History says that we never learn by watching. Haiti suffered the worst nightmare created from Earthquake. The whole world watched, like Bangladesh. But did we learn anything from it? Despite the fact that buildings are collapsing here and there and experts are declaring "Bangladesh faces high earthquake risk" - do we give it that much of a care? Well, shouldn't we push the issue to a national level of urgency? Yes, you're right, we should. Here are some facts you should know about Earthquake to learn what a serious threat is on our head :
Earthquake: An earthquake (also known as a quake, tremor, temblor or seismic activity) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph.
The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event — whether a natural phenomenon or an event caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
Naturally occurring earthquakes:
Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behavior. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interio Naturally occurring earthquakes.
Fault types
Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior
Earthquakes and volcanic activity:
Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helens eruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltimeters (a device which measures the ground slope) and used as sensors to predict imminent or upcoming eruptions.
Earthquake clusters:
Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.
Aftershocks:
An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.
Earthquake swarms:
Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.
Earthquake storms:
Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.
Size and frequency of occurrence:
There are around 500,000 earthquakes each year. 100,000 of these can actually be felt.Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years.This is an example of the Gutenberg-Richter law.
The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.
The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.
With the rapid growth of mega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.
Tsunami:
Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 miles), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373–497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.
Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.
Human impacts:
Earthquakes may lead to disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings. Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage, as in the "Year Without a Summer" (1816).
Major earthquakes:
The largest earthquake that has been measured was the 9.5 magnitude one in Chile in 1960.
Preparation:
In order to determine the likelihood of future seismic activity, geologists and other scientists examine the rock of an area to determine if the rock appears to be "strained". Studying the faults of an area to study the buildup time it takes for the fault to build up stress sufficient for an earthquake also serves as an effective prediction technique. Measurements of the amount of accumulated strain energy on the fault each year, time passed since the last major temblor, and the energy and power of the last earthquake are made.Together the facts allow scientists to determine how much pressure it takes for the fault to generate an earthquake. Though this method is useful, it has only been implemented on California's San Andreas Fault.
Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: earthquake engineering, earthquake preparedness, household seismic safety, seismic retrofit (including special fasteners, materials, and techniques), seismic hazard, mitigation of seismic motion, and earthquake prediction. Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries (US, Japan etc.) and late 1970s for many other parts of the world (Turkey, China etc.), many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. Furthermore, state-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world - such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines
BANGLADESH FACES HIGH EARTHQUAKE RISK, warn experts:
Dhaka, Aug 12 (IANS) The latest threat of earthquake-triggered tsunami has abated, but disaster-prone Bangladesh faces a high risk of moderate to strong quakes, experts have warned.
Bangladesh also faces the risk of tsunami as four active sources of earthquake in the Bay of Bengal can generate tremors with a magnitude of over 7 on the Richter scale, affecting the country seriously.
Earthquake risk in Bangladesh: Facing the reality
The observatory at Bangladesh University of Engineering and Technology (BUET) recorded 86 tremors of over four magnitude during January 2006-May 2009. Another four earthquakes took place with magnitude of over five during the period.
The meteorological department detected at least 90 earthquakes taking place in the country between May 2007 and July 2008, nine of them above five on the Richter scale and epicentres of 95 percent being within a 600 km radius of Dhaka city.
Experts say it is these minor tremors that indicate the possibility of much more powerful earthquakes hitting the country.
According to a seismic zoning map prepared by BUET, 43 percent areas in Bangladesh are rated high risk, 41 percent moderate and 16 percent low, Mehdi Ahmed Ansary told The Daily Star.
The map, which is being drawn up under the supervision of Ansary with funding provided by the science and information and communications technology ministry, divides the country into three earthquake vulnerability zones.
The current zoning map has, however, not been included in the Bangladesh National Building Code (BNBC) that needs to be urgently updated, pointed out Ansary who is also vice-president of the Bangladesh Earthquake Society.
In the zoning map of 1993, 26 percent of the country was high risk, 38 percent moderate and 36 percent low in terms of earthquake vulnerability.
A.S.M. Maksud Kamal, an expert on earthquake and tsunami preparedness, said four sources of earthquake in the Bay of Bengal are active and can generate tsunami.
He said one of the sources generated an earthquake of 7.8 magnitude in 1762 which generated waves in rivers and other closed water bodies, and around 100 people were killed in boat capsizes at that time in the Buriganga.
Kamal said all the four sources in the Bay called F1, F2, F3 and F4 have a capability of generating earthquakes of over seven magnitude creating tsunami which will affect Bangladesh.
In that case the sea level will rise by 4-5 metres at Nijhum Dwip. The water level will rise 2-3 metres at the Sundarbans, Cox's Bazar and the estuary of the Meghna.
Kamal said seven major earthquakes struck Bangladesh during the last 150 years and only two had the epicentre within the country.
The Srimangal earthquake July 8, 1918 was recorded at 7.6 on the Richter scale and its epicentre was in Balisera valley near Srimangal. Although there was damage, the intensity rapidly decreased due to the shallow focal depth and only minor effects were felt in Dhaka, he said.
