Here is a compilation of essays on ‘Earthquakes’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Earthquakes’ especially written for school and college students.
Essay on Earthquakes
- Essay on Introduction to Earthquake
- Essay on the Causes of Earthquakes
- Essay on the Classification of Earthquakes
- Essay on the Hazardous Effects of Earthquakes
- Essay on the World Distribution of Earthquakes
Essay # 1. Introduction to Earthquake:
An earthquake is a major demonstration of the power of the tectonic forces caused by endogenetic thermal conditions of the interior of the earth. ‘An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking buildings apart and causing gaping fissures to open in the ground.
The earthquake is a form of energy of wave motion transmitted through the surface layer of the earth in widening circles from a point of sudden energy release, the focus’. ‘An earthquake is a vibration or oscillation of the surface of the earth caused by a transient disturbance of the elastic or gravitational equilibrium of the rocks at or beneath the earth the surface.’
The magnitude or intensity of energy released by an earthquake is measured by the Richter Scale devised by Charles F. Richter in 1935. The number indicating magnitude or intensity (M) on Richter scale ranges between 0 and 9 but in fact the scale has no upper limit of number because it is a logarithmic scale.
It is estimated that the total annual energy released by all earthquakes is about 1025 ergs, most of this is from a small number of earthquakes of magnitude over 7. The 1934 Bihar earthquake measuring 8.4 and Good Friday Earthquake of March 27, 1964 in Alaska (USA) measuring 8.4 to 8.6 on Richter scale are among the greatest earthquakes of the world ever recorded.
The place of the origin of an earthquake is called focus which is always hidden inside the earth but its depth varies from place to place. The deepest earthquake may have its focus at a depth of even 700 km below the ground surface but some of the major Himalayan earthquakes, such as the Bihar-Nepal earthquake of August 21, 1988, have their focus around 20- 30 km deep.
The place on the ground surface, which is perpendicular to the buried ‘focus’ or ‘hypocentre’, recording the seismic waves for the first time is called epicentre. The waves generated by an earthquake are called ‘seismic waves’ which are recorded by an instrument called seismograph or seismometer at the epicentre. The science, that deals with the seismic waves, is called seismology.
Essay # 2. Causes of Earthquakes:
Earthquakes are caused mainly due to disequilibrium in any part of the crust of the earth. A number of causes have been assigned to cause disequilibrium or isostatic imbalance in the earth’s crust such as volcanic eruptions, faulting and folding, up-warping and down-warping, gaseous expansion and contraction inside the earth, hydrostatic pressure of man-made water bodies like reservoirs and lakes, and plate movements.
If we look at the world distribution of earthquakes (fig. 10.2) it appears that the earthquake belts are closely associated with the weaker zones and isostatically disturbed areas of the globe. It was generally believed that isostatically balanced and old and stable rigid masses were free from seismic events but the devastating earthquake of Koyna on 11 December, 1967, in Satara district of Maharashtra, Latur-Kilari earthquake of Sept. 30, 1993 of Maharashtra, dis proved this old connotation and made us believe that no part of the earth is immune from seismic events. A host of possible causes have been suggested to cause disequilibrium in the earth’s crust which trigger earth tremors of various sorts.
Volcanic activity is considered to be one of the major causes of earthquakes. In fact, vulcanicity and seismic events are so intimately related to each other that they become cause and effect for each other. In other words, each volcanic eruption is followed by earthquakes and many of the severe earthquakes cause volcanic eruptions.
In fact, earth tremors are major precursor events of possible volcanic eruption in immediate future in any region. The explosive violent gases during the process of vulcanicity try to escape upward and hence they push the crustal surface from below with great force and thus is caused severe earth tremor of high magnitude.
Whenever these gases become successful in breaking the weak crustal surface they appear on the earth’s surface with violent explosion and great force causing devastating volcanic eruption which causes sudden disequilibrium in the crustal surface to invite severe earth tremors. It may be pointed out that the magnitude of such earthquakes depends upon the intensity of volcanic eruptions.
The violent eruption of Krakatoa volcano (between Java and Sumatra) caused such a severe earthquake the impact of which was experienced as far away as Cape Horn (some 12,800 km away). The devastating earthquake generated 30 to 40 m high tsunamis waves which killed 36,000 people in the coastal areas of Java and Sumatra.
ii. Faulting and Elastic Rebound Theory:
The horizontal and vertical movements caused by endogenetic forces result in the formation of faults and folds which in turn cause isostatic disequilibrium in the crustal rocks which ultimately causes earthquakes of varying magnitudes depending on the nature and magnitude of dislocation of rock blocks caused by faulting and folding. In fact, sudden dislocation of rock blocks caused by both tensile and compressive forces triggers immediate earth tremors due to sudden maladjustment of rock blocks.
The 1950-earthquake of Assam was believed to have been caused due to disequilibrium in crustal rocks introduced by crustal fracture. The 1934-earthquake of Bihar was also considered to have been triggered by faulting activity underneath. Underground active fault zone was suggested as one of the possible causes of Koyna earthquake (Maharashtra) of December 11, 1967.
The occurrence of severe devastating earthquake of San Francisco (USA) in 1906 led H.F. Reid, one of the official investigators of the San Fransisco earthquake disaster, to advance his famous and much appreciated elastic rebound theory to explain the mode and causes of earthquakes mainly caused by fractures and faults in the earth’s crust and upper mantle.
