Tsunami

A tsunami is a large destructive ocean wave that can be generated by any disturbance that rapidly displaces a large mass of water, such as an earthquake, a volcanic eruption, a landslide or a meteorite impact. However, the most common cause is an undersea earthquake, where the sea floor itself moves and the whole column of water above it is shifted.

The name tsunami comes from the Japanese words ‘Tsu’ and ‘nami’, meaning ‘harbour’ and ‘wave’ respectively. Tsunamis are therefore harbour waves, named for where they are noticed rather than for where they begin.

Tsunamis occur most frequently in the Pacific Ocean, but they are a global phenomenon. They are possible wherever large bodies of water are found, including inland lakes, where they can be caused by landslides.

Curiosity Corner

Q3. How can a shock deep under the sea drown a coast that lies far away from it?

A. An undersea earthquake shifts a large mass of water all at once. That displacement travels outward as a large destructive ocean wave, so a coastline thousands of kilometres from the break can be struck by it.

The 2004 Indian Ocean earthquake triggered a series of lethal tsunamis on December 26, 2004 that killed approximately 2,75,000 people, more than 168,000 in Indonesia alone, making it the deadliest tsunami in recorded history. The distribution of that toll is itself geography: the country nearest the break lost the most people, and the waves went on to reach coasts far beyond it.

Diagram

Fig. 8.7 Caption: Tsunami. Brief: Source a photograph of a tsunami wave surging inland across a coastal road, with water carrying vehicles and debris and the hillside and buildings of the shore behind; show the leading edge of the water clearly so the height of the surge is readable.

Measuring The Largest Earthquakes

The Richter scale standardised earthquake magnitude measurements in the 1930s, which made it far easier to compare the relative sizes of earthquakes recorded in different places. However, the moment magnitude scale, written as \(M_w\), is considered the most accurate and reliable method for measuring earthquake size, especially for large earthquakes. It is based on the same principle as the Richter scale, and it measures the total amount of energy released during an earthquake. Table 8.1 lists the ten largest earthquakes ever recorded on this scale.

Table

Table 8.1 Caption: The ten largest earthquakes ever recorded, ranked by moment magnitude. | | Location | Date | Magnitude (\(M_w\)) | |—|—|—|—| | 1 | Valdivia, Chile | 22-05-1960 | 9.5 | | 2 | Alaska, USA | 27-03-1964 | 9.2 | | 3 | Sumatra | 26-12-2004 | 9.1 to 9.3 | | 4 | Tohoku | 11-03-2011 | 9.1 | | 5 | Kamchatka, Russia | 05-11-1952 | 9.0 | | 6 | Chile | 27-02-2010 | 8.8 | | 7 | Rat Island | 03-02-1965 | 8.7 | | 8 | Assam-Tibet | 15-08-1950 | 8.6 | | 9 | Aleutian Islands | 01-04-1946 | 8.6 | | 10 | Indian Ocean | 11-04-2012 | 8.6 |

Read It

Table 8.1 Read down the magnitude column rather than the rank column. All ten sit between 8.6 and 9.5, so the gaps look small on paper but stand for very large differences in energy. One entry is a range, 9.1 to 9.3, because the largest events are hard to pin to a single value. The table ranks size only, never damage or loss of life.

Flash Floods

Strong shock waves damage dams and embankments, and the water stored behind them spreads speedily, causing flash floods. The danger does not end when the shaking does.

Debris produced during the earthquake also blocks the flow of the river. The river flow then gets diverted from its original course, and this diversion causes flash floods that damage life and property, sometimes in villages that were never touched by the shaking itself.

8.6 Distribution Of Earthquakes

Most earthquakes originate from plate boundaries, which is why a world map of earthquakes is a map of lines rather than of scattered dots. In regions where an ocean plate and a land plate converge, submarine ridges are formed on the sea floor. The underlying rocks there are unstable and subjected to movement, and any such movement triggers earthquakes in the continental areas nearby.

These submarine ridges join with the fold mountain ranges on the continents, so the shaking follows a continuous track that runs from the ocean floor onto land. The main earthquake belts of the world are three, and between them they account for all of the world’s earthquakes.

The Circum Pacific Mountain Belt has 70% of all earthquakes. A part of the San Andreas Fault in the USA lies in this belt, and this fault-zone makes California in the USA highly prone to earthquakes, while the areas of Japan, the Philippines and Indonesia also form parts of this belt and are prone to severe earthquakes.
The Mid-world Mountain Belt stretches from Eastern Europe and covers the Alpine-Himalayan ranges in Europe and Asia. About 20% of all earthquakes occur in this zone, which is the belt northern India sits inside.
The Mid-Atlantic Ridge comprises areas along the mid-oceanic ridges as well as many islands near the ridges of the Atlantic Ocean, and the remaining 10% of earthquakes occur in this belt. In this ridge two tectonic plates move in opposite directions, some moderate earthquakes are recorded along it, and the Rift Valley of East Africa is considered an extension of the belt.

