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Small earthquakes don’t reduce risk of a major earthquake
We believe that this earthquake did not occur on the megathrust, nor is it directly related to it. Therefore, it does not significantly affect the risk of a major earthquake, says Michael S Steckler.
en.prothomalo.com
Small earthquakes don’t reduce risk of a major earthquake
Michael S Steckler is a geophysicist. He is a Lamont Research Professor and Associate Director of Marine and Polar Geophysics at the Lamont–Doherty Earth Observatory of Columbia University in the United States. His primary field area is currently Bangladesh, where he works on the heavily sedimented Indo-Burma Subduction Zone (IBSZ). He has worked in Bangladesh and adjacent parts of India and Myanmar for almost 25 years, taking part in and leading numerous field campaigns.His interview for Prothom Alo was conducted by Partha Shankar Saha.
We believe that this earthquake did not occur on the megathrust, nor is it directly related to it. Therefore, it does not significantly affect the risk of a major earthquake. During the monsoon, shallow and unconsolidated sediments, particularly sand, can behave like a liquid and lose strength. This may explain why some buildings in Dhaka tilted slightly during the earthquake. A major earthquake may occur on the megathrust or a major fault within the next few years or it may not. We all hope it happens later rather than sooner. Retrofitting old buildings to make them earthquake-resistant is very costly, but new buildings can be made much stronger without substantially increasing construction expenses.
Prothom Alo: You and your colleagues have been monitoring Bangladesh’s ground movement since 2003. How does the recent series of earthquakes near Dhaka fit into the long-term GPS observations, and what does it reveal about ongoing seismic activity in the region?
The GPS observations show shortening across the fold belt of 11-12 mm per year, which builds stress, potentially towards an earthquake. Recently, we have noticed motion that may or may not indicate the anticlines (hills) deforming in a way that might slightly lower the hazard.
Under the foldbelt, the Indian plate is subducting. The boundary between the IndoBurma foldbelt above and the Indian plate below is the megathrust. This is the fault that we are concerned could have a large earthquake. We revised the size downwards to M 8.0-8.2 in a 2023 followup paper to my 2016 paper. In addition, as the lower plate deforms, it can also have earthquakes. We believe this recent earthquake was in the lower plate and may not affect the hazard from the megathrust.
Prothom Alo: Recent reports indicate that the ground beneath Bangladesh is moving north eastward at about two inches per year. How is this movement contributing to stress accumulation near Dhaka, and could it be linked to the recent tremors?
Yes, Bangladesh moves about 5.3 cm per year to the northeast. It is the relative movement of the plates that give rise to earthquakes. It is not that Bangladesh and India are moving to the northeast, but that Asia is moving differently towards India causing the Himalayas and Shillong to the north, and the IndoBurma subduction zone to the east.
Prothom Alo: Considering that there has not been a major earthquake in Bangladesh for at least 400 years, how do these recent smaller earthquakes influence your understanding of the potential for a large-scale event?
Two points: 1) we don’t think this was on or related to the megathrust, so probably doesn’t significantly influence that hazard. 2) The magnitude scale is logarithmic, with every increase of 1 unit, there is 32 times more energy. For 2 units, say from 5.5 to 7.5, it is an increase of 1000 times more energy, so the smaller events don’t have much impact or relieve much of the stress.
Prothom Alo: The recent earthquakes occurred near densely populated areas. Based on your models, which areas around Dhaka are most at risk if a major megathrust earthquake were to occur, and why? Much of Dhaka is built on centuries of soft river sediments. How might this loose ground amplify shaking during an earthquake, and how does it relate to your description of the city as “a bowl of Jell-O”?
When earthquake waves travel into slower velocity material, like soft sediments, the amplitude increases. When earthquake waves bounce inside a basin, some frequencies get amplified. The whole Bengal Basin is so large that the frequencies amplified may be too low for impacting buildings. Smaller basins, like the valleys between the hills in Sylhet may amplify waves at frequencies that affect buildings. I am not aware of detailed work on this in Bangladesh.
The other thing that may happen, particularly in the rainy season is that shallow, weak sediments, especially sands, may liquefy and lose strength. This may account for the tilting of buildings in Dhaka during this earthquake.
