| By Prof. Kapila Dahanayake | ||
A spate of natural disasters has assailed countries across the globe in quick succession in recent times. For Sri Lanka, with the death and destruction in the wake of the 2004 inundations of coastal areas, set off by a tsunamigenic earthquake seared into its collective memory, any natural disaster causes anxiety. Here we delve not only into the twin disasters of earthquakes and volcanoes but also look at Sri Lanka's vulnerability. Earthquakes and volcanoes are as old as the earth which was formed more than 4 billion years ago. There can be thousands of earthquakes and several volcanic eruptions in a given year. Only stronger earthquakes and tremors that cause widespread loss of life and property come to our attention with those of low magnitude going unnoticed. So what is an earthquake? Earthquakes refer to vibrations that occur suddenly on earth, when seismic waves travel inside and outside the earth at speeds of 15,000 to 20,000km per hour causing the ground to shake. These waves originate due to a fault or fracturing of rocks within the earth. The location of fracturing is known as the focus of the earthquake. During a fault, two blocks of rock move relative to each other. This movement can be vertical, horizontal or at an angle. The focus can be located at depths of several hundreds of kilometres from the earth's surface or ocean bed. The epicentre of an earthquake is identified at a point on the earth's surface or ocean bed directly above the focus. An earthquake may generate a tsunami if its focus is located at a point less than 10km below the ocean bed; the fault movement is vertical and its magnitude-M is generally greater than 6 or 6.5. A layman on the beach feeling a strong earthquake should immediately observe the sea level and if it gradually starts to rise should go to high ground for safety. The earth trembles in varying severities in different geographical locations depending on the magnitude of the earthquake and the underground geological conditions. The magnitude of the earthquake is calculated using data from seismometer networks located in different parts of the world. The magnitude as measured on the Richter scale developed in 1935 can have negative or positive values. Earthquakes (tremors) of magnitudes less than 3.4 may not be felt by all people while those above 6 can be categorized as dangerous. Those above 6.5 may develop tsunamis that can kill vast populations living in coastal areas sometimes thousands of kilometres from the epicentre like the December 26, 2004 Boxing Day earthquake which registered a magnitude of 9.3. In 1988, there were 99 earthquakes worldwide above the magnitude 6.0 with many of them causing extensive damage. When taking a closer look at volcanoes, it is evident that on average about 55 of the earth's 550 or so historically active volcanoes erupt each year. Earthquakes and volcanoes are not randomly scattered over the earth's surface but concentrated along edges of certain continents (eg. western margins of the Americas). According to the theory of plate tectonics, we know that the earth is a very dynamic planet, with its outermost shell (lithosphere) made up of a patchwork of a dozen or so (12 or 13) large, cool rigid slabs called tectonic or lithospheric plates. They are identified as either oceanic or continental plates and move horizontally relative to one another at a speed varying from less than 1 to 10cm per year. The thickness of the plates average about 80 kilometres and are composed primarily of the earth's thin surface layer (crust) and the topmost layer of the earth's interior (part of the mantle). The plates are thicker under the continents than under the oceans. These plates slide and collide with each other in very slow motion on top of the asthenosphere -- a 200km thick layer of the mantle that is hot, weak and capable of viscous flow characterized by convection currents thereby providing a mechanism for the plates to glide on top. The stress that builds up where the plates grind against each other is relieved periodically through earthquakes that occur when rocks break along faults. Volcanoes, meanwhile, are formed when molten rock material or magma rises to the surface near a plate boundary and erupts along fissures or vents. The theory of plate tectonics provides a tool to explain the distribution of earthquakes and volcanoes. They are related to boundaries between tectonic plates and geologists recognize three principal types: Divergent plate boundaries - these occur where the oceanic plates move away from each other. E.g mid oceanic ridges are impressive submarine mountain ranges identified in all ocean floors. The ocean floor spreads apart at these ridges and hot magma from the earth's mantle is injected to form new oceanic crust. Almost all of the world's divergent plate boundaries and associated abundant volcanoes are hidden by the oceans. The exception is Iceland where the mid-ocean ridge is exposed above sea level and active volcanoes are found on land. Earthquake activity along mid-ocean ridges is low and earthquakes occur at shallow depths. Convergent plate boundaries - these boundaries occur when two similar or different plates collide. Most boundaries formed due to two different plates with different densities -- oceanic and continental -- generate subduction zones because one of the plates sinks or is subducted beneath the other, forming a deep trench in the ocean floor. Collision of continental plates had formed the Himalaya mountains and such a convergence produces shallow and intermediate depth earthquakes but little volcanic activity. Earthquakes occur at the converging junction of two plates producing over 75% of the world's earthquakes at these boundaries. About two thirds of the world's sub-aerial historically active volcanoes occur at convergent plate boundaries and typically erupt explosively like the one at Krakatowa in Indonesia. Transform plate boundaries - these boundaries form when separate plates slide horizontally past one another. The friction between plates is so great that very large strains can build up in the rocks before they are periodically removed by large earthquakes. Transform boundaries produce earthquakes rather than volcanic activity. One of the best examples is the San Andreas fault zone in California, United States, which is the boundary between the Pacific and North American plates. The San Andreas fault zone serves as a field laboratory for the United States Geological Survey for its extensive earthquake research studies. Earthquakes: Looking back at Sri Lanka’s history Stories of ancient earthquakes in Sri Lanka abound in historical texts. An earthquake had killed more than 2,000 people near Colombo Fort in 1615 while in the last few decades more than 60 earthquakes (with magnitudes between 5 and 6 on the Richter scale) have been reported in and around the island of Sri Lanka.
