Wobbly Wellington

New Zealand's Basic Geology

New Zealand lies on the boundary between the Pacific Plate (which almost covers a quarter of the earth’s surface) and the Australian Plate. In New Zealand these two plates converge at a rate of 4cm a year, in a East - West direction. In the North Island the Pacific Oceanic Plate is subducting beneath the Continental Australian Plate. The boundary where the plates meet is called a Convergent Boundary, this is where one plate is forced under another. Fig. 1

Fig. 1: A cutaway diagram showing the layout of the underlying geology

The Australian Plate is known as a Continental Plate, and mainly consists of granite. The Pacific Plate is an Oceanic Plate and is made of basalt rock; which is cooled lava. Basalt is denser than granite and Oceanic Plates are also thicker that Continental Plates. So when the two plates meet, the larger Pacific Plate is forced under the weaker Australian plate which creates a subduction zone, and under the immense stress the Australian plate cracks causing many earthquake faults.
New Zealand’s North Island lies on this subduction zone which is the reason for the North Islands central volcanic region (Fig. 2). and the plate boundary runs diagonally through the South Island where the Southern Alps have been pushed up.

Fig. 2: Central North Island volcanoes

The Wellington Fault Line

The North islands’ longest fault line stretches from the Cook Strait all the way to the Bay of Plenty. The northern section of the fault is the Mōhaka Fault, and the southern section is the Wellington Fault. The Wellington Fault is extremely hazardous as it runs straight through New Zealand’s capital city, Wellington. The fault line dominates the greater Wellington region’s landscape and is the most predominant geological feature in the area. It is extremely dangerous, as the fault is surrounded big residential areas, infrastructure and at some parts is crossed by roads, pipelines and bridges, this could prove devastating in the event of a large earthquake along the fault. In addition over 75% of all the people that live in the Wellington region live within 10 kilometers of the fault.

Fig. 3: The Wellington Fault as it passes through Lower Hutt

Wellington’s Faulty Features

The Wellington region is very geologically diverse. The region is dominated by many faults which over time have greatly influenced the appearance of the landscape. The Wellington fault (fig 2) runs along the base of the Hutt Valley’s western hills, roughly along the motorway. Here one can clearly see the fault as right next it, the western hills shoot up.

The stretch of motorway that travels along the coast towards Wellington is built on a shelf created by the erosion of the fault scarp, This shelf was exposed in the 1855 and 1460 uplifts that were caused by the Wairarapa fault. The 1855 earthquake along the Wairarapa fault also brought up the land on which the Wellington International Airport.

 The Hutt River also flows along parts of the fault (See Fig. 3 above).You can physically stand on the fault scarplet formed by the Wellington fault at Totara Park in Upper Hutt, you can clearly see a step in ground where the earth has shifted (Figs. 4 & 5). You can also clearly view the fault from the lookout along Wainuiomata Road (Fig. 6), which goes up the Hutt Valley’s Eastern Hills, on the opposite side of the valley to the Wellington fault.

Internal Processes

One of the most dominant and obvious features/changes in the landscape that the fault has caused is the uplift of the land on it's western side. It is clearly visible between the south Wellington coast and the Rimutaka Range. The movement along the Wellington Fault is a combination of vertical and horizontal movements . The horizontal movements have shifted the land (on the western side) in a sideways direction. And vertical movements have elevated the land (on the western side).

Compared to the uplifted and hilly terrain of the western side of the fault, the eastern side has been buckled into a series of lows and highs. The lows have formed valleys and basins, and the highs have formed the ribs of ridges. These features have been created due to the down faulting movements of the land with the greatest downward movement being right on the fault. This has formed a "fault-angle depression" which has caused the entire eastern landscape sloping downward, towards the fault.

Fig. 4: Totara Park scarplet - Picture By Tim Noakes

Fig. 5: Survey poles arranged across the Wellington fault at Totara Park - Picture By Tim Noakes

Fig. 6: Wellington Fault viewed from Wainuiomata Road.
(Note the dramatic rise in land west of the fault) - Picture By Tim Noakes

The Faults Future

300 to 500 years ago is the last estimated time the Wellington Fault ruptured and caused a major earthquake. Geologists predict that the fault will cause a large earthquake every 500 to 100 years. However there are other active faults around Wellington. For example Oharia Fault, and the Wairarapa (Which had it's last major earthquake in 1855). Due to the number of faults in the area, the frequency of earthquakes is much higher, this in turn has made the average time that a severe earthquake can be expected in the lower North Island to approximately every 150 years.