How earthquakes have shaped our infrastructure

Being one of the most seismically active countries in the world, Aotearoa needs to be at the leading edge of protecting people, infrastructure and the economy from the impact of earthquakes. As part of WSP’s 150 anniversary, senior engineer Murray Triggs looks at how earthquakes have shaped our infrastructure and continue to challenge what we know. 

Despite the devastating earthquakes in Christchurch and Kaikōura happening so close together, there is no evidence the frequency of earthquakes has changed. GNS measures about 15,000 earthquakes per year and of these 150 can be felt, about one every three days. However, because our recent large earthquakes have been close to major population centres, the impact has been greater. 

This is a real issue. Fatalities resulting from earthquakes are predominantly caused by building failures and tsunamis. Globally there has been an increase in earthquake fatalities, not because of increased seismicity, but because of urbanisation. And the risk is growing. 

By 2025 more than 5,500 million people will live in cities; more than the entire 1990 global rural and urban population. This puts an increased concentration of people in harm’s way – and is a huge expense in repairing critical infrastructure. 

Earthquakes have cost Aotearoa close to $50 billion during the last decade, with the Government spending around $20 billion following Christchurch and $2 billion on Kaikōura. 

The challenge for engineers is to ensure our infrastructure can withstand the impact of seismic activity, reducing the risk of fatalities and providing resiliency. Ultimately good design is the difference between earthquake survival and disaster.

And we’re doing it.  Aotearoa is a world leader in seismic engineering because we’ve learnt from events and constantly seek to improve. Acknowledging that any loss of life is awful, we can also acknowledge improvements. The Christchurch and Napier earthquakes were similarly sized but Christchurch had a significant reduction in fatalities, something that can largely be attributed to the huge improvements in buildings’ seismic resistance. Where Napier’s fatalities occurred in multiple buildings, the tragic loss of life in Christchurch predominantly occurred in two buildings.  

Christchurch taught us new lessons and, lest we become complacent, Mother Nature tested us with the Kaikōura earthquake and highlighted new vulnerabilities. 

 

 

kaikoura Earthquake - Source: NCTIR

Frequency of shaking

Every building has a natural frequency. If you give it a strong enough shove, it will naturally rock back and forward with a frequency. A short building may have a period of 0.2 seconds, but a tall building may be at over 2 seconds per sway. If the frequency of the earthquake’s shaking coincides with the building’s own frequency, it will experience much more extensive damage.

Typically, short, sharp shaking abates near the quake. The Kaikoura earthquake resulted in strong frequency shakes of 0.8-1.2 seconds that lasted for an unusually long time and particularly affected Wellington buildings in the five to ten storey range. The earthquake was stronger and longer than the design standards these buildings were designed to.

These same buildings wouldn’t have been vulnerable with a major earthquake close to the centre. Conversely, the one and two storey, unreinforced masonry buildings that were untouched by the Kaikoura quake would be more likely to be damaged and potentially cause loss of life in a closer quake.

Engineers can design buildings that can withstand substantial shaking, but the reality is that if a fault line rips through a building, there is little prospect of it remaining safe. 
We can’t eliminate all the risks that come from random seismic events and another Napier, Christchurch, Kaikoura will strike again in the future.

However, with smart science, innovative engineering and pragmatic policies, we can reduce the loss of life and the cost. 

 
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