Esme Stallard
Climate and science reporter, BBC News
Moment Bangkok high-rise collapses following Myanmar earthquake
A major earthquake in Myanmar on Friday has caused more than 1,600 deaths and led to the collapse of numerous structures.
Even though the south-east Asian nation is a high risk region for earthquakes, neighbouring Thailand and China – which were also affected by the quake – are not.
The Thai capital, Bangkok, sits more than 1,000km (621 miles) from the epicentre of Friday’s earthquake – and yet an unfinished high-rise building in the city was felled by it.
Here we will explain what caused this earthquake, and how it was able to have such a powerful effect so far away.
What caused the earthquake?
The earth’s upper layer is split into different sections, called tectonic plates, which are all moving constantly. Some move alongside each other, whilst others are above and below each other.
It is this movement that causes earthquakes and volcanoes.
Myanmar is considered to be one of the most geologically “active” areas in the world because it sits on top of the convergence of four of these tectonic plates – the Eurasian plate, the Indian plate, the Sunda plate and the Burma microplate.
The Himalayas were formed by the Indian plate colliding with the Eurasian plate, and the 2004 Tsunami as a result of the Indian plate moving beneath the Burma microplate.
Dr Rebecca Bell, a reader in tectonics at Imperial College London, said that to accommodate all of this motion, faults – cracks in the rock – form which allow tectonic plates to “slither” sideways.
There is a major fault called the Sagaing fault, which cuts right through Myanmar north to south and is more than 1,200km (746 miles) long.
Early data suggests that the movement that caused Friday’s 7.7-magnitude earthquake was a “strike-slip” – where two blocks move horizontally along each other.
This aligns with the movement typical of the Sagaing fault.
As the plates move past each other, they can become stuck, building friction until it is suddenly released and the earth shifts, causing an earthquake.
Why was the earthquake felt so far away?
Earthquakes can happen at up to 700km (435 miles) below the surface. This one was just 10km from the surface, making it very shallow. This increases the amount of shaking at the surface.
The earthquake was also very large – measuring 7.7 on the moment scale. It produced more energy than the atomic bomb dropped on Hiroshima, according to the US Geological Survey.
The size of the earthquake was because of the type of fault, said Dr Bell.
“The straight nature [of the fault] means earthquakes can rupture over large areas – and the larger the area of the fault that slips, the larger the earthquake,” she explained.
“There have been six magnitude 7 or greater earthquakes in this region in the last century.”
This straight fault also means a lot of the energy can be carried down its length – which extends for 1200km south towards Thailand.
How earthquakes are felt at the surface is also determined by the type of soil.
In soft soil – which is what Bangkok is built on – seismic waves (the vibrations of the earth) slow down and build up, getting bigger in size.
So Bangkok’s geology would have made the ground shaking more intense.
Why did just one skyscraper collapse in Bangkok?
While dramatic footage has emerged of high-rise buildings in Bangkok swaying during the quake – knocking water from rooftop pools – the unfinished headquarters for the auditor-general’s office Bangkok’s Chatuhak district appears to be the only skyscraper to collapse.
Prior to 2009, Bangkok did not have a comprehensive safety standard for constructing buildings to withstand earthquakes, according to Dr Christian Málaga-Chuquitaype, a senior lecturer in earthquake engineering at Imperial College London.
This means that older buildings would have been particularly vulnerable.
This is not unusual, as earthquake-resistant buildings can be more expensive to construct and Thailand, unlike Myanmar, does not frequently experience earthquakes.
Dr Emily So, a professor of architectural engineering at the University of Cambridge, noted that older buildings can and have been strengthened, such as in California, western Canada and New Zealand.
Watch: Dashcam captures moment Bangkok building collapses
Prof Amorn Pimarnmas, president of the Structural Engineers Association of Thailand, said that while there were regulations in 43 provinces on earthquake-proofing buildings, less than 10% of buildings are estimated to be quake-resistant.
Yet the building that collapsed was new – in fact, it was still under construction when the earthquake hit – and the updated building standards would have applied.
Dr Pimarnmas said Bangkok’s soft soil may have also played a part in its collapse, as it can amplify ground motions three or four times over.
He added: “However, there are other assumptions such as material (concrete and reinforcements) quality and some irregularity in [the] structural system. These remain to be investigated in detail.”
Having studied the video, Dr Málaga-Chuquitaype said it appears a “flat slab” construction process was being favoured – which is no longer recommended in earthquake-prone areas.
“A ‘flat slab’ system is a way of constructing buildings where floors are made to rest directly on columns, without using beams,” he explained.
“Imagine a table supported only by legs, with no extra horizontal supports underneath.
“While this design has cost and architectural advantages, is performs poorly during earthquakes, often failing in a brittle and sudden (almost explosive) manner.”
What about the buildings in Myanmar?
Mandalay in Myanmar was much closer to where the ground slipped and would have experienced significantly more severe shaking than Bangkok.
Although Myanmar regularly experiences earthquakes, Dr Ian Watkinson, a lecturer in earth sciences at Royal Holloway University, thought it was unlikely that many buildings were constructed to be earthquake-proof.
“General poverty, major political upheaval, alongside other disasters – e.g. the Indian Ocean tsunami in 2004 – has distracted the country from concentrating on the unpredictable risks from earthquakes,” he said.
“This means that, in many cases, building design codes are not enforced, and construction happens in areas that could be prone to enhanced seismic risk, for example flood plains and steep slopes.”
Parts of Mandalay and its buildings also lie along the floodplain of the Ayerwaddy River. This makes them very vulnerable to a process called liquefaction.
This happens when the soil has a high water content, and the shaking causes the sediment to lose its strength and behave like a liquid. This increases the risk of landslides and building collapses, as the ground can no longer hold them up.
Dr So warned that there was “always a chance” of further damage to buildings near a fault line due to aftershocks – tremors that follow an earthquake, which can be caused by the sudden transfer of energy into nearby rock.
“Most of the time aftershocks are smaller than the main shock, and tend to decrease in size and frequency over time,” she said.
