04.12.2019 : Andrew Baird

Seismic risks in ASEAN?

At 6:50am on 21 November, a magnitude 6.1 earthquake occurred near the Thailand-Laos border. Having lived in Christchurch through the 2010-2011 earthquake sequence, I have experienced thousands of earthquakes, but this was the first time I had felt one outside of New Zealand, and the first time from as high as the 22nd floor in Bangkok, where I have lived for the last two and a half years. Despite being over 600km from the epicentre, there were thousands of accounts of people feeling the quake in Bangkok, as social media was abuzz with videos of swinging lights and chandeliers

How was it that this earthquake was felt in the Thai capital?

Much like our voices create sound waves with a variety of low and high frequencies, earthquakes produce seismic waves over a range of frequencies. High frequency seismic waves, which cause short and sharp shaking, get damped out over long distances so are only felt close to the earthquake source. Low frequency seismic waves, which cause long and slow shaking, can travel long distances – the same waves that travelled all the way to Bangkok.

Bangkok is underlain with thick layers of soft clay that amplify seismic waves as they travel up to the ground surface. In addition, these seismic waves become trapped within the basin of soft soils, reflecting waves back to the surface like an echo chamber. When combined, these effects cause ground shaking three to four times stronger at the ground surface compared to areas without soft soil. 

How do buildings respond to the ground-shaking?

All buildings have a frequency that they naturally oscillate at, which is dependent on several variables but is closely linked to the building height. Short buildings have a high natural frequency, and tall buildings have a low natural frequency. How a building responds to an earthquake is determined by how much energy is present in the ground shaking at the building’s natural frequencies. For example, tall buildings (> 15 stories) will respond to low frequency shaking. This is a phenomenon called “resonance” and is like being on a swing - if the push is applied at the same time with each swing, the amplitude of the swing will increase. All these effects meant that the 21 November earthquake was felt by many in Bangkok, particularly those in the tall buildings despite the epicentre being located hundreds of kilometres away.

What are the seismic risks in ASEAN?

As a structural engineer from New Zealand, earthquakes have been at the centre of both my education and time working as a consulting engineer. New Zealand is often referred to as the Shaky Isles due to the regularity of earthquakes. As I write this, a M5.1 earthquake occurred on New Zealand’s east coast. 

In Southeast Asia, while earthquakes are rare, the consequences can be severe when they do occur. It is estimated that 37% of the total ASEAN population are exposed to the risk of an earthquake of Modified Mercalli Intensity (MMI) 7 and above. This is a measure of shaking that corresponds to slight to moderate damage in well-built structures and considerable damage in poorly built or badly designed structures. Compounding the impact, ASEAN countries have on average 146 people per sq km, eight times more than New Zealand, and this increases sharply in the capital cities. In Bangkok, the population density is 5,300 people per sq km and in Jakarta a whopping 14,500 people per sq km.

With a combination of high-density, high-risk and many high-rises, it is unsurprising that earthquakes and tsunami rank as the number one natural disaster threat to ASEAN residents. The M7.5 earthquake and subsequent tsunami that hit Palu, Indonesia in September 2018 left 4,340 either dead or missing. This came just one month after the M6.9 Lombok earthquake which left 564 dead or missing. These two earthquakes accounted for nearly half of all deaths from natural disasters worldwide in 2018. 

Public awareness for seismic risks in Bangkok

Unfortunately, public awareness of the potential seismic risk to Bangkok is limited. This is quite understandable since Bangkok does not have a history of destructive earthquakes like other parts of the world. The nearest active fault zone able to generate a M7.5 earthquake is located 120km from the city. In many circumstances, this distance would be enough to protect Bangkok from severe shaking, however, due to the soft soil effects previously mentioned, taller buildings in Bangkok are exposed to a higher risk than many realise.

The M8.1 earthquake that devastated Mexico City in 1985 serves as a frightening example of what could happen in Bangkok. The epicentre of that quake was over 350km away from Mexico’s capital, but still killed over 5,000 people. The soft soil beneath Mexico City amplified the earthquake shaking causing the collapse of many mid-rise reinforced concrete buildings.

The most effective way to reduce seismic risk is to design for it. In Thailand, the first seismic design regulations were enforced in 2007, which required buildings over 15 metres tall to be designed for earthquakes. Prior to this, most buildings in Bangkok were only designed for lateral wind loads. In 2009, the Department of Public Works and Town & Country Planning (DPT) introduced an earthquake design standard for buildings in Bangkok that modernised the approach to seismic design, considering effects like resonance in mid-rise and taller buildings, and more recently, a new DPT design standard has been made public to replace the 2009 version. This new standard requires buildings to be designed for stronger shaking, as well as increased requirements on the steel reinforcement inside reinforced concrete, especially at the parts of the building that are subjected to seismic loads, such as walls and foundations.

These design requirements mean the newest generation of skyscrapers being built in Bangkok will be more resilient to earthquake shaking. Strengthening existing buildings for earthquakes presents a more significant challenge, not only in Bangkok, but in many other ASEAN cities. While the probability of a large earthquake occurring is low, the one thing we know for certain is that it’s not a question of ‘if’ but ‘when’ it will occur.  

 

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About the Author

Andrew Baird

Senior Structural Engineer

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