Why New Zealand is called the Shaky Isles

[Parihaka]

Firstly a comparison. On the left California, in the middle New Zealand (henceforward known as Aotearoa) and on the right the UK and Ireland.
All images taken from the same altitude.

[Parihaka]

As you can see from the image above, Aotearoa lies on or near the meeting of the Pacific and Indo_Australian plates. The Te Wai Pounamu lies upon a region where the Indo_Australian plate is being subducted under the Pacific plate and Te Ika A Maui to the west of where the Pacific plate is being subducted under the Indo_Australian plate.

[Parihaka]

So, to Te Wai Pounamu, the South Island. As this part of the pacific plate bulldozes the I_A plate under, large portions of this old and highly compressed rock are broken up and stack up into mountains, tough rock but heavily fractured. For a size comparison here's shots of the Alps and the southern alps, again from matched altitudes.

[Parihaka]

As you can see, Te Wai Pounamu is formed from the Alps being upthrust and eroded down to the coastal plains in the east via braided rivers

[Parihaka]

Apart from a couple of millennia-old volcanos still visible (Christchurch and Dunedin peninsulas) Te Wai Pounamu is comprised of these growing, heavily fractured mountain ranges and the alluvial plains to the east. This gives us some pretty spectacular views, and some memorable earthquakes.
I'm sure you recall the recent Christchurch earthquakes, the only thing unusual in these were they were virtually under one of our cities and very shallow, so created a lot of damage and deaths.

[Parihaka]

Fiords of course

[Parihaka]

and some quite high mountains

[Parihaka]

So, to Te Ika A Maui, the North Island, an altogether more boring place scenery-wise though it does have it's excitements. Consisting of the scrapings of the Pacific plate, it's basically a big pile of mud and ash heated from below by some fairly significant upwellings of lava, hence the recent volcanic farting.
To wit, Tongariro volcano on the right and its two brothers Ruapehu (left) and Ngarahoe (middle, classic cinder cone)

[Parihaka]

From above you can see they are all three merely side vents on the lip of the main caldera Taupo, currently filled with a lake (not the little lake, the big one)

[Parihaka]

Lake Taupo lies in a caldera created by a supervolcanic eruption which occurred approximately 26,500 years ago. According to geological records, the volcano has erupted 28 times in the last 27,000 years. It has ejected mostly rhyolitic lava, although Mount Tauhara formed from dacitic lava.
The initial event 26,500 years ago is the largest eruption and is known as the Oruanui eruption. It ejected an estimated 1170 cubic kilometres of material and caused several hundred square kilometres of surrounding land to collapse and form the caldera. The caldera later filled with water, eventually overflowing to cause a huge outwash flood.[3] It is possible that the Lake Taupo event contributed to starting the Last Glacial Maximum.[citation needed]
Several later eruptions occurred over the millennia before the most recent major eruption, which is traditionally dated as about 180 CE from Greenland ice-core records. Tree ring data from two studies suggests a later date of 232 CE ± 5.[4] Known as the Hatepe eruption, it is believed to have ejected 100 cubic kilometres of material, of which 30 cubic kilometres was ejected in the space of a few minutes. This was one of the most violent eruptions in the last 5000 years (alongside the Tianchi eruption of Baekdu at around 1000 and the 1815 eruption of Tambora), with a Volcanic Explosivity Index rating of 7; and there appears to be a correlation, to within a few years, of a year in which the sky was red over Rome and China. The eruption devastated much of the North Island and further expanded the lake. The area was uninhabited by humans at the time of the eruption, since New Zealand was not settled by the Māori until about 1280. Possible climatic effects of the eruption would have been concentrated on the southern hemisphere due to the southerly position of Lake Taupo.[5] Taupo's last known eruption occurred around 30 years later, with lava dome extrusion forming the Horomatangi Reefs, but that eruption was much smaller than the 180 CE eruption.
Underwater hydrothermal activity continues near the Horomatangi vent,[6] and nearby geothermal fields with associated hot springs are found north and south of the lake, for example at Rotokawa and Turangi. These springs are the site of occurrence of certain extremophile micro-organisms, that are capable of surviving in extremely hot environments.[7]
The volcano is currently considered to be dormant rather than extinct.

...

[Parihaka]

So, all very exciting and as you can see volcanism covers a large portion of Te Ika A Maui, mostly active, and of course a large number of earthquakes to boot.

[Parihaka]

All very interesting you say and won't it be a bugger when we have to evacuate everybody north of Taupo to Australia when it blows again. (consider yourself warned BF et al.)

This however isn't the main and immediate threat. Let me introduce you to our principle city of Auckland.

[Parihaka]

This shot is low angle with topographical features exaggerated by a factor of three to make the major volcanic cones more visible within the city. All of them are capable of erupting any time.

[Parihaka]

IIRC there are 30 something potential eruption points within Auckland, some of them highlighted here

[ambidex]

Time to merge with Australia ? ;)

NZ shaky most of the time.