The Bengal earthquake of July 14, 1885 caused considerable damage in the Sirajganj-Bogra region and perhaps more severe destruction in Jamalpur-Sherpur-Mymensingh region. The magnitude of the earthquake was more than seven on the Richter scale and the epicentre was at Manikganj, he added.
During the 1762 earthquake in Chittagong-Arakan coast the magnitude was 7.6 but the exact epicentre remained unclear.
The great Indian earthquake of June 12, 1897 that had a magnitude of 8.7 with the epicentre in the central part of the Shillong plateau was recalled as one of the world’s worst.
He said any minor earthquake might be due to the activity in the local small fault zones, thus increasing the chances of a major jolt happening. Besides, Ansary felt a strong earthquake could occur in the plate boundaries as the 100-year alarm bells have passed.
Bangladesh is close to the meeting point of the Indian, Eurasian and Burma (Myanmar) plates. The movement of Indian and Eurasian plates has been locked at the foot of the Himalayas for many years, storing strain energy, he said.
When the lock is released, it will let out the strain energy causing major earthquakes that will affect Bangladesh, northeastern India and Myanmar, Ansary explained.
Three strong earthquakes were recorded from the Indian-Eurasian plate, which jolted Bangladesh within 150 years, said Ansary.
Due to the Indian and Eurasian and Myanmar plates, the Bihar-Nepal earthquake took place in 1934 and it was felt as far away as Dinajpur and Rangpur, he said.
The Assam earthquake Aug 15, 1950 had a magnitude of 8.6 on the Richter scale. The tremor was felt throughout Bangladesh but miraculously no damage was reported anywhere.
But the Mandalay earthquake that struck in 1858 with a magnitude of 7.9 affected Chittagong division, he added.
MAJOR EARTHQUAKES IN BANGLADESH:
With the south-eastern parts of Bangladesh experiencing a number of small-to-moderate tremors since July 27 2006, it is feared that a major earthquake will occur in the region in the near future1. From 1996 to July 2003, the Chittagong region has experienced more than 200 small to moderate earthquakes that may be precursors to a major quake. The last serious quake in the region was the Srimangal earthquake of 1918, which measured 7.6 on the Richter scale1. Stress accumulation over the past 80 years may result in a magnitude 7-8 quake at any time
Seismic Zones in Bangladesh and their Earthquake Potentia
Zones Operational Basis Magnitude (Richter)
Max. Credible Magnitude (Richter) Depth of Focus (km)
Asam-Meghalaya fault
zone: 8.0 8.7 0-70
Tripura fault zone: 7.0 8.0 0-70
Sub-Dauki fault zone: 7.3 7.5 0-70
Bogra fault zone: 7.0 7.5 0-70
A future large earthquake also threatens the country's largest dam and HEP plant sited in Kaptai town near Rangamati1.
Earthquake: An earthquake (also known as a quake, tremor, temblor or seismic activity) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph.
The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event — whether a natural phenomenon or an event caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
Naturally occurring earthquakes:
Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behavior. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interio Naturally occurring earthquakes.
Fault types
Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior
Earthquakes and volcanic activity:
Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helens eruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltimeters (a device which measures the ground slope) and used as sensors to predict imminent or upcoming eruptions.
Earthquake clusters:
Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.
Aftershocks:
An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.
Earthquake swarms:
Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.
Earthquake storms:
Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.
Size and frequency of occurrence:
There are around 500,000 earthquakes each year. 100,000 of these can actually be felt.Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years.This is an example of the Gutenberg-Richter law.
The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.
The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.
With the rapid growth of mega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.
Tsunami:
Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 miles), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373–497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.
Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.
Human impacts:
Earthquakes may lead to disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings. Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage, as in the "Year Without a Summer" (1816).
Major earthquakes:
The largest earthquake that has been measured was the 9.5 magnitude one in Chile in 1960.
Preparation:
In order to determine the likelihood of future seismic activity, geologists and other scientists examine the rock of an area to determine if the rock appears to be "strained". Studying the faults of an area to study the buildup time it takes for the fault to build up stress sufficient for an earthquake also serves as an effective prediction technique. Measurements of the amount of accumulated strain energy on the fault each year, time passed since the last major temblor, and the energy and power of the last earthquake are made.Together the facts allow scientists to determine how much pressure it takes for the fault to generate an earthquake. Though this method is useful, it has only been implemented on California's San Andreas Fault.
Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: earthquake engineering, earthquake preparedness, household seismic safety, seismic retrofit (including special fasteners, materials, and techniques), seismic hazard, mitigation of seismic motion, and earthquake prediction. Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries (US, Japan etc.) and late 1970s for many other parts of the world (Turkey, China etc.), many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. Furthermore, state-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world - such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines
BANGLADESH FACES HIGH EARTHQUAKE RISK, warn experts:
Dhaka, Aug 12 (IANS) The latest threat of earthquake-triggered tsunami has abated, but disaster-prone Bangladesh faces a high risk of moderate to strong quakes, experts have warned.
Bangladesh also faces the risk of tsunami as four active sources of earthquake in the Bay of Bengal can generate tremors with a magnitude of over 7 on the Richter scale, affecting the country seriously.
