Article post Myanmar earthquake

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Adam Pascale

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Mar 31, 2025, 5:11:21 AMMar 31
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If you’re on LinkedIn, click this link to read some briefd thoughts from a UK colleague following the Myanmar earthquake. Feel free to leave a comment there to discuss. 

If you can’t access it to comment, here’s the text, and feel free to discuss here. 

Adam

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Friday's M7.7 Mandalay, Myanmar earthquake - do we need to rethink earthquake hazard models?

Hours after Friday's earthquake, media reports focussed on the unexpectedely large damage to buildings in Bangkok, Thailand, some 1000 km south-east of the epicentre. There will likely be near-devastation to areas close to the rupture in Myanmar, and due to the ongoing coup and civil war, we might never get a complete picture of local damage. 
Let's focus on the distant shaking in Thailand, and let me summarise in simple terms the 5 key seismological factors behind it.

(1) *LONG-PERIOD GROUND MOTION" The further seismic waves travel away from their source, the higher frequencies get lost ("attenuated"), leaving longer-period waves that sustain large distances. Because of their height, urban highrise buildings have lower resonant frequencies, making them more sensitive to shaking from distant earthquakes.

(2) *BASIN AMPLIFICATION* Bangkok lies in the Chao Phraya River delta basin, filled with soft alluvial sediments, which can trap & amplify these long-period seismic waves, causing a higher intensity and duration of shaking.

(3) *RUPTURE LENGTH* News articles typically talk about earthquake epicentres. However, large quakes do not rupture at a single point so we need to consider the finiteness of their ruptures. Imagine ripping a piece of paper - the first point where the paper breaks is like the epicentre, but you can end up with a substantially long tear. The larger the earthquake magnitude, the longer the portion of the ruptured fault. For a magnitude ~7 earthquake, rupture lengths are ~100km; for magnitude 9, we think about ~1000 km long ruptures. Slip models, back-projection analysis and aftershock distributions point to Friday's quake rupturing ~300 km south of the epicentre, bringing Bangkok much closer.

(4) *DIRECTIVITY* Because the fault unzipped towards the south, this will have led to a focussing of seismic energy in that direction (called "directivity"), potentially amplifying the shaking intensity in a swathe including Bangkok.

(5) *SUPERSHEAR RUPTURE* Early results shared online indicate that this southward-directed rupture may have travelled at "supershear" speed - the seismological equivalent of a sonic boom from an aeroplane. Instead of a jet breaking the sound barrier, Friday's rupture unzipped at speeds exceeding the speed at which seismic waves travel through the crust, trapping them and focussing even more energy in the direction of rupture, leading to increased shaking in a "Mach cone" that included Bangkok.

Seismic hazard models that guide building regulations do not include these directivity & supershear effects. If they contributed to the strong shaking and collapsed buildings in Bangkok, then we need to better understand these mechanisms, understand rupture directionality, & identify which faults are likely to "go supershear" to ensure that buildings are developed to withstand abnormally strong shaking

Kevin McCue

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Mar 31, 2025, 5:47:33 AMMar 31
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As well as the seismology, you need to look at engineering to explain the damage in Bangkok. The only building that collapsed was under construction at the time, its most vulnerable state. But why did it collapse, the only building to do so. Must have been something about its state at the time. That’s what I would focus on first. For the seismologist, did it collapse under shear waves or surface waves? It was certainly in the maximum shear wave quadrant and directivity may well have contributed to the collapse as well as the basin effect. I think a lot more analysis should be undertaken of this unique collapse before you start talking about changing hazard models.

As for the damage in Myanmar, most of the buildings there were built before the code was implemented.  The ground shaking would have been very strong and the duration tens of seconds. Again foundation conditions would have been critical - some buildings were left leaning over but not collapsed. Are there any strong motion records in Mandalay or near the fault? How can we learn if there weren’t? Lesson for Australia. 

Has anyone looked at surface faulting yet? That may have been another factor in the damage.
 