Although there had been a few tsunamigenic earthquakes reported, not much damage had occurred except on December 26, 2004 in which 35,000 lost their lives and the coastal areas suffered extensive damage to property. The tsunami recorded in ancient texts about 2,300 years ago during the reign of King Kelanitissa resulted in the sacrifice of his daughter, Viharamahadevi to stop the ill effects of the disastrous event. About 3,000 years ago there had been subsidence of land in coastal areas due to earthquakes. Earthquake events occurring thousands of kilometres from Sri Lanka could affect the country by way of tsunamis as it happened in 2004 and also by landslides following the onset of heavy rains at locations where ground shaking was felt. Old volcanoes have not been reported in Sri Lanka and past tectonic events do not indicate such possibilities. As such volcanism can be ruled out but the occurrence of hot water wells in the eastern part of the country warrant in-depth studies. However, volcanism in the Java-Sumatra Indonesian region has slightly affected Sri Lanka by way of mild tsunamis. |
Tuesday, April 19, 2011
Earthquakes, volcanoes and Sri Lanka’s vulnerability
Sri Lanka Earthquake Facts:
Sri Lanka Earthquake in 2004 was a result of the earthquake in the Indian Ocean, which was also known as the Sumatra-Andaman earthquake. The Sri Lanka Earthquake was an undersea phenomenon and had an epicenter somewhere off the western coast of Sumatra. Sri Lanka Earthquake left a deep impact of the Sri Lanka History.
Sri Lanka Earthquake Facts:
The earthquake resulted in a number of deadly tsunamis, which wrecked havoc along the coastal regions around the Indian Ocean. The Sri Lanka Earthquake was also responsible causing similar and even greater damage to countries of Southeast Asia, which included India, Indonesia and Thailand.
Sri Lanka Earthquake Deaths:
The death toll crossed 275,000 and more than thousands were missing. The U.N. stated the Sri Lanka Earthquake to be one of the worst natural disasters seen by man. The Sri Lanka Earthquake was recored at a Richter scale of 9.0, which is actually an extreme magnitude, when nothing is spared.
Sri Lanka Earthquake and Tsunami:
The Sri Lanka Earthquake and tsunami combined their forces together to take the live of uncountable people and also destroyed property. The origin of Sri Lanka Earthquake, was in the Indian Ocean and it also triggered numerous earthquakes in other places as well. The waves of the tsunamis rose to the height of multistoreyed buildings.
The Sri Lanka Earthquake lasted for nearly 10 minutes and the time is enough to shake the whole of the planet.
Sri Lanka Earthquake Facts:
The earthquake resulted in a number of deadly tsunamis, which wrecked havoc along the coastal regions around the Indian Ocean. The Sri Lanka Earthquake was also responsible causing similar and even greater damage to countries of Southeast Asia, which included India, Indonesia and Thailand.
Sri Lanka Earthquake Deaths:
The death toll crossed 275,000 and more than thousands were missing. The U.N. stated the Sri Lanka Earthquake to be one of the worst natural disasters seen by man. The Sri Lanka Earthquake was recored at a Richter scale of 9.0, which is actually an extreme magnitude, when nothing is spared.