According to Reid the underground rocks are elastic like rubber and expand when stretched and pulled. The stretching and pulling of crustal rocks due to tensile forces is slow process. The rocks continue to be stretched so long as the tensile forces do not exceed the elasticity of the rocks but as the tensile forces exceed the rocks elasticity, they are broken and the broken rock blocks try immediately to occupy their previous positions so that they may adjust themselves. All these processes occur so rapidly that the equilibrium of the concerned crustal surface is suddenly disturbed and hence earth tremors are caused.
Reid’s elastic rebound theory very well explains the occurrences of seismic events in Californian valley which is very much frequented by faulting activity. The famous earthquake of 1872 of California was caused due to creation of a massive fault in the Oven Valley. Similarly, the Californian earthquake of April 18, 1906, was caused due to the formation of 640 km long San Andreas Fault. The 1923 earthquake of Sagami Bay of Japan was also believed to have been triggered by big fault.
N. Krishna Brahman and Janardhan G. Niyogi, the two scientists of the National Geophysical Research Institute, have opined that the seismic events near Bhatsa Dam and Koyna Dam are very much active due to active faulting beneath the Deccan Traps. They have claimed to have identified two active rift faults in Maharashtra beneath the Deccan Traps viz. Kurduvadi rift and Koyna rift.
According to them Koyna rift begins from Kaladgi in Karnataka and runs for a distance of 540 km through Koyna and terminates 40 km west of Nasik. The 390 km long Kurduvadi rift begins from 40 km south-west of Solapur and after running through Kurduvadi it merges with the Koyna rift to the north of Pune. According to them Bhatsa Dam is located at the junction of Tawi and Koyna faults.
They are of the opinion that gradual increase in the seismic events in Bhatsa Dam area since 1983 is because of active faulting beneath the basaltic crust. The 1950 Assam earthquake, 1934 Bihar earthquake and 2001 Bhuj earthquake (Gujarat) of India were caused mainly by faulting.
iii. Hydrostatic Pressure and Anthropogenic Causes:
Though the earthquakes are natural phenomena and are caused by the endogenetic forces coming from within the earth but certain human activities such as pumping of groundwater and oil, deep underground mining, blasting of rocks by dynamites for constructional purposes (e.g., for the construction of dams and reservoirs, roads etc.), nuclear explosion, storage of huge volume of water in big reservoirs etc. also cause earth tremors of serious consequences.
The introduction of additional artificial superincumbent load through the construction of large dams and impounding of enormous volume of water in big reservoirs behind the dams cause disequilibrium of already isostatically adjusted rocks below the reservoirs or further augment the already fragile structures due to faults and fractures underneath.
Many major seismic events have been correlated with dams and reservoirs all over the world such as earthquake of 1931 in Greece due to Marathon Dam constructed in 1929; initiation of earth tremors since 1936 around Hoover Dam (USA) due to creation of Mead Lake in 1935; Koyna earthquake of 1967 (in Satara district of Maharashtra) due to Koyna reservoir constructed in 1962; other examples of earthquakes caused by dams and reservoirs are of Monteynard and Grandvale in France, Mangla in Pakistan, Kariba in Zambia, Manic in Canada, Hendrick Verwoerd in South Africa, Nourek in earst-while USSR, Kurobe in Japan etc.
It may be pointed out that the intensity of earthquake has been positively correlated with the levels of water in the reservoirs. The earthquakes caused by hydrostatic pressure of reservoirs are called ‘reservoir-induced earthquakes’.
iv. Plate Tectonic Theory:
Recently, plate tectonic theory has been accepted as the most plausible explanation of the causes of earthquakes. As per theory of the plate tectonics the crust or the earth is composed of solid and moving plates having either continental crust or oceanic crust or even both continental-oceanic crust.
The earth’s crust consists of 6 major plates (Eurasian plate, American plate, African plate, Indian plate, Pacific plate and Antarctic plate) and 20 minor plates. These plates are constantly moving in relation to each Other due to thermal convective currents originating deep within the earth.
Thus, all the tectonic events take place along the boundaries of these moving plates. From the stand point of movement and tectonic events and creation and destruction of geomaterials the plate boundaries are divided into:
(i) Constructive plate boundaries,
(ii) Destructive plate boundaries, and
(iii) Conservative plate boundaries.
Constructive plate boundaries represent the trailing ends of divergent plates which move in opposite directions from the mid-oceanic ridges, destructive plate boundaries are those where two convergent plates collide against each other and the heavier plate boundary is sub-ducted below the relatively lighter plate boundary and conservative plate boundaries are those where two plates slip past each other without any collision.
Major tectonic events associated with these plate boundaries are ruptures and faults along the constructive plate boundaries, faulting and folding along the destructive plate boundaries and transform faults along the conservative plate boundaries. All sorts of disequilibrium are caused due to different types of plate motions and consequently earthquakes of varying magnitudes are caused.
Normally, moderate earthquakes are caused along the constructive plate boundaries because the rate of rupture of the crust and consequent movement of plates away from the mid-oceanic ridges is rather slow and the rate of upwelling of lavas due to fissure flow is also slow. Consequently, shallow focus earthquakes are caused along the constructive plate boundaries or say along the mid-oceanic ridges.
The depth of ‘focus’ of earthquakes associated with the constructive plate boundaries ranges between 25 km to 35 km but a few earthquakes have also been found to have occurred at the depth of 60 km. It is, thus, obvious that the earthquakes occurring along the mid-Atlantic Ridge, mid- Indian Oceanic Ridge and East Pacific Rise are caused because of movement of plates in opposite directions (divergence) and consequent formation of faults and ruptures and upwelling of magma or fissure flow of basaltic lavas (fig. 10.1).