My recommendation would be that engineering codes should demand that designers of major and important buildings everywhere adopt base isolation, surely we learned that from Christchurch. 

Kevin








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Michael Griffith

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Mar 31, 2025, 9:59:55 PMMar 31
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Further to Kevin’s comments, I should note that base isolation can be very effective if it shifts the natural period of a conventional fixed-base building to a longer period away from the dominant shaking frequencies of the strong ground shaking.  However, the natural period of tall structures (say > 10 stories) are normally outside of the damaging shaking frequencies.  To use base isolation that is flexible enough to shift the natural period of a high rise to something significantly  > 1sec would mean that they would move significantly under every day wind loads.

 

Mike

 

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Tatheer Zahra

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Mar 31, 2025, 10:06:28 PMMar 31
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Kevin McCue

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Mar 31, 2025, 10:59:52 PMMar 31
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Thanks Mike, that is all true but….
I have watched the Bangkok tall building collapse on tv a number of times and it reminded me of the collapse of the Twin Towers in New York. I suspect a similar mechanism, upper structure collapse caused by higher mode induced stresses and then progressive failure of the whole building under the dynamic vertical load.
So we would not be concerned with shifting the natural period but with filtering out the higher modes, using base isolation.
This could be modelled in the lab quite nicely I think.
Cheers
Kevin

Hong Hao

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Mar 31, 2025, 11:06:38 PMMar 31
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It looks like progressive collapse occurred. The collapse started from the top floor, impacted and damaged the floor below and caused progressive collapse. Because of the flat slab design of the building without beams, the impact damage to the flat slab floor caused the column lost stability.

 

Bangkok basin has the natural vibration period of about 1 second according to a paper published by Prof. Pennung Warnitchai in 12WCEE in 2000. Prof. Warnitchai was also the leader in developing Thai seismic design code. The code published in 2018 divides Bangkok into 10 microzones. It has specific requirements for flat slab design to resist progressive collapse, similar to US DoD method. Unfortunately this building still collapsed. This building was designed by a consulting firm from Italy and is under construction by a Chinese contractor.

 

The epicentral distance to Bangkok is about 1000 kilometers, we can expect long period motions and site amplifications. More than 30 highrise buildings in Bangkok suffered different levels of damage, but only this one collapsed.  Resonation contributed to these large responses.  

 

Hong

Hong Hao                                                                     

BS, MS, PhD                                                                                                 IMCAE, DistFIAPS, FTSE, FASCE, FISEAM
ARC Laureate Fellow, John Curtin Distinguished Professor | Department of Civil Engineering Building 204
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Web | https://staffportal.curtin.edu.au/staff/profile/view/Hong.Hao                                                         Web | http://structuraldynamics.curtin.edu.au/ 

Professor, Earthquake Engineering Research and Test Center                                                     Guangzhou University                                                                        Email: hao...@gzhu.edu.cn

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geor...@bigpond.net.au

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Mar 31, 2025, 11:21:01 PMMar 31
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Kevin

 

I agree with you that it appeared to fail from the top down like the World Trade Centre.  However I doubt very much that it was due to higher mode excitation.  This was a building still under construction. My suspicion is that they had poured the slab forming the floor of the next higher story, but the columns supporting it had not been fully braced with the lift well walls and maybe shear walls, which would been following a story or two behind.  The upper story would then have had a lower stiffness that the lower ones which would have resulted in something like a whiplash behaviour of the upper floor.  We will have to wait until the results of an engineering investigation to find out if this is what happened, but I suspect that it will be due to a temporary lack of stiffness due to the construction procedure.

 

Regards

 

George

 

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Leo Noicos

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Mar 31, 2025, 11:39:17 PMMar 31
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To add another cause for the collapse of the building under construction is that perhaps the lower columns were still "green" well below the 28day strength??
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Leo Noicos

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richar...@gmail.com

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Mar 31, 2025, 11:49:16 PMMar 31
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Great conversation,

 

I agree with George. For example, the concrete might have been poured recently and not be at its full strength or perhaps even still not fully set. Internal brick walls would be missing. Ductility might be missing from the floor to column connections. Load paths might be partially through formwork and scaffolding. Lots of opportunities for vulnerability.