Sri Lanka Earthquake and Tsunami:
The Sri Lanka Earthquake and tsunami combined their forces together to take the live of uncountable people and also destroyed property. The origin of Sri Lanka Earthquake, was in the Indian Ocean and it also triggered numerous earthquakes in other places as well. The waves of the tsunamis rose to the height of multistoreyed buildings.
The Sri Lanka Earthquake lasted for nearly 10 minutes and the time is enough to shake the whole of the planet.
First Recorded Earthquake in Sri Lanka: 14th April, 1615
On the evening of April 14th, 1615, a strong earthquake struck Sri Lanka unleashing widespread damage and casualties in western sections of the island, most notably in Colombo. It is thought that 200 houses collapsed. A part of the western city wall of the Colombo Fort collapsed destroyed. A bastion also collapsed and destroyed a neighbouring house killing 4 persons. A stone bridge was also destroyed in the earthquake. Deep fissures opened in the earth. According to a historical text (reproduced below), flames and sulphur are said to have been emitted from these fissures. It is thought that 2000 persons were killed in and around Colombo as a result of this earthquake. If the description of the damage described in this text is to be believed, this earthquake would most likely have a maximum intensity (MMI) of VIII or perhaps even IX.
Knowledge of this earthquake is derived from a 4 page pamphlet published in Lisbon in 1616, the contents of which were brought to light by late Fr. S.G. Pereira, SJ a pioneer historian, proficient in several languages, Prof. in Missionology Georgian University Rome, etc. The pamphlet is titled, " "Re acion Veradadera del Espantoso Terrempoto que el ano passado de 1615 se vio en la Isla de Ceilao en las Indias Orientales. Dase cuenta de los edifices q arruyno, y muerte de muchas personas, y assi mismo de los presagios y sena les que antes del se vieron por muchos dias, assi en el cielo, como em la tierra con otras muchas cosas del mismo proposito...Impresso com licenca em Lisboa por Jorge Rodrigues, e vendese en sua casa, Anno 1616."
"A true relation of the Terrible Earthquake that in the past year of 1615 took place in the Isle of Ceylon in the East Indies: giving an account of the buildings that were destroyed, and the deaths of many persons, as also of the portents and signs that precede it for many days both in the heavens and on earth, with many other things on the same subject...Printed with permission at Lisbon by Jorge Rodriguez and sold in his shop, in the year 1616."
Sadly the writer does not give specific details but wraps the incident in a number of inane reflections and general statements. Reproduced herewith is text from that document, which appeared in an article in the Sri Lankan newspaper, the Sunday Observer, on 4th June 2000.
"The Isle of Ceylon is one of the best in the East, as well for the fertility of its soil as for the cinnamon it produces, which is the best that comes from those parts. It enjoyed for many years past the greatest prosperity both in health and abundance of harvest, with the result that the inhabitants became so proud that they almost forgot that those gifts were distributed by the hands of one who was able to turn to chastisement if they did not give him thanks for them.
"At last one day, which was the 7th of March 1615, soon after prayers there appeared in the air a terrible comet, which continued for many days to the great wonder of many but to the amazement of none. The comet had three tails, the end of which were like the heads of arrows, and so fiery and red that they seemed to be emitting rays of fire threatening therewith the total destruction of that Island.
"This sign in the heavens ceased, God showed many others on land which being nearer to men might produce in them an effect which the other had not. There died animals on the fields and men in the city without any other cause of death than the infection of the atmosphere, without even time to confess their sins.
"There were thrown up from the sea numbers of dead fish, so poisonous that all who ate them died, which caused such terror to all that there was no one who ventured to the beach to give them burial: thus the putrefaction and bad odour so infected the air that not men but even the birds of the air fell dead.
"In this way the Island continued to be depopulated as some quitted it for fear and others gave themselves to the hands of death, but these signs and rigorous chastisements did not produce the amazement of those to whom they were sent. But God is a divine surgeon and as such he heals men who are sick in soul with mild remedies till, when a member is putrefied and cankered, he applies fire and the iron to amputate it, and so he acted with these folk for seeing that they were so hardened that they did not mend at sight of the signs in the heavens nor of the chastisements on land, he wished to cut them and destroy them by a terrible earthquake, which took place on the fourteenth of April in the following manner:
"On that day the sun set half an hour earlier than on other days, for it would seem that he even hastened the chastisement as if weary of the obstinacy of the inhabitants of the Island. It was seven in the evening when thunder shook the air with such force and the earth quaked so violently that, unable to remain in the houses people rushed out to the streets fearing to be buried under the falling ruins of the buildings. Then bolts of thunder fell from the heavens, whereupon the terrified people ran into the houses hoping to shelter themselves from the rigour and justice of God. The thunderbolts had their effect, destroying and laying low not only the most sumptuous edifices but also the meanest cottages levelling all, and the few that the fire had not consumed were destroyed by the earthquakes."