Earthquakes of high magnitude and deep focus are caused along the convergent or destructive plate boundaries because of collision of two convergent plates and consequent subduction of one plate boundary along the Benioff zone. Here mountain building, faulting and violent volcanic eruptions (central explosive type of eruptions) cause severe and disastrous earthquakes having the focus at the depth up to 700 km.
This process, convergence of plates and related plate collision, explains the maximum occurrence of earthquakes of varying magnitudes along the Fire Ring of the Pacific or the Circum-Pacific Belt (along the western and eastern margins of the Pacific Ocean or say along the western coastal margins of North and South Americas and thus the Rockies to Andes Mountain Belt and along the eastern coastal margins of Asia and island arcs and festoons parallel to the Asiatic coast).
The earthquakes of the Mid-Conti- nental Belt along the Alpine-Himalayan chains are caused due to collision of Eurasian plates and African and Indian plates. The earthquakes of the western marginal areas of North and South Americas are caused because of subduction of Pacific plate beneath the American plate and the resultant tectonic forces whereas the earthquakes of the eastern margins of Asia are originated because of the subduction of Pacific plate under Asiatic plate.
Similarly, the subduction of African plate below European plate and the subduction of Indian plate under Asiastic plate cause earthquakes of the mid-continental belt. The severe earthquake of Bhuj of Jan. 26, 2001 (Gujarat, India) was caused due to reactivated subsurface faults due to subduction of Indian plate below Asiatic plate.
Creation of transform faults along the conservative plate boundaries explains the occurrence of severe earthquakes of California (USA). Here one part of California moves north-eastward while the other part moves south-westward along the fault plane and thus is formed transform fault which causes earthquakes.
Essay # 3. Classification of Earthquakes:
It has become apparent after the discussion of the causes of seismic events that there is wide range of variation in the nature and magnitude of earthquakes. Each earthquake differs from the other and thus it becomes difficult to classify all the earthquakes into certain categories.
Inspite of these limitations earthquakes are classified on the basis of common characteristics as given below.
i. Classification on the basis of Causative Factors:
(A) Natural earthquakes are those which are caused by natural processes i.e., due to endogenetic forces.
These are further divided into four subcategories:
(i) Volcanic earthquakes are caused due to volcanic eruptions of explosive and fissure types. Generally, volcanic earthquakes are confined to volcanic areas. The intensity and magnitude of such earthquakes depend on the intensity and magnitude of volcanic eruptions. Examples, severe earthquakes caused by violent explosions of Krakatao volcano in 1883 and Etna volcano in 1968.
(ii) Tectonic earthquakes are caused due to dislocation of rock blocks during faulting activity. Such earthquakes are very severe and disastrous. Examples, 1872 earthquake and 1906 earthquake of California (USA), 1923 earthquake of Sagami Bay (Japan), 2001 earthquake of Gujarat etc.
(iii) Isostatic earthquakes are triggered due to sudden disturbance in the isostatic balance at regional scale due to imbalance in the geological processes. Generally, the earthquakes of active zones of mountain building are included in this category.
(iv) Plutonic earthquakes are infact deep-focus earthquakes which occur at greater depths. The centres (foci) of these earthquakes are generally located within the depths ranging from 240 km to 670 km.
(B) Artificial or man-induced earthquakes or anthropogenic earthquakes are caused by human activities such as pumping of water and mineral oil from underground aquifers and oil reserves respectively, deep underground mining, blasting of rocks by dynamites for constructional purposes (e.g., for the construction of dams and reservoirs, roads etc.), nuclear explosion, storage of huge volume of water in big reservoirs etc.
Examples, 1931 earthquake of Greece due to Marathon Dam, 1936 earthquake of Hoover Dam (USA) due to Lake Mead, Koyna earthquake (Maharashtra, India) of 1967 due to Koyna reservoir etc.
ii. Classification on the basis of Focus:
Guttenberg has divided the world seismic centres on the basis of the depths of their foci into 3 types viz.:
(i) Moderate earthquakes—foci are located at the depths from the ground surface (0 km) to 50 km,
(ii) Intermediate earthquakes-seismic foci at the depths between 50 km and 250 km and
(iii) Deep focus earthquakes-seismic foci at the depths between 250 km and 700 km. Moderate and intermediate earthquakes are also called as shallow focus and intermediate focus earthquakes respectively.
iii. Classification on the basis of Human Casualties:
Earthquakes are grouped into 3 categories on the basis of their hazardous impacts in terms of human casualties:
(i) Moderately hazardous earthquakes- When human deaths caused by severe seismic tremors are below 50,000 mark. Examples, Kamakura earthquake of Japan of 1293 A.D. (22,000 deaths), Tabas earthquake of Iran of 1978 A.D. (25,000 deaths), Armenian earthquake of erstwhile USSR of 1988 (26,000 deaths), Lisbon earthquake of Portugal in 1531 A.D. (30,000 deaths), Chile earthquake of 1939 A.D. (40,000 deaths), Quito earthquake of Ecudador in 1797 A.D. (41,000 deaths), Calabria earthquakes of Italy in 1783 A.D. (50,000 deaths), North Iranian earthquake of 1990 A.D. (50,000 deaths) etc.