 

Regards,

 

 

Richard

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Kevin McCue

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Apr 1, 2025, 12:52:01 AMApr 1
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Thanks George and Richard
So the earthquake had nothing to do with the collapse? at least you haven’t specified what role it played but I can’t envisage how the fundamental mode could have caused the problem without structural elements falling outside the footprint of the building. It looked to collapse vertically. Interstorey drift would have been minimal in the fundamental mode at a distance of 1000km. 

geor...@bigpond.net.au

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Apr 1, 2025, 1:21:49 AMApr 1
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Kevin

 

The earthquake had everything to do with the collapse.  It would have excited the vibration of the Bangkok basin at its natural period of about 1 sec according to Hong Hao, which in turn would have caused resonant response in buildings with similar natural periods. Presumably the building that collapsed was one of these. What probably made this building different was that because it was still under construction the interstory stiffness of the assumed uncompleted top story may have been markedly less than that of the completed stories below. If the stiffness of the top story of a building is markedly less than the stiffness of the lower stories, then the interstory amplitude of vibration of the top story in the fundamental mode can be much greater than that of the stories below it, making it much more susceptible to collapse. I call it the whiplash effect. It can also be a problem for flexible towers mounted on the top of buildings, not only in earthquakes but also when tall buildings supporting flexible towers are excited into vibration by wind.

Kevin McCue

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Apr 1, 2025, 6:43:18 AMApr 1
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Thanks George, I just couldn’t find why in your earlier mention. The trouble is that a 33 storey building would be expected to have a natural period of 3.3s (N/10) roughly, but given it was under construction the period was probably longer maybe 3 to 5s discounting the 1s basin resonance possibility. But your whiplash effect could easily have been generated by a higher mode and then all the other factors you mention would come into play.

richar...@gmail.com

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Apr 1, 2025, 6:46:47 PMApr 1
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Yes Kevin,

 

It would be the displacement demand that strains the top floor that leads to the initial damage and consequent collapse. Unless there is not sufficient ductility and resilience to absorb the energy, the top floor collapses.

 

The weight then impacts on the next floor which is probably only a week or two old and so not full strength anyway.

 

So, it may not matter what frequencies are present as long as the displacement demand is large enough to breach the practical ductility present – scaffolding, wet or 2 day old concrete slab or columns or column/slab connecctions (pumchiing shear in a weak concrete state).

 

 

Richard

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geor...@bigpond.net.au

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Apr 1, 2025, 7:42:43 PMApr 1
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Kev

 

Looking at the swaying building in the video circulated by Adam the period of that building is much larger than 1 second – more like 5-6 seconds – which suggests the natural period of the basin is probably of this order at this location.  It depends on the depth of the alluvium at the location so this may vary.  Because of the form of construction – flat slab and columns, not beam and columns – the period of this building would probably be more than N/10 which is a very empirical estimate. I don’t think higher modes of vibration would have been the problem.

 

In the Newcastle earthquake a similar phenomenon occurred at a substation where some structures failed and others didn’t, which we investigated at CSIRO.  It turned out it was due to the structures which failed being located on soft soil of a depth which produced a period of vibration of the soil column that closely matched their natural frequency, and these structures also being characterised by having an upper component much more flexible than the rest of the structure which was the component that failed.  Unfortunately I cannot find the report we wrote on it.

geor...@bigpond.net.au

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Apr 1, 2025, 9:10:21 PMApr 1
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Kev

 

There is an interesting photo on Wikipedia of the building 3 months ago. See https://en.wikipedia.org/wiki/2025_Bangkok_skyscraper_collapse#/media/File:State_Audit_Office_of_Thailand_2024.jpg

 

It looks as if the top story was different form the lower stories, with a much higher story height and therefore probably much less interstory stiffness.  This may be a major contributing factor.

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