My graduate student, Hasnat Jaman (also professor at Barisal University), studied this for his Master’s thesis and found quite a bit of risk of liquefaction in Dhaka and was able to map it. The stiffer Madhupur Terrace is better, areas of anthropogenic fill and recent sediments are weaker. My impression was that when designing a building in Dhaka, assume the upper 2-3 meters could liquefy, so make sure the foundation below that can still support the building.
Prothom Alo: The recent seismic activity may raise public awareness. In your view, what immediate preparedness measures should Dhaka implement, particularly regarding building codes and emergency response?
One important thing is to enforce building codes. It is very expensive to retrofit buildings, but for new construction, it is only a modest increase in cost to make it more resilient. If people accept these costs, then Dhaka can become more resilient over the next decades to a century. The reduction in fatalities from cyclones, similarly took decades.
Prothom Alo: From your GPS and geological data, can we link the recent earthquakes to specific faults or to the locked megathrust beneath the Indo-Burman Ranges? How certain can we be about identifying these sources?
This earthquake took place on a fault that is buried beneath many kilometers of sediment. We are not able to identify such faults, but the deformation rates and therefore seismicity should be relatively low.
The megathrust has the greatest potential for a major earthquake reaching M8. The hills in Sylhet, Tripura and the Chittagong Hill Tracts are all strata that are being deformed over the megathrust and most or all of the hills are associated with thrust faults that may be capable of a M7+ earthquake.
Prothom Alo: Given the recent activity near Dhaka, what lessons can policymakers and the public learn about the timing, magnitude, and potential impact of future earthquakes in Bangladesh? How should these insights shape planning and mitigation efforts?
A future megathrust earthquake could happen in the near future or not for a 1000 years or more. We all hope for the latter.
This makes it difficult to determine how much resources should be put towards earthquake hazard reduction given all the other more immediate issues in Bangladesh to deal with. Public information so people support the extra costs to make buildings more earthquake resistant is needed. Planning that includes open spaces and some roads wide enough to not be blocked by debris are needed.
The standard construction of rectangles of columns and slab floors do poorly in earthquakes, but there are design changes that can help, reinforcing junctions, diagonal elements, shear walls. There are other experts that know much more about this than I do.
Prothom Alo: Thank you for your time.
Thank you too.
Michael S Steckler is a geophysicist. He is a Lamont Research Professor and Associate Director of Marine and Polar Geophysics at the Lamont–Doherty Earth Observatory of Columbia University in the United States. His primary field area is currently Bangladesh, where he works on the heavily sedimented Indo-Burma Subduction Zone (IBSZ). He has worked in Bangladesh and adjacent parts of India and Myanmar for almost 25 years, taking part in and leading numerous field campaigns.His interview for Prothom Alo was conducted by Partha Shankar Saha.
We believe that this earthquake did not occur on the megathrust, nor is it directly related to it. Therefore, it does not significantly affect the risk of a major earthquake. During the monsoon, shallow and unconsolidated sediments, particularly sand, can behave like a liquid and lose strength. This may explain why some buildings in Dhaka tilted slightly during the earthquake. A major earthquake may occur on the megathrust or a major fault within the next few years or it may not. We all hope it happens later rather than sooner. Retrofitting old buildings to make them earthquake-resistant is very costly, but new buildings can be made much stronger without substantially increasing construction expenses.
Prothom Alo: You and your colleagues have been monitoring Bangladesh’s ground movement since 2003. How does the recent series of earthquakes near Dhaka fit into the long-term GPS observations, and what does it reveal about ongoing seismic activity in the region?
The GPS observations show shortening across the fold belt of 11-12 mm per year, which builds stress, potentially towards an earthquake. Recently, we have noticed motion that may or may not indicate the anticlines (hills) deforming in a way that might slightly lower the hazard.
Under the foldbelt, the Indian plate is subducting. The boundary between the IndoBurma foldbelt above and the Indian plate below is the megathrust. This is the fault that we are concerned could have a large earthquake. We revised the size downwards to M 8.0-8.2 in a 2023 followup paper to my 2016 paper. In addition, as the lower plate deforms, it can also have earthquakes. We believe this recent earthquake was in the lower plate and may not affect the hazard from the megathrust.