"Some of those who escaped declared that they heard many voices which sounded so terrible amidst the fire that when they reached the ears that they caused fear and panic in addition to that of imminent death in which they were, though the same seemed to others to be the cries of those who were perishing under the fire of heaven and the falling stones, which rolled from side to side like light feathers or as if some strong hand were moving and hurling them about. "This lasted till about morning, which dawned bright and serene, either to show that its Author had already received satisfaction for the offences of the inhabitants of that land or in order that the survivors may seem more clearly the calamities of the dead."
Fissures in the Earth "There were seen in many parts of the Island vast openings and fissures in the earth, some of which so deep that no one could find the bottom, nor were they closed up for a long time, from which there issued at certain times flames of fire as of sulphur, as terrible and awful that some people thought they were the mouths of hell and that God showed them open to engulf them if they did not reform. "The damage which this earthquake did was moreover general throughout the Island not only to fruits, trees and crops but also to cattle and other things necessary for human life. "Great grief was caused by the destruction of a large stone bridge, built at great cost and very necessary as it was the most important passage in that Island, the stones of which were afterwards found many leagues away. "They made up for it with boats and are already considering the re-erection of the bridge because of their need of it."
"There fell a good bit of the Western wall as well as the whole of the bastion, which was so strong that they thought that nothing could destroy it. "This building brought down with it a house which was near it and in which lived some of those Moors who lived in those parts in peace, killing four and there escaped only a boy of about six years of age, who was afterwards baptized for he said he was saved from death by a most beautiful lady, the same as the Christians have in their Churches, for so he called the Virgin Mary our Lady, for whom he had a great devotion."
Knowledge of this earthquake is derived from a 4 page pamphlet published in Lisbon in 1616, the contents of which were brought to light by late Fr. S.G. Pereira, SJ a pioneer historian, proficient in several languages, Prof. in Missionology Georgian University Rome, etc. The pamphlet is titled, " "Re acion Veradadera del Espantoso Terrempoto que el ano passado de 1615 se vio en la Isla de Ceilao en las Indias Orientales. Dase cuenta de los edifices q arruyno, y muerte de muchas personas, y assi mismo de los presagios y sena les que antes del se vieron por muchos dias, assi en el cielo, como em la tierra con otras muchas cosas del mismo proposito...Impresso com licenca em Lisboa por Jorge Rodrigues, e vendese en sua casa, Anno 1616."
"A true relation of the Terrible Earthquake that in the past year of 1615 took place in the Isle of Ceylon in the East Indies: giving an account of the buildings that were destroyed, and the deaths of many persons, as also of the portents and signs that precede it for many days both in the heavens and on earth, with many other things on the same subject...Printed with permission at Lisbon by Jorge Rodriguez and sold in his shop, in the year 1616."
Sadly the writer does not give specific details but wraps the incident in a number of inane reflections and general statements. Reproduced herewith is text from that document, which appeared in an article in the Sri Lankan newspaper, the Sunday Observer, on 4th June 2000.
"The Isle of Ceylon is one of the best in the East, as well for the fertility of its soil as for the cinnamon it produces, which is the best that comes from those parts. It enjoyed for many years past the greatest prosperity both in health and abundance of harvest, with the result that the inhabitants became so proud that they almost forgot that those gifts were distributed by the hands of one who was able to turn to chastisement if they did not give him thanks for them.
"At last one day, which was the 7th of March 1615, soon after prayers there appeared in the air a terrible comet, which continued for many days to the great wonder of many but to the amazement of none. The comet had three tails, the end of which were like the heads of arrows, and so fiery and red that they seemed to be emitting rays of fire threatening therewith the total destruction of that Island.
"This sign in the heavens ceased, God showed many others on land which being nearer to men might produce in them an effect which the other had not. There died animals on the fields and men in the city without any other cause of death than the infection of the atmosphere, without even time to confess their sins.
"There were thrown up from the sea numbers of dead fish, so poisonous that all who ate them died, which caused such terror to all that there was no one who ventured to the beach to give them burial: thus the putrefaction and bad odour so infected the air that not men but even the birds of the air fell dead.
