Earthquakes

You may have wondered, why do we get so many earthquakes in New Zealand?

To explain, let's take a step back.

Earth's outermost layer, the crust, is made up of massive slabs of solid rock, ranging from a few hundred to thousands of kilometres across. These are known as tectonic plates, and there are seven major ones, and they're constantly moving, at slightly different speeds and in slightly different directions, continuously reshaping Earth's landscapes.

And it's at the boundaries between these plates that earthquake and
volcanic activity is concentrated.

Aotearoa, New Zealand, is at the boundary between two of these major plates, the Australian Plate and the Pacific Plate. How the plates move against each other changes along this curving boundary.

In the North Island, the Pacific Plate is being pushed under the Australian Plate. This setting, with one plate going down beneath another, is called subduction.

At the southern end of the South Island, the opposite situation occurs, with the Australian Plate diving down beneath the Pacific Plate.

Through most of the South Island, the two plates grind past each other, creating the 600-kilometre-long main plate boundary fault. We call this the Alpine Fault, as it runs along the Southern Alps.

A fault is a fracture or a line of weakness in the crust, and it can be as short as a few metres or as long as a thousand kilometres. Much of the movement on
the Alpine Fault is sideways, but there's also some compression or pushing together and upward motion.

The upward movement is what causes the sudden change in the landscape between Westland and the high peaks of the Southern Alps, and the mountains continue to be driven upwards at a rate of about seven millimetres per year.

In the North Island, we have a subduction zone on the East Coast, and that can cause large earthquakes, such as those that have occurred in Japan or in Indonesia. It can also create smaller, but still major, earthquakes close to the coast, such as that in Napier in 1931. 

Through the centre of the North Island, we have the volcanic region, where the land is being pulled apart. Earthquakes are generally smaller in
this region, but can often occur in swarms. Swarms can include hundreds of events and can continue for days, weeks or months, such as the
activity we saw in Taupō in 2022. Earthquake swarms in
these regions can indicate volcanic eruptions
and other volcanic activity.

But why does this mean we have earthquakes? While the plates are trying
to push against or past each other, often they're stuck, and stress is just building up. When the stress is big enough, the faults move suddenly and cause earthquakes.

And here in Aotearoa New Zealand, we have faults everywhere. There are the big plate boundary ones, like the Alpine Fault, but there are also many hundreds of smaller faults on the North and South Island and extending out into the oceans, and each of them can host earthquakes.

Every year, GNS Science records over 20,000 earthquakes in Aotearoa New Zealand. On average, about 100 to 150 of these earthquakes are big enough to be felt, though that can vary if we have a big sequence. The majority of the earthquakes, the smaller ones, we only know about because they're
recorded by our large network of instruments.

Living in an earthquake-prone country means you've always got to be prepared. And remember, if the earth starts shaking, drop, cover and hold.