(ii) Highly hazardous earthquakes causing human deaths ranging between 51,000 and 1,00,000 occurred in 1268 (in Silicia, Asia Minor, death toll, 60,000), in 1667 (in Shemaka, Caucasia, death toll 60,000), in 1693 (Catania, Italy, 93,000 deaths), in 1693 (Naples, Italy, 93,000 deaths), in 1932 (Kansu, China, human deaths, 70,000), in 1935 (Quetta, Baluchistan, death toll, 60,000), in 1970 (Chimbote, Peru, 67,000 deaths), in 2001 (Bhuj, Gujarat, 50,000-1,00,000 death) etc.
(iii) Most hazardous earthquakes causing human casualitis above 1,00,000 mark occurred in the year 1290 (in Chihli, China, 1,00,000 deaths), in 1556 (in Shen-Shu, China, 8,30,000 deaths), in 1737 (Kolkata, India, 3,00,000 deaths), in 1908 (in Messina, Italy, 1,60,000 deaths), in 1920 (in Kansu, China 1,80,000 deaths), in 1923 (in Tokyo, Japan, 1,63,000 deaths), in 1967 (in Tang-Shan, China 7,50,000) deaths etc.
Essay # 4. Hazardous Effects of Earthquakes:
It may be pointed out that the intensity of earthquakes and their hazardous impacts are determined not on the basis of the magnitude of seismic intensity as determined by Richter scale but are decided on the basis of quantum of damages done by a specific earthquake to human lives and property.
An earthquake becomes hazard and desaster only when it strikes the populated area. The direct and indirect disastrous effects of earthquakes include deformation of ground surfaces, damage and destruction of human structures such as buildings, rails, roads, bridges, dams, factories, destruction of towns and cities, loss of human and animal lives and property, violent devastating fires, landslides, floods, disturbances in groundwater conditions etc.
i. Slope Instability and Failures and Landslides:
The shocks produced by earthquakes particularly in those hilly and mountainous areas which are composed of weaker lithologies and are tectonically sensitive and weak cause slope instability and slope failure and ultimately cause landslides and debris falls which damage settlements and transport systems on the lower slope segments.
The shocks generated by Peruvian earthquake of May, 1970 triggered off the collapse of ice caps seated on the peak of high mountain called Huascaran of 6654 m height near the town of Yungay in Peru.
The huge masses of falling ice dislodged thousands of tonnes of rock mass from the said mountain and thus was generated a gigantic debris flow down the slope of Huascaran mountain travelling at the speed of 320 km per hour. The enormous mass of debris flow covered a distance of 15 km within few minutes and buried many buildings and human structures of Yungay town and killed about 25,000 people.
ii. Damage to Human Structures:
Earthquakes inflict great damage to human structures such as buildings, roads, rails, factories, dams, bridges, and thus cause heavy loss of human property. It may be pointed out that in the ground surface composed of unconsolidated geomaterials, such as alluvium, colluvium, artificially infilled and levelled depressions, swamp deposits reclaimed through the dumping of coarse sands and city garbages the vibrations of earthquakes last longer and the amplitudes of seismic waves are greater than in the structures of consolidated materials, and bedrocks. Thus, the earthquakes cause more damages in the areas of unconsolidated ground than their counterparts in the regions of solid structures and bedrocks.
Two major earthquakes of Bihar-Nepal border in 1934 and 1988 can explain the impact of earthquake disasters on human structures and human lives. The damage caused by the Bihar earthquake of 15 January, 1934, measuring 8.4 on Richter scale, include 10,700 human deaths, landslides and slumping in an area of 250 km length and 60 km width, ruptures and faults in the ground surface etc. which caused irreparable damage to human structures.
The Darbhanga (Bihar) earthquake of 21 August, 1988 measuring only 6.5 magnitude on Richter scale (1000 times smaller than the great earthquake of 1934 in intensity) damaged 25,000 houses due to unconsolidated Gangetic alluvium which in fact acted as a seismic amplifier. The disastrous earthquake of Mexico city of 1985 (September) caused total collapse of 400 buildings, damage to 6,000 buildings and moderate damage to 50,000 buildings.
Besides, the infrastructures of the city were seriously damaged, for example, water pipes were broken, telecommunication lines and systems were severely damaged, power and water supplies were disrupted, inner vehicular transport was halted etc.
The severe earthquake of 9 February, 1971 in the San Fernando valley, located to the north-west of Los Angeles (USA) caused total collapse of Olive New Hospital in Sylmar. This damage shocked everybody because this building was constructed in conformity with the earthquake resistance standards. Uttar Kashi (Uttaranchal) earthquake of 1991 and Latur-Kilari quake (Maharashtra) of 1993 (India) flattened many buildings.
iii. Damages to the Towns and Cities:
Earthquakes have their worst effects on towns and cities because of highest density of buildings and large agglomerations of human populations. The earth tremors of higher magnitudes shake the ground to such an extent that large buildings collapse and men and women are hurried under large debris and rubbles of collapsed structural materials of buildings, ground water pipes are bent and damaged and thus water supply is totally disrupted, electric poles are uprooted and electric and telephone wires and cables are heavily damaged causing total disruption of electric supply, obstruction and destruction of sewer systems causes epidemics, road blocks throw the transport systems out of gear etc.
Kolkata city was severely damaged due to severe earthquake of 11 October, 1737 as thousands of buildings were severely damaged and 3,00,000 people were killed. The sad tale of the destruction of Mexico city due to the earthquake of 1985 has already been described. Recent Bhuj earthquake of Gujarat (Jan. 26, 2001) flattered towns of Anjar and Bhuj destroying more than 90 percent buildings.
iv. Loss of Human Lives and Property:
It may be pointed out that it is not the intensity (magnitude of Richter scale) of earthquake alone which matters more as regards the human casualities but it is the density of human population and houses which matter more in terms of human deaths and loss of property.
