Story: Active faults
Page 3 – New Zealand’s longest active faults
The Wellington–Mōhaka Fault
The North Island’s longest active fault runs from Cook Strait to the Bay of Plenty. The southern section is the Wellington Fault; the northern section is called the Mōhaka Fault. The Wellington section presents a major hazard, as it goes through the heart of New Zealand’s capital city and is crossed by numerous bridges, roads and pipelines. Over 75% of people in the Wellington region live within 10 kilometres of the fault.
Wellington owes its distinctive landscape to this fault. Near the coast, the sea has flooded into the fault depression to create Wellington Harbour. During earthquakes along the fault, land along the north-western side of Wellington Harbour and the Hutt Valley moves upward, while in areas south-east of the fault land subsides.
Further inland, the Hutt River flows down the depression and has filled the Lower and Upper Hutt areas with hundreds of metres of sediment.
Movement along the 75-kilometre-long segment of the Wellington Fault from Cook Strait through Wellington and the Hutt Valley to Kaitoke is considered likely to cause a major earthquake in the future. At least two earthquakes have occurred on this part of the Wellington Fault in the last 1,000 years, with the most recent about 400 years ago. During these earthquakes, sections of land on opposite sides of the fault moved past each other by about 4 metres. Such movement would produce earthquakes of the order of magnitude 7.5.
Large earthquakes on this section of the fault are estimated to occur about every 500 to 770 years.
The Alpine Fault
The western ramparts of New Zealand’s Southern Alps define a remarkable straight line visible from space – the trace of the Alpine Fault. It is the longest active fault in New Zealand. Onshore it extends 650 kilometres from Blenheim to Milford Sound.
The Alpine Fault is a major plate boundary, where the moving Pacific and Australian plates collide and scrape past each other. In 1948 geologist Harold Wellman realised that rocks that were once adjacent to each other had been separated by 480 kilometres as a result of movement along the Alpine Fault.
No major earthquakes have occurred on the Alpine Fault since Europeans settled in New Zealand. Its most recent movements have been determined by tree-ring dating and radiocarbon dating of plant material in trenches dug across the fault. Dates from earthquake-triggered landslides and forest disturbance indicate an earthquake around 1460 AD. Another quake occurred about 1630, when there was movement along the fault between the Paringa and Ahaura rivers (about 250 kilometres). The most recent earthquake was about 1717, when over 300 kilometres of the fault ruptured, from Milford to the Haupiri River.
On these occasions there was up to 8 metres of horizontal movement and 1 to 2 metres of uplift along the fault, producing earthquakes with magnitudes of about 8. The quakes have not occurred at regular intervals, nor have they been on the same section of the fault. The length of time between earthquakes has varied from less than 100 years to over 285 years. The period from 1717 to the present is the longest interval between known movements.
Rise and fall
New Zealand’s Southern Alps are one of the fastest-rising mountain ranges in the world, and over the last 5 million years they have been moving up at an average rate of about a centimetre per year. This uplift is not continuous – much of it occurs in jumps of several metres at a time during earthquakes along the Alpine Fault. Scientists estimate that the land east of the Alpine Fault has risen by as much as 20 kilometres. The mountains, however, have never been much higher than they are at present – a little over 3 kilometres – because erosion wears them down about as fast as they go up.