Those three figures, 70%, 20% and 10%, add to the whole hundred. That is worth saying out loud, because it means an earthquake outside these belts is not merely uncommon; on this reckoning there is no share of the total left over for it.

Why Here

Why California Shakes Factors: The Circum Pacific Mountain Belt runs along it; a part of the San Andreas Fault lies inside that belt; the belt follows plate boundaries, where faults form along lines of weakness. The decisive one: The plate boundary. The fault exists because the boundary is there. Move it: Shift the plate boundary offshore and California becomes ordinary ground.

Fig. 8.1 shows how these belts lie across the world.

Map

Fig. 8.1 Caption: Distribution of Earthquake belts. Brief: World map at the printed scale of about 1:69,000,000 showing the major earthquake belts as continuous shaded bands and the lava plateaus in a second shade, with a key for both; label the Circum Pacific Mountain Belt and the Midworld Mountain Belt, the named plateaus as printed, Vesuvius and Mauna Loa, the Equator, the two tropics, the Arctic and Antarctic circles and the Prime Meridian; show the full extent of India as officially defined; do not mark individual earthquake epicentres.

Read It

Fig. 8.1 Check the key first: one shade is the earthquake belts, the other the lava plateaus. Trace the belts and notice they follow coasts and mountain chains rather than country borders. At this scale a band is hundreds of kilometres wide, so the map shows where earthquakes cluster, never which town will shake or when.

One Belt, One Night In The Sunda Strait

A tsunami struck Java and Sumatra in Indonesia on December 22, 2018, resulting in more than 300 deaths. It was likely caused by undersea landslides due to the ongoing eruption of the Anak Krakatau volcano, which stands in the Sunda Strait, the channel of sea between Java and Sumatra.

Indonesia is an archipelago, a country made up of a great many islands, and it lies on the Pacific Ring of Fire. It includes thousands of volcanic islands, which have been created over time as plates shifted and molten magma exerted pressure. Living on that belt is what gives Indonesia its islands and its soil, and it is the same fact that puts its coasts at risk.

8.7 Predicting Earthquakes

Earthquake prediction in the past was left to astrologers and mystics, because nobody had an instrument that could see into the crust. Today it is a respectable scientific pursuit, carried out by seismologists, the scientists who study earthquakes and the waves they send out.

The starting point was the waves themselves. The Russians were the first to discover P and S seismic waves in the 1960s, and predictions were made on the basis of these waves. They still are the only means available to seismologists to predict earthquakes.

The theory of plate tectonics offers another means of prediction on scientific lines, since the boundaries where plates meet are the places worth watching. The Japanese use the methods of measuring changes in sea level and variations in the earth’s magnetic field to predict earthquakes. The earth behaves as though it carries a giant magnet, and instruments can pick up small changes in that field; the reasoning is that rock under rising strain does not behave exactly as it did before.

However, no method is still foolproof, as has been proved by recent earthquakes in Japan, India and the USA. These are three countries with careful monitoring and long records, and earthquakes have arrived in all three without a warning that was good enough. Prediction, for the present, means knowing where and roughly how often, not knowing the day.

How Do We Know

Whether An Earthquake Can Be Predicted Evidence: Records of P and S waves, plate boundary positions, and Japanese measurements of sea level change and magnetic field variation. Reasoning: Rock under rising strain alters what these instruments record, so a change is read as a warning. Limit: No method is foolproof. Recent earthquakes in Japan, India and the USA arrived anyway.

New Words

Seismic focus — the point of origin of seismic waves. Epicentre — the point on the earth’s surface directly above the seismic focus. Fold — a bend in layered rocks in the crust; folds are most commonly found in sedimentary rocks, but they can also occur in igneous and metamorphic rocks. Fault — a fracture in the earth’s crust along which the blocks of crust on either side have moved relative to one another. Fault plane — the planar, that is flat, surface along which there is a slip during an earthquake; the more or less planar surface of a fault.

