Understanding how tremors are measured: What scientists mean by a 5.7 or 3.7 earthquake
Experts warn that a 2024 Rajuk study grimly predicts that a 6.9 magnitude earthquake on the Madhupur fault could collapse up to 64.83% of Dhaka's buildings, with a potential death toll of up to 5 lakh
The 5.7 magnitude earthquake that violently shook the nation on 21 November, with its epicentre in Narsingdi's Madhabdi, a mere 13 kilometres east of the BMD office in the capital's Agargaon, has brought the science of seismology into sharp focus.
As residents grapple with unprecedented proximity to a significant tremor for two days in a row, with yesterday's one being 3.7 magnitude, understanding how these powerful events are measured becomes critical to comprehending their devastating potential.
Magnitude vs intensity
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While often used interchangeably, magnitude and intensity are distinct measures of an earthquake's strength. Magnitude is a scientific measure of the energy released by an earthquake at its source, while intensity describes the effects of an earthquake at a particular location, says Michigan Technological University.
Think of it like a lightbulb. Its wattage (magnitude) is constant, but its brightness (intensity) varies depending on how far you are from it.
If this earthquake had lasted just five to seven seconds longer, many buildings in Dhaka would have collapsed
Md Momenul Islam, director (current charge) of the Bangladesh Meteorological Department (BMD) and a seismology researcher, emphasised this distinction in the context of 21 November's quake.
"This earthquake's epicentre was very close to Dhaka, which is why it was felt so intensely," he told The Business Standard. He elaborated that even a magnitude 7 or higher earthquake occurring farther away, such as in Rangamati or Khagrachhari, would not produce such a strong jolt in Dhaka.
Momenul issued a chilling warning, "However, if this earthquake had lasted just five to seven seconds longer, many buildings in Dhaka would have collapsed."
Factors influencing intensity and damage include: the magnitude of the earthquake, distance from the epicentre, earthquake depth, building design, and the type of surface material (rock or soil) the buildings rest on. Buildings on solid rock generally fare better than those on loose, waterlogged sediment, a critical concern for Dhaka.
A deceptive simplicity
Earthquake magnitudes are primarily measured using scales based on the amplitude of seismic waves recorded by seismometers. The most widely used scale today is the Moment Magnitude Scale (Mw), which has largely replaced the older Richter scale due to its accuracy over a wider range of earthquake sizes and global applicability.
The moment magnitude scale is based on the total 'moment release' of an earthquake – a product of the distance a fault moved and the force required to move it.
Islam highlighted a common misconception about these scales.
"It may commonly seem that a magnitude 6 earthquake is just one point higher than a magnitude 5. But that is not the case." He explained the logarithmic nature of the scale, "A magnitude 6 earthquake has a vibrating tendency 10 times greater than a magnitude 5. Here, every fraction means its intensity is proportionally higher. Consequently, this earthquake's tendency has intensity over at least a 100-kilometre radius."
To put this into perspective, for each whole number increase on the magnitude scale, the amplitude of ground motion goes up ten times, and roughly 32 times as much energy is released.
A magnitude 1 quake releases energy equivalent to 6 ounces of TNT, while a magnitude 8 quake releases as much energy as detonating 6 million tonnes of TNT.
According to a geological expert, "The Richter scale is logarithmic. A magnitude of 1 to 2 means about 10 times more energy released. A magnitude of three means 100 times more energy than a magnitude one."
Earthquakes of 3 or 4 magnitudes are often not felt by people and do not have a significant impact on the typical RCC structures of Bangladesh. However, anything closer to magnitude 6 can pose a risk to weak structures or old, fragile buildings.
"A magnitude 6 or 6.5 earthquake could cause problems for Old Dhaka's weak buildings. Magnitude 7 is too much for us."
7.5 magnitude equals 72,000 buildings collapsed
Rubaiyat Kabir, head of BMD's Earthquake Monitoring and Research Centre, underscored Bangladesh's precarious geological position. The country sits on the Indian Plate, bordered by the Eurasian Plate to the north and the Myanmar Plate to the east. This complex tectonic setting, further complicated by the major Dauki Fault to the north and the Sagaing Fault in Myanmar, makes the region highly earthquake-prone.
"And earthquakes happen in this region quite often," Kabir told TBS, though he clarified that quakes of yesterday's specific proximity and intensity are less frequent. He added a chilling thought, "Even considering the repeat time of earthquakes, there is a possibility of a major earthquake occurring in this region."
Historically, the Bengal region has endured devastating quakes, including the 1897 Great Indian Earthquake (M8.0), which killed over 1,500, and the 1918 Srimangal Earthquake (M7.6), causing extensive damage. While past tremors with epicentres near Dhaka (M4.5 in 2012, M4.8 in 2008, M4.5 in 2001) did not cause major casualties, 21 November's 5.7 magnitude quake, so close to the capital, serves as an unprecedented warning.
Experts warn that a 2024 Rajuk study grimly predicts that a 6.9 magnitude earthquake on the Madhupur fault could collapse up to 64.83% of Dhaka's buildings, with a potential death toll of up to 5 lakh.
A Comprehensive Disaster Management Programme (CDMP) survey similarly warned a 7.5 magnitude quake could collapse 72,000 buildings and severely damage 1.5 lakh more, particularly older buildings on weak soil and high-rises, ignoring building codes.