For example, the Kangra earthquake of India in 1905 recorded 8.6 magnitude on Richter scale but it could cause deaths of only 20,000 people whereas 1976 Tang-Shan earthquake of China measuring 7.8 to 8.1 on Richter scale killed 7,50,000 people.
More than 40,000 people lost their lives in the devastating earthquake of Turkey (August 17,1999) which recorded 7.4 on Richter scale. The loss of human lives caused by earthquakes has been enumerated in the preceding section on the classification of earthquakes based on human casualities (see also tables 10.1, 10.2, 10.3).
The strong vibrations caused by severe earthquakes strongly shake the buildings and thus strong oscillations cause severe fires in houses, mines and factories because of overturning of cooking gas cylinders, contact of live electric wires, churning of blast furnaces, displacement of other electric and fire- related appliances. For example, the house wives were cooking their lunches in the kitchens when disastrous killer earthquake struck in the vicinity of Tokyo and Sagami Bay in 1923.
Consequently, severe fire broke out which claimed the lives of 38,000 people out of total fatalities of 1,63,000 caused by the earthquake through various processes. This earthquake resulted into total loss of property worth 2,500 million US dollars. The severe earthquake of San Fransisco (USA), which occurred on April 18, 1906, caused widespread fires in several parts of the city.
No water could be made available immediately to extinguish the fire because water pipes were also broken and displaced by the earthquake. Two biggest oil refineries of Turkey were completely devastated due to fire caused by the killer earthquake of August 17, 1999 (7.4).
vi. Deformation of Ground Surface:
Severe earth tremors and resultant vibrations caused by severe earthquakes result in the deformation of ground surface because of rise and subsidence of ground surface and faulting activity. For example, the Alaska (USA) earthquake of 1964 caused displacement of ground surface upto 10-15 metres.
The 1897-Assam earthquake caused a large fault measuring 10.6 m (35 feet) wide and 19.3 km (12 miles) long. Several faults were created in the mouth areas of the Mississippi river because of the earthquakes of 1811, 1812 and 1813 in the Mississippi valley. The alluvial-filled areas of the flood plains of the Mississippi were fractured at many places which forced ground surface at few places to collapse. This process resulted in the formation of lakes and marshes.
The ground surface was greatly deformed in the delta area of the Indus River (in Pakistan) due to the earthquake of 1819 as an area of 4,500 square kilometres was submerged beneath sea water and this land area disappeared for ever. It may be pointed out that subsidence in one area is followed by emergence of the land in other area.
This also happened in the Indus delta area as a large area measuring 80 km in length and 26 km in width was raised by 3 m from the surrounding area. Similarly, the coastal land of Chile was raised from 6m to 13 m because of the earthquake of 1835. The seafloor of Sagami Bay of Japan was subsided from 305 m to 457 m because of the earthquake of 1923.
vii. Flash Floods:
Strong seismic events result in the damages of dams and cause severe flash floods. Severe floods are also caused because of blocking of water flow of rivers due to rock blocks and debris produced by severe tremors on the hill slopes facing the river valleys. Sometimes, the blockade of the rivers is so immense that even the main course of the river is changed.
The 1950 earthquake of Assam produced barrier in the Dihang river, the tributary of the Brahmaputra River, due to accumulation of huge debris caused by landslides triggered by earth tremors and thus caused severe flash floods in the upstream sections. Similarly, the dam on Subansiri River broke in and resultant flash flood submerged an area of 770 square kilometres.
The seismic waves, caused by the earthquakes travelling through sea water, generate high sea waves and cause great loss of life and property. Since the Pacific Ocean is girdled by the ring of earthquakes and volcanoes tsunamis are more common in the Pacific with a minimum frequency of 2 tsunamis per year. The Kutch earthquake of June 16, 1819 generated strong tsunamis which submerged the coastal areas and inflicted great damage to ships and country-made boats of the fishermen.
The land area measuring 24 km in length was raised upward because of tectonic movement triggered by the said earthquake which provided shelter to the stranded and marooned people. This is why the people called this raised land as Allah’s Bund (bund created by the God). The great tsunamis caused by the Lisbon earthquake of the year 1755 (in Portugal) generated about 12 m high sea waves which damaged most parts of Lisbon city and killed 30,000 to 60,000 people.
The impact of this earthquake was so enormous that the waters of inland lakes like Looh Lomond and Looh Ness continued to oscillate for several hours. The strong tsunamis triggered by Lisbon earthquake also caused 3.5 m to 4.5 m high waves as far away as the West Indies. The earthquake caused by violent volcanic eruption of Karakatoa in 1883 caused enormous tsunamis which generated 36.5 m high sea waves which ravaged the coastal areas of Java and Sumatra and killed 36,000 people.
Tsunami: Historical Perspective:
The waves generated in the oceans triggered by high magnitude earthquakes in the ocean floors (exceeding 7.5 on Richter scale), or by violent central volcanic eruptions, or by massive landslides of the coastal lands or of submerged continental shelves and slopes or in deep oceanic trenches, are called tsunami, which is a Japanese word meaning thereby harbour waves.
The tsunamis are long waves (with longer wavelengths of 100 km or more) which travel at the speed of hundreds of kilometers per hour but are of shallow in depth in deeper oceans and seas. As these waves approach coastal land, the depth of oceanic water decreases but the height of tsunamis increases enormously and when they strike the coast, they cause havoc in the coastal areas.