8.8 Summary

An earthquake is a tremor below the surface of the earth which causes shaking of the crust, and about 20,000 earthquakes occur every year across the world.
Earthquakes are caused by strain in the earth’s crust, and when the crust can no longer accommodate that strain the result is a sudden release of tremendous energy as a violent shock. Four factors build that strain.
1. Volcanoes push hot gases upward against the crust during an eruption, and volcanic activity and earthquakes are interrelated, each following the other.
2. Plate tectonics is the most frequent cause, because moveable plates slip past or collide against each other and produce faults along the lines of weakness.
3. Folding and faulting, caused by compressional and tensional forces, displace rocks in the crust and the imbalance sets off earthquakes.
4. Man-made causes include huge dams, nuclear explosions, blasting of rocks and mining near fault zones.
The energy released travels outward as seismic waves, of which three types are identified.
1. P waves arrive first, are compressional, and pass through solids, liquids and gases.
2. S waves arrive next, are distortional, travel through the earth’s interior, but cannot be transmitted by liquids.
3. L waves arrive last, travel along the surface, and are of two types, Love waves and Rayleigh waves.
The point of origin of the seismic waves is the seismic focus, and the point on the earth’s surface directly above it is the epicentre. Most earthquakes have their seismic focus between 8 to 35 km in depth.
Three means are used to measure earthquakes.
1. The seismograph traces wave movements with a pen on a travelling strip of paper, and the interval between the arrival of the P and S waves gives the distance from the station to the epicentre.
2. The Richter scale measures the power of an earthquake on a scale of 1 to 9, with each point upward standing for a 10-fold increase in the amplitude of shock waves.
3. The Mercalli scale grades earthquakes on the basis of observed effects, on a 12-point scale.
A higher magnitude does not automatically mean greater damage. The Assam earthquake of 1897 measured 8.7, yet the Gujarat earthquake of 2001, at only 7.7, caused the greatest damage.
In geological terms earthquakes are endogenic and constructive, while from the human point of view they are destructive.
1. Constructively, they release the earth’s stored up energy and they build landforms, as when Sagami Bay of Japan was uplifted 200 m in 1923 and when landslides dam valleys to form lakes in the Himalayan region.
2. Destructively, they collapse structures, submerge coastal parts as at Dwarka, change the course of rivers, kill people and animals, trigger landslides, start fires, generate tsunamis and cause flash floods.
A tsunami is a large destructive ocean wave generated by any disturbance that rapidly displaces a large mass of water, most commonly an undersea earthquake. The tsunamis triggered by the 2004 Indian Ocean earthquake on December 26, 2004 killed approximately 2,75,000 people and were the deadliest in recorded history.
Most earthquakes originate from plate boundaries, and the world’s earthquakes fall into three belts.
1. The Circum Pacific Mountain Belt has 70% of all earthquakes.
2. The Mid-world Mountain Belt, covering the Alpine-Himalayan ranges, has about 20%.
3. The Mid-Atlantic Ridge has the remaining 10%, with the Rift Valley of East Africa considered an extension of it.
Prediction is now a scientific pursuit. P and S waves remain the only means available to seismologists, plate tectonics offers a second line, and the Japanese measure changes in sea level and in the earth’s magnetic field, but no method is still foolproof.

Flowchart

Fig. 8.A4 Caption: One chain runs through the whole chapter, from strain in the rock to the damage on the surface. Nodes: strain builds in the crust, from volcanoes, plate movement, folding and faulting, or human activity near fault zones → crust cannot accommodate the strain → rock breaks at the seismic focus, 8 to 35 km deep → energy released as seismic waves → P waves first → S waves next, stopped by liquids → L waves last, along the surface → waves recorded by a seismograph and graded on the Richter and Mercalli scales; the same waves reach the surface at the epicentre → constructive effects → energy released and landforms raised or lowered; → destructive effects → collapse, submergence, changed river courses, landslides, fires, tsunami and flash floods → strain begins to build again along the same plate boundary → back to strain builds in the crust

What This Chapter Cannot Tell You

The belt map in this chapter is drawn at about 1:69,000,000, and at that scale a band is hundreds of kilometres wide. It can show you that northern India lies in an earthquake belt; it cannot show you which valley, which town or which street.

A magnitude describes the energy the earth released, not the harm that followed. Nothing in the Richter reading knows what the houses were built of, how crowded the district was, or whether the shaking came at night, and the Gujarat and Assam comparison shows how far apart those two things can be.

Above all, the chapter can say where earthquakes happen and roughly how often, but not when. No method of prediction is foolproof, and that limit is not a gap in this chapter alone.

8.9 Check Your Concepts

Study Table 8.1.
1. One of the ten earthquakes has its magnitude given as a range instead of a single value. Name it, and say what this tells you about measuring the largest earthquakes.
2. Three entries in the table share the same magnitude. What does this tell you about how far apart in size the ten earthquakes really are?
Each point upward on the Richter scale represents a 10-fold increase in the amplitude of shock waves. How many times larger is the amplitude of a magnitude 8 earthquake than that of a magnitude 6 earthquake? Show every step of your working.
Study Fig. 8.5.
1. Explain how the instrument manages to draw a record of the shaking when the ground beneath it is moving as well.
2. State how the record it produces is used to work out the distance between the recording station and the epicentre.
The Assam earthquake of 1897 measured 8.7 on the Richter scale, while the Gujarat earthquake of 2001 measured only 7.7, yet the Gujarat earthquake caused the greatest damage. What does this comparison tell you about what a magnitude measures and what it does not?
What is the difference between the seismic focus and the epicentre of an earthquake?
Give a geographical reason for each of the following:
1. Most earthquakes occur on account of plate movements.
2. Primary waves are the first earthquake waves to be recorded on a seismogram.
3. Volcanic activity and earthquakes occur in the same belt.
Explain the three types of earthquake waves.
The destructive effects of an earthquake are far more than its constructive effects. Give reasons to support your answer.
Is it possible to predict an earthquake? Give reasons to support your answer.
On an outline map of the world, show the world distribution of earthquakes. Mark the earthquake prone areas in India, and give the reasons for earthquakes in each region you have marked.