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How Earthquake-Safe are Singapore’s Buildings? Assistant Professor David Lallemant Explains
In light of the recent earthquake in Sumatra, the structural safety of buildings in Singapore has become an issue of greater concern. We sat down with Assistant Professor David Lallemant, a Principal Investigator at the Earth Observatory of Singapore (EOS), to find out how earthquake-safe Singapore’s buildings really are.
In the Q&A below, Asst. Prof Lallemant discusses earthquake risks present for Singapore, how buildings can be designed to resist earthquake damage, and what building damage Singaporeans can expect in the event of an earthquake.
Q1. What makes buildings in Singapore resistant to earthquake damage? Is it possible to make a building “earthquake-proof?”
It is not possible to construct buildings that are completely "earthquake-proof”. Building codes with proper earthquake-resistant design standards are developed to ensure a low probability of significant damage from earthquakes, but even the best building codes don’t guarantee that buildings are ever “earthquake-proof.”
Building standards in Singapore were improved in 2013 to more explicitly account for possible tremors from distant earthquakes. The 2013 standards require that buildings taller than 20 metres, and located on soft soils, should be designed specifically to resist earthquake forces. This typically means that they should have more lateral bracing – think of the back part of an Ikea shelf, the metal 'x', which keeps the shelf from swaying – and more ductility – think of a rubber-band that can stretch without breaking.
Q2. How do we know when an earthquake will cause damage to buildings? Would we expect an earthquake like the one that recently struck Sumatra to cause damage to buildings in Singapore?
The level of damage that an earthquake will cause to a building is determined by (1) the intensity of shaking and (2) the resistance of the building. The first depends on the magnitude of the earthquake, the distance from the fault rupture, the soil characteristics, and quite a bit of randomness. These factors will all contribute to the intensity, duration, and characteristic of the shaking. The resistance of the building is determined by its specific engineering design, materials, and construction quality.
Buildings in Singapore are well-constructed and the earthquake hazard is low. As such, we would expect buildings to be safe even in the case of a large (yet distant) earthquake.
However, while “safe” means “a low probability of failure,” it doesn’t mean that the buildings would be completely undamaged. An earthquake could cause damage to the facade of a building, break its windows, or cause objects and equipment inside to fall, even if the building structure itself is undamaged. This can be a major concern for museums or buildings with high-value or high-sensitivity contents (like servers, sculptures, or hospital equipment).
Even modern building codes in high-seismic zones are only meant to ensure “life-safety”, but do not guarantee that buildings will be undamaged, or even functional after an earthquake.
Q3. Some Singaporeans have reported that they have felt buildings sway because of earthquake tremors. Others haven’t felt anything. Why?
It is not surprising that people have felt buildings swaying in Singapore. It is likely that those people were on high floors of tall buildings. Large earthquakes cause strong seismic waves at a wide range of frequencies. The low-frequency waves can travel very far, even when the high-frequency waves get dampened and disappear.
Tall buildings act like amplifiers to these same low-frequency waves, because their natural period of vibration is similar to that of low-frequency seismic waves. This amplification of vibration is also known as “resonance,” and is the same phenomenon that explains the working of a singing meditation bowl: when the rim of the bowl is rubbed at its natural frequency, it will cause resonance of the rim, producing strong vibrations and a distinctive singing sound.
The same can happen in earthquakes. This is why in 2008, people at the top of Taipei 101 in Taiwan - the 2nd tallest building in the world - felt swaying from the M 8.0 Sichuan earthquake, 1,800km away! This swaying is often very uncomfortable and scary for occupants, but it is only dangerous if it is strong enough to cause damage to buildings.
Q4. The closest fault to Singapore is around 400 km away. If an earthquake the strength of the 2004 Boxing Day one were to strike there, how much damage would buildings in Singapore sustain?
The 2004 Boxing Day earthquake had a magnitude of 9.3. The closest known fault to Singapore is the Sumatran fault (~400km away), but all indications suggest that it could not generate an earthquake larger than about M 7.7 - 7.8. Such an earthquake would be felt in Singapore, but the distance of 400km would significantly reduce the force of the shaking.
The new 2013 building regulations target tall buildings which would otherwise be susceptible to the low-frequency seismic waves which could reach Singapore. So we would expect buildings to be safe even in the event of a major distant earthquake. That being said, as I mentioned before, “safe” doesn’t mean that they would be completely undamaged. The cost of widespread low-level damage could still be very significant.
Q5. Are there other faults that we should be worried about in terms of their potential to generate earthquakes that could damage Singapore’s buildings?
Our knowledge is limited and therefore it is possible that an earthquake could occur on an unknown fault, which is smaller but closer to Singapore than the Sumatran fault. This is why continued research in this field is so important. We still have many knowledge gaps when it comes to natural hazards in the region; gaps that institutions like EOS are actively working to fill.
It should also be noted that a large earthquake in the region would be of concern to Singaporeans even if Singapore is completely undamaged. Singapore is connected to its neighbouring countries through numerous economic, social and humanistic ties. It is thus crucial that we consider how best to promote the resilience of the entire region.
Asst. Prof Lallemant was interviewed on this same topic for an article that appeared in the Straits Times. The article can be accessed here.