Prothom Alo: Recent reports indicate that the ground beneath Bangladesh is moving north eastward at about two inches per year. How is this movement contributing to stress accumulation near Dhaka, and could it be linked to the recent tremors?
Yes, Bangladesh moves about 5.3 cm per year to the northeast. It is the relative movement of the plates that give rise to earthquakes. It is not that Bangladesh and India are moving to the northeast, but that Asia is moving differently towards India causing the Himalayas and Shillong to the north, and the IndoBurma subduction zone to the east.
Prothom Alo: Considering that there has not been a major earthquake in Bangladesh for at least 400 years, how do these recent smaller earthquakes influence your understanding of the potential for a large-scale event?
Two points: 1) we don’t think this was on or related to the megathrust, so probably doesn’t significantly influence that hazard. 2) The magnitude scale is logarithmic, with every increase of 1 unit, there is 32 times more energy. For 2 units, say from 5.5 to 7.5, it is an increase of 1000 times more energy, so the smaller events don’t have much impact or relieve much of the stress.
Prothom Alo: The recent earthquakes occurred near densely populated areas. Based on your models, which areas around Dhaka are most at risk if a major megathrust earthquake were to occur, and why? Much of Dhaka is built on centuries of soft river sediments. How might this loose ground amplify shaking during an earthquake, and how does it relate to your description of the city as “a bowl of Jell-O”?
When earthquake waves travel into slower velocity material, like soft sediments, the amplitude increases. When earthquake waves bounce inside a basin, some frequencies get amplified. The whole Bengal Basin is so large that the frequencies amplified may be too low for impacting buildings. Smaller basins, like the valleys between the hills in Sylhet may amplify waves at frequencies that affect buildings. I am not aware of detailed work on this in Bangladesh.
The other thing that may happen, particularly in the rainy season is that shallow, weak sediments, especially sands, may liquefy and lose strength. This may account for the tilting of buildings in Dhaka during this earthquake.
My graduate student, Hasnat Jaman (also professor at Barisal University), studied this for his Master’s thesis and found quite a bit of risk of liquefaction in Dhaka and was able to map it. The stiffer Madhupur Terrace is better, areas of anthropogenic fill and recent sediments are weaker. My impression was that when designing a building in Dhaka, assume the upper 2-3 meters could liquefy, so make sure the foundation below that can still support the building.
Prothom Alo: The recent seismic activity may raise public awareness. In your view, what immediate preparedness measures should Dhaka implement, particularly regarding building codes and emergency response?
One important thing is to enforce building codes. It is very expensive to retrofit buildings, but for new construction, it is only a modest increase in cost to make it more resilient. If people accept these costs, then Dhaka can become more resilient over the next decades to a century. The reduction in fatalities from cyclones, similarly took decades.
Prothom Alo: From your GPS and geological data, can we link the recent earthquakes to specific faults or to the locked megathrust beneath the Indo-Burman Ranges? How certain can we be about identifying these sources?
This earthquake took place on a fault that is buried beneath many kilometers of sediment. We are not able to identify such faults, but the deformation rates and therefore seismicity should be relatively low.
The megathrust has the greatest potential for a major earthquake reaching M8. The hills in Sylhet, Tripura and the Chittagong Hill Tracts are all strata that are being deformed over the megathrust and most or all of the hills are associated with thrust faults that may be capable of a M7+ earthquake.
Prothom Alo: Given the recent activity near Dhaka, what lessons can policymakers and the public learn about the timing, magnitude, and potential impact of future earthquakes in Bangladesh? How should these insights shape planning and mitigation efforts?
A future megathrust earthquake could happen in the near future or not for a 1000 years or more. We all hope for the latter.
This makes it difficult to determine how much resources should be put towards earthquake hazard reduction given all the other more immediate issues in Bangladesh to deal with. Public information so people support the extra costs to make buildings more earthquake resistant is needed. Planning that includes open spaces and some roads wide enough to not be blocked by debris are needed.
The standard construction of rectangles of columns and slab floors do poorly in earthquakes, but there are design changes that can help, reinforcing junctions, diagonal elements, shear walls. There are other experts that know much more about this than I do.
Prothom Alo: Thank you for your time.
Thank you too.
