Wellington Harbour and Fault Scarp

The Wellington Fault is believed to extend all the way down to the plate boundary interface, the gently northwest-dipping fault between the Pacific and Australian plates, about 25-30 kilometres under Wellington and the Hutt Valley.

The fault is thought to have become established about 750,000 to 1 million years ago in this region. Because it is the most active of the major faults in the region (see map image) it is the one that has had the most dramatic effect on the topography.

Initiating several kilometres below the surface, a fault rupture tears through many kilometres of bedrock with a ripping effect that travels at speeds of several kms per second, radiating shockwaves that rattle the crust, refracting and reflecting from different densities of rock in all directions.The rupture becomes a plane of weakness that, once established, is where future ruptures will repeatedly take place.

During a Wellington fault earthquake, the Western Hills usually get pushed up and sideways. Looking across the fault, the far side moves to the right. Because of the vertical component the movement can be further described as 'dextral oblique slip' (meaning: 'rightwards slanting movement"). Although the earthquakes will vary in magnitude, a 'typical' Wellington Fault example might have 4 or 5m of sideways movement and variable amounts of vertical, rupturing along perhaps 100km of the fault.

Whilst the Wellington Fault does push the valley and harbour downwards, an earthquake on the Wairarapa Fault further to the east, such as that of 1855, will push the region upwards, so the up or down movement does depend on which fault has ruptured.

Because of the sideways pressure along the fault, the downthrown side has also been compressed into folds, with Wellington City situated on a high, the Harbour and Hutt Valley on a low, Taita Gorge being the next high and Upper Hutt basin another low. Along the segment between Taita and Thorndon, the original land surface has been pushed down to atleast 300 metres below the present sea level in some places adjacent to the fault. This depression has been filled up with sediments to create the relatively flat surface of the valley and harbour floor.

Earthquakes occur on the Wellington Fault approximately every 700 to 1000 years on average, with the last between 170 and 370 years ago. Erosion on the other hand is continuous, causing the hills to retreat from the actual fault line and developing the landscape we see today – steep hills cut and shaped by streams and gullies.

From the viewpoint, you can see several features very well:
- The uplifted surface of the Western Hills has been broken up by subsidiary faults so that it has a stepped appearance. The top of the faulted blocks form flat areas in the hills that have been largely covered with housing.
- The front of the Wellington Fault scarp is eroded into a number of stream valleys, with ridges in between them. As the scarp has been eroded back by the meandering Hutt River and the seawater in the harbour, these ridges have been cut back, creating steep triangular facets called "truncated spurs".
- The harbour is a depression caused by subsidence along the fault, combined with downfolding due to sideways compression along the direction of the fault.
- Notice the stepped appearence of Somes Island in the harbour. Each of the steps (marine terraces) was once at sea level, and show the progressive uplift over the last three or four glacial cycles. Atleast one fault to the side of the island has caused it to be uplifted within the harbour.
- Finally notice the flatness of Petone and the Hutt Valley, a depression that has filled with water laden sediments that underlie the area. These will tend to amplify shaking during earthquakes and also are susceptible to liquefaction. This along with a potential for subsidence and even a tsunami during a Wellington Fault rupture, makes Petone particularly vulnerable to such an event. The probability of rupture over the next hundred years is estimated at 10%.