The big ones

Like the bogeyman lurking in the basement in a horror movie, the next big earthquake hides in our collective conscious – too scary to consider for many. Earthquakes are unpredictable and scary and the impacts of significant past earthquakes are etched into our history. But there is no avoiding it, we live on shaky islands – New Zealand records an average of 20,000 earthquakes each year – and the more we know, the better prepared we can be for the next ‘Big One’, whenever and wherever it comes. So, let’s take a deep breath, head down to the basement and turn on the lights to see what’s really there…

But when it comes to impacts, the magnitude of an earthquake isn't the only factor, here are some others: 

Depth – earthquakes can occur anywhere from the surface to 700km deep. The shallower the earthquake the closer it is to us on the surface and so the more intense the shaking.

Ground conditions – different ground conditions (rock and soil types) react to shaking differently, e.g. soft sediments can experience liquefaction.

Building Construction – with every earthquake, something new is learned by engineers and building designers about how they will react to a quake and this is translated into the New Zealand Building code.

Time – the time of day and time of year an earthquake takes place can impact the number of casualties, damage and ongoing effects. In winter, for example, woodfires can create a hazardous situation during an earthquake and hypothermia becomes a greater risk for those with limited shelter in the aftermath.

Secondary hazards – Landslides, tsunami, liquefaction, aftershocks, fire and rockfall are common secondary hazards associated with large earthquakes.

Location – Obviously, the more remote an earthquake, the fewer impacts for people. A very large earthquake in Fiordland would hardly be noticed beyond Te Anau and surrounding settlements, whereas a much moderate one in Wellington would have serious implications for a much bigger population and the condensed urban environment.

Past big ones

The largest earthquake ever recorded in New Zealand was the magnitude 8.2 quake on the Wairarapa Fault in January 1855. Some of the land around Wellington was uplifted by 6.4m and in other places it moved 18m horizontally. This was an event of international significance – before this we didn’t know that earthquakes caused shifts of the landscape! This earthquake also caused a tsunami in Cook Strait, which inundated many buildings around Wellington Harbour.

Our little islands have experienced many large earthquakes in the past. Māori oral history includes many references to the shaking of Papatuānuku, and there is a wealth of geological evidence that reveals when some of our big ones occurred (including the last rupture of the Alpine Fault in 1717). While earthquakes have been around for as long as New Zealand has existed, the human impacts have increased over the last 200 years with increased urbanisation and population. Most casualties in earthquakes are from building collapse and/or fire.

The map below shows the most significant quakes recorded since 1848, many of which will be familiar to you. The good news is that each big earthquake increases our understanding of the inner workings of our landscape and how best to prepare for the next one. 

New Zealand’s plate boundary has a complicated twist – in the north at the Hikurangi Trench, the Pacific Plate is moving under the Australian Plate, whereas in the south at the Puysegar, it’s the opposite!

When will it happen?

By focusing research along the plate boundary scientists are learning more all the time about where the next big one might happen. So now lets take a look at what we know about the when, and thats based on looking at the pattern of what has happened in the past. 

The Good news: really big and damaging earthquakes don’t happen very often. While GNS Science locates an average of 20,000 earthquakes in New Zealand each year, only about 100-150 are large enough to be felt - the rest are only perceptible to seismometers. 

The Bad news:  we didn’t have a magnitude 8 earthquake last century. In fact, the last magnitude 8 earthquake was New Zealand’s largest in 1855 on the Wairarapa Fault (part of the Hikurangi subduction zone). Before that, there was one in 1826 in Fiordland (part of the Puysegur Trench subduction zone) and in 1717 on the Alpine Fault.

The Hikurangi Subduction Zone and the Alpine Fault represent the two biggest sections of our plate boundary. Let's take a look at what we know about when a big earthquake is likely to occur on each.