Earthquake risk in Bangladesh: Facing the reality
The observatory at Bangladesh University of Engineering and Technology (BUET) recorded 86 tremors of over four magnitude during January 2006-May 2009. Another four earthquakes took place with magnitude of over five during the period.
The meteorological department detected at least 90 earthquakes taking place in the country between May 2007 and July 2008, nine of them above five on the Richter scale and epicentres of 95 percent being within a 600 km radius of Dhaka city.
Experts say it is these minor tremors that indicate the possibility of much more powerful earthquakes hitting the country.
According to a seismic zoning map prepared by BUET, 43 percent areas in Bangladesh are rated high risk, 41 percent moderate and 16 percent low, Mehdi Ahmed Ansary told The Daily Star.
The map, which is being drawn up under the supervision of Ansary with funding provided by the science and information and communications technology ministry, divides the country into three earthquake vulnerability zones.
The current zoning map has, however, not been included in the Bangladesh National Building Code (BNBC) that needs to be urgently updated, pointed out Ansary who is also vice-president of the Bangladesh Earthquake Society.
In the zoning map of 1993, 26 percent of the country was high risk, 38 percent moderate and 36 percent low in terms of earthquake vulnerability.
A.S.M. Maksud Kamal, an expert on earthquake and tsunami preparedness, said four sources of earthquake in the Bay of Bengal are active and can generate tsunami.
He said one of the sources generated an earthquake of 7.8 magnitude in 1762 which generated waves in rivers and other closed water bodies, and around 100 people were killed in boat capsizes at that time in the Buriganga.
Kamal said all the four sources in the Bay called F1, F2, F3 and F4 have a capability of generating earthquakes of over seven magnitude creating tsunami which will affect Bangladesh.
In that case the sea level will rise by 4-5 metres at Nijhum Dwip. The water level will rise 2-3 metres at the Sundarbans, Cox's Bazar and the estuary of the Meghna.
Kamal said seven major earthquakes struck Bangladesh during the last 150 years and only two had the epicentre within the country.
The Srimangal earthquake July 8, 1918 was recorded at 7.6 on the Richter scale and its epicentre was in Balisera valley near Srimangal. Although there was damage, the intensity rapidly decreased due to the shallow focal depth and only minor effects were felt in Dhaka, he said.
The Bengal earthquake of July 14, 1885 caused considerable damage in the Sirajganj-Bogra region and perhaps more severe destruction in Jamalpur-Sherpur-Mymensingh region. The magnitude of the earthquake was more than seven on the Richter scale and the epicentre was at Manikganj, he added.
During the 1762 earthquake in Chittagong-Arakan coast the magnitude was 7.6 but the exact epicentre remained unclear.
The great Indian earthquake of June 12, 1897 that had a magnitude of 8.7 with the epicentre in the central part of the Shillong plateau was recalled as one of the world’s worst.
He said any minor earthquake might be due to the activity in the local small fault zones, thus increasing the chances of a major jolt happening. Besides, Ansary felt a strong earthquake could occur in the plate boundaries as the 100-year alarm bells have passed.
Bangladesh is close to the meeting point of the Indian, Eurasian and Burma (Myanmar) plates. The movement of Indian and Eurasian plates has been locked at the foot of the Himalayas for many years, storing strain energy, he said.
When the lock is released, it will let out the strain energy causing major earthquakes that will affect Bangladesh, northeastern India and Myanmar, Ansary explained.
Three strong earthquakes were recorded from the Indian-Eurasian plate, which jolted Bangladesh within 150 years, said Ansary.
Due to the Indian and Eurasian and Myanmar plates, the Bihar-Nepal earthquake took place in 1934 and it was felt as far away as Dinajpur and Rangpur, he said.
The Assam earthquake Aug 15, 1950 had a magnitude of 8.6 on the Richter scale. The tremor was felt throughout Bangladesh but miraculously no damage was reported anywhere.
But the Mandalay earthquake that struck in 1858 with a magnitude of 7.9 affected Chittagong division, he added.
MAJOR EARTHQUAKES IN BANGLADESH:
With the south-eastern parts of Bangladesh experiencing a number of small-to-moderate tremors since July 27 2006, it is feared that a major earthquake will occur in the region in the near future1. From 1996 to July 2003, the Chittagong region has experienced more than 200 small to moderate earthquakes that may be precursors to a major quake. The last serious quake in the region was the Srimangal earthquake of 1918, which measured 7.6 on the Richter scale1. Stress accumulation over the past 80 years may result in a magnitude 7-8 quake at any time
Seismic Zones in Bangladesh and their Earthquake Potentia
Zones Operational Basis Magnitude (Richter)
Max. Credible Magnitude (Richter) Depth of Focus (km)
Asam-Meghalaya fault
zone: 8.0 8.7 0-70
Tripura fault zone: 7.0 8.0 0-70
Sub-Dauki fault zone: 7.3 7.5 0-70
Bogra fault zone: 7.0 7.5 0-70
A future large earthquake also threatens the country's largest dam and HEP plant sited in Kaptai town near Rangamati1.
