Myanmar has been hit by a magnitude 7.7 earthquake, which also affected neighbouring Thailand, its tremors felt as far afield as Cambodia and India.

Much of the devastation caused by Friday’s quake appeared to be in Myanmar’s ancient capital of Mandalay, close to the epicentre in the Sagaing region, where buildings toppled and infrastructure buckled. More than 140 people were killed in the country, according to state media.

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Myanmar has been struck by several quakes since a magnitude 7.3 earthquake in the southern city of Bago in 1930, which killed at least 550 people, according to a United Nations seismic risk assessment.

So, what makes this Southeast Asian country, which has been blighted by nearly four years of civil war, so vulnerable to earthquakes and how big was this one?

What causes earthquakes?

First of all, a quick explanation of what an earthquake actually is. The Earth is made up of three parts: a molten, mostly metallic core at the centre, surrounded by a hot, nearly solid layer of rock called the mantle, with a jigsaw-like crust on the outside that is made up of constantly shifting tectonic plates.

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This movement of the plates on the slippery mantle, at different speeds and in different directions, causes energy to build up. The release of this energy causes the intense shaking of the planet’s surface that we call an earthquake. When the energy is released below the ocean, it creates a series of huge waves known as a tsunami.

Aftershocks are triggered “because of changes to stress in the Earth from the main shock,” according to Will Yeck, a seismologist with the US Geological Survey (USGS).

What lies beneath the surface in Myanmar?

Myanmar’s location between two tectonic plates – the India and Eurasia plates – places it at particular risk of earthquakes.

The boundary between the two plates is called the Saigang Fault. Experts describe it as a long, straight line running approximately 1,200km (745 miles) from north to south through cities such as Mandalay and Yangon, placing millions of people at risk.

According to the USGS, the Myanmar earthquake occurred because the India and Eurasia plates were rubbing sideways against each other, a motion described as “strike-slip faulting”.

Dr Rebecca Bell, a tectonics expert at Imperial College London, cited by the London-based Science Media Centre, compared the boundary between the two plates to the famous San Andreas Fault in California, which caused the deadly Northridge earthquake in 1994.

“The straight nature means earthquakes can rupture over large areas – and the larger the area of the fault that slips, the larger the earthquake,” she was quoted as saying.

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How big was the earthquake?

The strength of the earthquake is measured on the Moment Magnitude Scale, which largely replaced the famous Richter scale in the 1970s.

Friday’s quake of 7.7 was considered powerful, unleashing chaos in Myanmar and Thailand.

In Thailand’s capital, Bangkok, a 33-storey high-rise that was still under construction crumbled, killing at least eight and trapping dozens of construction workers under the rubble.

In Myanmar’s Mandalay, buildings were toppled, the royal palace was damaged, and the road-and-rail Ava Bridge collapsed. There was also damage in the modern capital, Naypyidaw, and the former capital, Yangon. State media said at least 144 people had been killed across the country.

The USGS estimates that nearly 800,000 people in Myanmar may have been within the zone of the most violent shaking, with the death toll expected to rise sharply over the coming days.

How much damage is expected?

The earthquake took place at a relatively shallow depth –  just 10km (six miles) deep.

Dr Ian Watkinson, from the Department of Earth Sciences at Royal Holloway, University of London, was cited by the Science Media Centre as saying that shallow earthquakes can create a lot of damage, given that “the seismic energy is not dissipated much by the time it reaches the surface”.

While some regions of the world along active fault lines, including California and Japan, have building codes designed to withstand earthquakes, the infrastructure in the region hit by Friday’s quake is less well equipped.

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As Watkinson puts it, Myanmar has gone through “rapid urbanisation”, with “a boom in high-rise buildings constructed from reinforced concrete”.

He believes Friday’s earthquake could create levels of destruction comparable to the 2023 magnitude 7.8 quake in southern Turkiye, where many buildings collapsed after years of unregulated construction.