The best example of tsunami induced by violent volcanic eruption is from Krakatao eruption which occurred in 1883. Severe earthquake caused by Krakatao eruption generated furious tsunami waves ranging in 30 to 40 meters in height (average being 120 feet or 36.5 m). These waves were so violent that they ravaged the coasts of Java and Sumatra and killed 36,000 people.
Since the Pacific Ocean is girdled by convergent plate boundaries and the ring of earthquakes and volcanoes, tsunamis are more common in the Pacific with a minimum frequency of 2 tsunamis per year. The great tsunamis caused by the Lisbon earthquake (Portugal) of the year 1755 generated about 12 m high sea waves which damaged most parts of Lisbon city and killed 30,000 to 60,000 people.
The Kutch earthquake of June 16, 1819 generated strong tsunamis which submerged the coastal areas. The land area measuring 24 km in length was raised upward because of tectonic movements. The raised land was called as Allah’s Bund (bund created by the God).
The following are the significant tsunamis in the second half of the 20th century and 21st century:
(1) Aleutian tsunami:
April 1,1946, generated by Aleutian earthquake of the magnitude of 7.8 on Richter scale, the resultant tsunami with a height of 35 m killed many people in Alaskan and Hawaiian coastal areas.
(2) Kamchatka tsunami:
Nov. 4,1952, earthquake of the magnitude of 8.2, generated Pacific-wide tsunami with a wave height of 15 m.
(3) Aleutian tsunami:
March 9, 1957, earthquake of the magnitude of 8.3 on Richter scale, generated a Pacific-wide tsunami of 16 m height and adversely affected Hawaii islands.
(4) Chilean tsunami:
May 22, 1960, a strong earthquake of the magnitude of 8.6 on Richter scale, generated Pacific-wide tsunamis and claimed 2,300 human lives in Chile.
(5) Alaskan tsunami:
March 28,1964, a strong earthquake of the magnitude of 8.4 on Richter scale, generated 15 m high tsunami and killed more than 120 people in Alaska.
(6) Papua New Guirea tsunami:
July 17, 1998, a moderate intensity (7.00n Richter scale) submarine earthquake followed by massive submarine landslides generated 30m high tsunami killing thousands of people living along the lagoon.
(7) Sumatra tsunami:
December 26, 2004, a powerful earthquake of the magnitude of 9 on Richter scale, off the coast of Sumatra with its epicenter at Simeulue in the Indian Ocean occurred at 00:58:53 (GMT), 7:58:53 (Indonesian Local Time) or 6.28 a.m. (Indian Standard Time, 1ST) and generated a powerful tsunami with a wavelength of 160 km and initial speed of 960 km/hr. The deep oceanic earthquake was caused due to sudden subduction of Indian plate below Burma plate upto 20 meters in a boundary line of 1000 km or even more (2000 km upto southern China).
This tectonic movement caused 10 m rise in the oceanic bed which suddenly displaced immense volune of water causing killer tsunami. This earthquake was largest (highest on Richter scale) since 1950 and the 4th largest since 1900 A.D. The Andaman and Nicobar group of islands were only 128 km (80 miles) away from the epicenter (Simeulue) and the east coasts of India were about 1920 km (1200 miles) away from the epicenter.
The furious tsunami with a height of about 10 m adversely affected 12 countries bordering the Indian Ocean; worst affected areas included Tamil Nadu coast and Andman-Nicobar Islands of India, Sri Lanka, Indonesia and Thailand. The strong tsunami took about 3 hours to strike Tamil Nadu coast. The killer tsunami claimed more than200,000 human lives in the affected countries wherein Indonesia, Sri Lanka and India stood 1st, 2nd and 3rd in the number of human casualties.
Japan Tsunami, 2011:
Date : March, 11, 2011; time : Japan time = 2.46 A. M., 1ST = 6.15 A. M.; undersea earth quake of 8.9 magnitude; epicenter 130 km off the coast of Sendai City near Lameng Village and 380 km north-east of Tokyo, at the depth of 10 km on sea bed; tsunami wave height 10m; more than 10,000 people killed; many cities like Miyako, Miyagi, Kesennuma were flattened; Sendai airport was inundated with heaps of cars, trucks, buses and mud deposits; aircrafts including fighter planes standing on airport were washed out by gushing tsunami waves; rotation speed of the earth increased by 16 microseconds; day length decreased by 1.6 microseconds; Honshu island was displaced by 2.4 m due to monstrous quake; earth rotational axis was displaced by 10 centimeters; 2100 km stretch of eastern coastlines having several villages, cities and towns were battered by killer tsunami; nuclear power plants in Fukushima severely damaged resulting into leakage of killer radiactive radiation; more than 5 lakh people in the radius of 20 km from Fukushima power plants were evacuated and shifted to safer places.
Essay # 5. World Distribution of Earthquakes:
If we look at the world distribution map of earthquakes (fig. 10.2) it appears that the seismic centres are closely related to certain zones of the globe. Earthquakes are, in fact, associated with the weaker and isostatically disturbed areas of the globe.
Most of the world earthquakes occur in:
(i) The zones of young folded mountains,
(ii) The zones of faulting and fracturing,
(iii) The zones representing the junction of continental and oceanic margins,
(iv) The zones of active volcanoes, and
(v) Along different plate boundaries.
