The President of the New Zealand Society for Earthquake Engineering has stressed that no building can be 100% earthquake-proof.

"The hard thing for the public to comprehend is the way the frequency in the energy of the earthquake affected buildings," Peter Smith said, in the wake of the recent 7.8 magnitude earthquake. 

The more flexible buildings, often taller, suffered more damage because of the way the frequency of the building matched with the frequency of the earthquake.

Retired structural and civil engineer Laurie Petherick put it like this: "If you shake the bottom of a pencil at your desk slowly for a couple of seconds, nothing happens right? Well, if you do it for a longer period of time the stop starts to sway, like a metronome. This is what happened in this earthquake."

The reason why buildings reacted the way they did is because the earthquake itself was unusual, and "quite an event", for several reasons.

Most obviously, it was longer than most earthquakes are, meaning the energy shaking flexible buildings started to amplify while the energy directed at more solid buildings remained consistent.

Another issue is the unusual scope of the earthquake.

According to Petherick, "An earthquake is exactly the same as throwing a stone into a pond. The rings will go out in concentric circles. The rings that go out are the waves that you feel as they move past you".

This particular earthquake, however, ruptured over a length of over a hundred kilometres and triggered a second, simultaneous earthquake.

Flexible buildings were being attacked, effectively, from multiple angles. The longer and more slender the building, the worse the damage.

"That's the problem – there are a thousand different ways the ground can move. That's not even factoring in P-waves and S-waves – one moves along the surface of the road, and the other ones go way down into bedrock and bounce back up in a zig-zag."

"You can't plan or design for every type of earthquake that might happen."

Smith, too, stressed that "every earthquake is different".

"Some are very short, some are very long. What you're matching is the energy input of the earthquake to the natural vibration of the building."

Commonly – though not, Smith insisted, invariably – the further away from the epicentre of the quake, the more likely flexible buildings will be affected because of the type of unleashed energy that carries.

If the earthquake had a closer epicentre to Wellington, the flexible buildings would have held while the stockier ones wouldn't have, Petherick said.

"They would have been levelled. Simple as that."

The New Zealand design code is built around a central credo that both Smith and Petherick recited: protection of life, and prevention of loss of life, are at its core.

This means life is foregrounded over a property’s value, meaning, said Petherick, "they're designed to get people out, not stop any damage".

"Many of them are designed to take significant damage from earthquake but be totally safe [afterwards] to get people out."

In that sense – because, as Petherick noted, people got out in spite of structural damage – the buildings, despite their damage, actually succeeded in their task: minimising loss of life at a cost to the buildings themselves.

In Smith's words, "every time we get an earthquake we understand buildings better. A lot of research is done on elements or components. The way those components are put together, and how they don't necessarily provide good resilience".

"We're probably going to learn more."

However, Smith is concerned that not all the lessons learned from previous earthquakes have been – or will be – put into practice.

So, the unfortunate truth: no building will ever be entirely earthquake-proof.

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