"In this way the Island continued to be depopulated as some quitted it for fear and others gave themselves to the hands of death, but these signs and rigorous chastisements did not produce the amazement of those to whom they were sent. But God is a divine surgeon and as such he heals men who are sick in soul with mild remedies till, when a member is putrefied and cankered, he applies fire and the iron to amputate it, and so he acted with these folk for seeing that they were so hardened that they did not mend at sight of the signs in the heavens nor of the chastisements on land, he wished to cut them and destroy them by a terrible earthquake, which took place on the fourteenth of April in the following manner:
"On that day the sun set half an hour earlier than on other days, for it would seem that he even hastened the chastisement as if weary of the obstinacy of the inhabitants of the Island. It was seven in the evening when thunder shook the air with such force and the earth quaked so violently that, unable to remain in the houses people rushed out to the streets fearing to be buried under the falling ruins of the buildings. Then bolts of thunder fell from the heavens, whereupon the terrified people ran into the houses hoping to shelter themselves from the rigour and justice of God. The thunderbolts had their effect, destroying and laying low not only the most sumptuous edifices but also the meanest cottages levelling all, and the few that the fire had not consumed were destroyed by the earthquakes."
"Some of those who escaped declared that they heard many voices which sounded so terrible amidst the fire that when they reached the ears that they caused fear and panic in addition to that of imminent death in which they were, though the same seemed to others to be the cries of those who were perishing under the fire of heaven and the falling stones, which rolled from side to side like light feathers or as if some strong hand were moving and hurling them about. "This lasted till about morning, which dawned bright and serene, either to show that its Author had already received satisfaction for the offences of the inhabitants of that land or in order that the survivors may seem more clearly the calamities of the dead."
Fissures in the Earth "There were seen in many parts of the Island vast openings and fissures in the earth, some of which so deep that no one could find the bottom, nor were they closed up for a long time, from which there issued at certain times flames of fire as of sulphur, as terrible and awful that some people thought they were the mouths of hell and that God showed them open to engulf them if they did not reform. "The damage which this earthquake did was moreover general throughout the Island not only to fruits, trees and crops but also to cattle and other things necessary for human life. "Great grief was caused by the destruction of a large stone bridge, built at great cost and very necessary as it was the most important passage in that Island, the stones of which were afterwards found many leagues away. "They made up for it with boats and are already considering the re-erection of the bridge because of their need of it."
"There fell a good bit of the Western wall as well as the whole of the bastion, which was so strong that they thought that nothing could destroy it. "This building brought down with it a house which was near it and in which lived some of those Moors who lived in those parts in peace, killing four and there escaped only a boy of about six years of age, who was afterwards baptized for he said he was saved from death by a most beautiful lady, the same as the Christians have in their Churches, for so he called the Virgin Mary our Lady, for whom he had a great devotion."
Large and Deadly Earthquakes This Year
| Year | Month | Day | Time UTC | Latitude | Longitude | Depth (km) | Magnitude | Region | |
|---|---|---|---|---|---|---|---|---|---|
| 1. | 2011 | 01 | 01 | 09:56:58 | -26.795 | -63.079 | 577 | 7.0 | Santiago del Estero, Argentina |
| 2. | 2011 | 01 | 02 | 20:20:18 | -38.313 | -73.269 | 24 | 7.1 | Araucania, Chile |
| 3. | 2011 | 01 | 13 | 16:16:41 | -20.623 | 168.459 | 9 | 7.0 | Loyalty Islands |
| 4. | 2011 | 01 | 18 | 20:23:23 | 28.727 | 63.933 | 68 | 7.2 | Southwestern Pakistan |
| 5. | 2011 | 03 | 09 | 02:45:20 | 38.435 | 142.842 | 32 | 7.3 | Near East Coast of Honshu, Japan |
| 6. | 2011 | 03 | 11 | 05:46:24 | 38.320 | 142.351 | 32 | 9.0 | Near East Coast of Honshu, Japan |
| 7. | 2011 | 03 | 11 | 06:15:39 | 36.270 | 141.145 | 35 | 7.9 | Near East Coast of Honshu, Japan |
| 8. | 2011 | 03 | 11 | 06:25:50 | 38.058 | 144.591 | 18 | 7.7 | Near East Coast of Honshu, Japan |
| 9. | 2011 | 04 | 07 | 14:32:41 | 38.253 | 141.640 | 49 | 7.1 | Near East Coast of Honshu, Japan |
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