The world map of the distribution of earthquakes prepared by the seismologists on the basis of computer analysis and simulation of 30,000 earthquakes that occurred between 1961 and 1967 very much coincides with the traditional map of world distribution of earthquakes (fig. 10.2) e.g.,
(1) Circum- Pacific Belt surrounding the Pacific Ocean,
(2) Mid- Continental Belt representing epicentres located along the Alpine-Himalayan Chains of Eurasia and northern Africa and epicentres of East African Fault Zones, and
(3) Mid-Atlantic Belt representing the earthquakes located along the mid-Atlantic Ridge and its offshoots. ‘The high-quality seismicity maps showed that narrow belts of epicentres coincide almost exactly with the crest of mid-Atlantic (Ridge).
The east Pacific, and the other oceanic ridges, where plates separate. Earthquake epicenters are also aligned along the transform faults, where plates slide past each other. But the earthquakes that occur at depths greater than about 100 km typically occur near margins where plates collide. It is a basic tenet of the theory of plate tectonics that these deep earthquakes actually define the positions of sub-ducted plates which are plunging back into the mantle beneath an overriding plate.
(1) Circum-Pacific Belt includes the epicentres of the coastal margins of North and South Americas and East Asia representing the eastern and western margins of the Pacific Ocean respectively. This belt accounts for about 65 per cent of the total earthquakes of the world.
This belt presents 4 ideal conditions for the occurrences of earthquakes viz.:
(i) Junction of continental and oceanic margins,
(ii) Zone of young folded mountains,
(iii) Zone of active volcanoes, and
(iv) Subduction zone of destructive or convergent plate boundaries.
The western marginal zones of North and South Americas are represented by Rockies and Andes folded mountain chains respectively. These zones are isostatically very sensitive zones because they are also the zones of convergent plate boundaries where the Pacific Oceanic plate is being continuously subducted below the American plates. Besides, these zones are also the areas of strong volcanic activity.
The earthquakes associated with the eastern coastal margins of Asia and the island arcs and festoons (Kamchatka, Sakhalin, Japan, Philippines) are caused due to the collision of the Pacific and Asiatic plates and consequent vulcanicity. Japan records about 1500 seismic shocks every year.
The recent earthquake of Mexico city in 1985 reveals the impact of collision of convergent (destructive) plate boundaries on the occurrences of earthquakes. The damage done by the devastating earthquake included death of 5,000 people, disappearance of 2,000 persons, injuries to 40,000 people, destruction of 4000 buildings, damages to 6,000 buildings, lesser damage to 50,000 buildings etc.
(2) Mid-continental belt is also known as Mediterranean Belt or Alpine-Himalayan Belt which represents the collision or subduction zones of continental plates. About 21 per cent of the total seismic events of the world are recorded in this belt.
This belt includes the epicentres of the Alpine mountains and their offshoots in Europe, Mediterranean Sea, northern Africa, eastern Africa and the Himalayan mountains and Burmese hills. This belt represents the weaker zones of folded mountains where isostatic and fault-induced earthquakes are caused due to subduction of African and Indian plates below Eurasian plate.
The Indian seismic foci are grouped into 3 zones viz.:
(i) Himalayan region,
(ii) Plain region, and
(iii) Plateau region.
The Himalayan region is a zone of maximum intensity in terms of the magnitude of seismic tremors because this zone is located in the subduction zones of the Asiatic and Indian plates where the process of mountain building is still in progress. Uttar Kashi earthquake of October 20, 1991 and Chamoli earthquake of 29 March, 1999 (all in Uttaranchal of India) are latest examples. The plain seismic region is a zone of comparatively moderate intensity.
Even the earthquakes of Assam are also included in this zone. The significant earthquakes recorded in the past in this region are 1934 earthquakes of Bihar, Assam earthquake of 1950, Kolkata earthquake of 1737 and Darbhanga earthquake (Bihar) of 1988. The peninsular Indian region is considered to be a zone of minimum intensity.
The Indian earthquakes along the Himalayas and foothill zones may be explained in terms of plate tectonics. The Asiatic plate is moving southward whereas the Indian plate is moving northward and hence the northern margin of the Indian plate is being subducted below the Asiatic plate.
The collision of Asiatic and Indian plates and resultant subduction of Indian plate and consequent folding and faulting and gradual rise of the Himalayas at the rate of 50 mm per year cause earthquakes of northern India, Tibet and Nepal.
According to J.G. Negi, P.K. Agrawal and O.P. Pandey (as reported in Hindu, September 8, 1988) the Indian subcontinent has deformed at places due to the Indian Ocean floor spreading process. India folds at places and when the energy reaches the elastic limit the rocks break up and trigger strike-slip and thrust fault earthquakes. The Himalayan fault zone is not actually one fault but a broad system of interactive faults. It consists of a complex grid of faults extending all along this colliding zone.
The earthquake belt extends through Sulaiman and Kirthar shear zones in the west, the Himalayas in the north and Burmese arc in the east. These tectonic events caused by plate movements cause earthquakes in the northern and north-eastern parts of India. Even the earthquakes of Peninsular India have been related to the active faults below deccan traps.
On the basis of magnitude of damage risk India is divided into five damage risk zones:
1. Zone I of least damage risk includes the places of some parts of Punjab and Haryana, plain areas of Uttar Pradesh, portions of plains of Bihar and west Bengal, delta area of the Godavari, coastal plain areas of Maharashtra and Kerala, desert areas of Rajasthan and most areas of Gujarat except Kutch area.
2. Zone II of low damage risk includes southern Punjab and Haryana, southern parts of plains of Uttar Pradesh, eastern Rajasthan, coastal districts of Orissa, Tamil Nadu etc.
3. Zone III of moderate damage risk represents the areas of southern and south-eastern Rajasthan, most of Madhya Pradesh, Maharashtra and Karnataka, southern Bihar(Jharkhand), northern and north-western Orissa etc.
4. Zone IV of high damage risk covers Jammu and Kashmir, Himachal Pradesh, northern Punjab and Haryana, Delhi, eastern Uttar Pradesh, ‘tarai’ and ‘bhabar’ regions and Himalayan regions of Uttaranchal and Bihar and Sikkim areas.
5. Zone V of very high damage risk includes parts of Jammu and Kashmir, some parts of Himachal Pradesh, Uttaranchal, western north Bihar (including Munger-Darbhanga), entire north eastern India and Kutch areas of Gujarat.
Though the plains of west Bengal comes under the zone of least damage risk but the devastating severe earthquake of Kolkata of 11 October, 1737, killing 300,000 people, puts a question mark against this concept. The zone of very high damage risk of Kutch region of Gujarat registered most devastating killer earthquake on Jan. 26, 2001 in its seismic history of past 182 years killing 50,000 to 100,000 people. The epicenter was located near Bhuj town.
Bhuj Earthquake (2001):
While the people of India were busy in celebrating the first republic day on Jan. 26, 2001 of the new century in different parts of the country and the programme of display of might of armed forces of the country was in progress in New Delhi, the nature demonstrated its might by rocking Kutch region of Gujarat when a severe earthquake struck at 8.45 A.M. and shook the region for almost a minute.
Within no time the villages and towns were flattened, high rise buildings collapsed, many villages and towns became heaps of debris, communication and power lines were completely disrupted, transport system was thrown out of gear and settlements became ruins. This was the second most devastating quake in the earthquake history of India after 1737 killer earthquake of Kolkata (300,000 people dead). The epicentre of this earthquake was located near Bhuj town (population, 150,000).
A moderate quake measuring 4.2 on Richter scale was registered on 24 December, 2000. The epicentre of this precursor quake was located only 22 km away from Bhuj town but no attention was paid to this precursor seismic event either by experts or by govt., agencies. The Bhuj quake of Jan. 26, 2001 was measured 6.9 on Richter scale by the Indian Meteorological Department (IMD) while the quake was measured 7.9 which was subsequently upgraded to 8.1 by the U.S.A. France and China.
National Geophysical Research Institute (NGRI) of India and Bhabha Atomic Research Centre (BARC) also confirmed the American measurement (8.1). According to Indian Meteorological Department the main reason for the difference in the magnitude of the quake was the application of different methodologies for the measurement of seismic magnitude by different countries and organizations.
It may be pointed out that the IMD uses body wave for the measurement of seismic magnitude while the USA uses shock waves for this purpose. This severe devastating earthquake killed 50,000 to 100,000 people and adversely affected 5,000,000 people. Bhachau and Anjar towns were totally flattened, 90, 60 and 50 per cent houses collapsed in Bhuj, Rajkot and Ahmedabad respectively.
If we look at the past seismic history of Gujarat, it appears that a severe earthquake occurs every 30 years e.g., Bhawnagar earthquake, 1872; Kutch earthquake, 1903; Dwarka earthquake, 1940; Broach earthquake, 1970 and Bhuj earthquake, 2001. Between 1845 and 1956 sixty six moderate earthquakes were registered in Kutch area but no one was killed, five severe and one very severe earthquakes rocked the area.
In fact, the sequence of destruction of Kutch began with the severe earthquake of June 19, 1819 (7.1 on Richter scale) when 2000 people were killed, Bhuj town was destroyed, famous mosquake of Ahmedabad was damaged, a 100 km long ridge known as Allah Bund was created (most of which is now in Sind of Pakistan, only 15 km ridge is in India) was formed etc.
The main reasons for the recent Bhuj earthquake of2001 are: sea floor spreading of Indian Ocean at the rate of 5 cm per year, gradual northward movement of Indian plate and reactivated faults below the surface. Two major connecting faults have been located in Kutch region. A 200 km long and 100 km wide fault runs east-west between Bhuj and Ahmedabad.
The second fault measuring 500 km in length and 100 km in width runs in north-south direction through Ahmedabad, Mehsana and Baroda and is known as Combay Graben. These subterranean faults intersect each other near Viramgam, Santhalpur and Radhanpur towns and become the pivot of seismic events whenever these are activated due to plate movement.
(3) Mid-Atlantic Ridge Belt includes the epicentres located along the mid-Atlantic Ridge and several islands nearer the ridge. This belt records moderate and shallow focus earthquakes which are essentially caused due to the creation of transform faults and fractures because of splitting of plates and their movement in opposite directions. Thus, the spreading of sea floor and fissure type of volcanic eruption cause earthquakes of moderate intensity.
It may be pointed out that the earthquakes that occur along the plate margins (boundaries) are well explained on the basis of plate tectonic theory but the earthquakes originating within the plates are difficult to be explained on the basis of this revolutionary theory.
For example, the earthquakes of New Madrid, Charleston, Boston, Tang-Shan, Koyna etc. are a few examples of intraplate earthquakes. Similarly, ‘the seismicity of the Indian Shield as revealed from Kutch (1819), Koyna (1967), Bhadrachalam (1969) and Broach (1970) cannot be explained easily by plate tectonics since they occurred away from plate boundary’.