Earthquakes occur on fractures, or faults, within the Earth. Active faults have ruptured repeatedly, and may move again. The most spectacular is the Alpine Fault, where the Southern Alps are being uplifted. The city of Wellington was built, unknowingly, on another active fault. Geologists have now identified these lines of weakness, and there is greater awareness of the hazards of building where the earth may suddenly shift again.
Most earthquakes occur when adjacent blocks of land move along fractures in the earth’s crust known as faults. The bedrock of New Zealand is cut through with faults, but most are geologically old and pose little threat of earthquake activity. Faults that have moved one or more times in the last 120,000 years, however, are considered likely to move again. These are classified as active faults. The evidence that a fault is active is that it repeatedly breaks the earth’s surface.
Features in the landscape can often indicate the presence of active faults. When land along a fault has ruptured it may produce a break in the surface, known as a fault scarp. For example, during the 1929 Murchison earthquake a scarp over 4.5 metres high formed along part of the White Creek Fault.
Repeated upward movement of land along a fault can create steep hillslopes and, over geological time, build mountains. Many of New Zealand’s mountain ranges have been uplifted by thousands of separate offset movements along active faults – for example, the Rimutaka and Tararua ranges along the Wairarapa Fault, the Inland and Seaward Kaikōura ranges along the Clarence and Hope faults, and the western side of the Southern Alps along the Alpine Fault.
The bedrock along a fault is often shattered by movements during repeated earthquakes. This broken-up rock along fault lines is easily eroded by streams or glaciers, producing distinctive linear valleys. Fault movement can also offset landscape features, and streams may develop right-angle bends where they cross faults.
To estimate the likelihood of future earthquakes along a given section of a fault, scientists must determine how often, on average, earthquakes have occurred there in the past, and the date of the last movement.
Several methods are used to establish the dates of past earthquakes and to estimate the amount of movement that occurred then. Earthquakes often trigger small slips and larger landslides that bury vegetation, and strip trees and soil from hillsides. Trenches dug across faults often reveal buried plant material that can be dated using radiocarbon techniques, while tree rings will reveal the number of years since new stands of trees began to grow again on landslide-scarred hillsides.
The amount of movement during past earthquakes can be estimated from the offset of surface features or from breaks in layers of sediment exposed in trenches dug across faults. From the extent of movement during a single earthquake, scientists can estimate the likely magnitude of the quake.
A number of faults in New Zealand are known to be active because movement has occurred along them during earthquakes in the period since European settlement. There was activity along at least 100 kilometres of the Awatere Fault in 1848, and along 140 kilometres of the Wairarapa Fault in 1855. Fault movement also ruptured the land surface in the following quakes:
Only one fault in New Zealand has been known to move twice within the period of written records: the Kaiapo Fault near Taupō, which moved during earthquake swarms in 1922 and 1983.
The faults that are considered to present the greatest earthquake hazard are those that move most frequently. The average frequency of movement can range from several hundred years to tens of thousands of years. More than 50 active faults in New Zealand are known to be the site of large earthquakes that occur at intervals of less than 2,000 years.
Some faults have moved in historical times, but are unlikely to cause earthquakes in the near future. The White Creek Fault, which caused the magnitude 7.8 Murchison earthquake in 1929, shows no evidence of having moved in the previous 20,000 years. It will probably be many centuries before enough stress builds up to cause a major quake along this fault again.
The number of New Zealand faults classified as active is increasing as known faults are investigated in detail and new ones are discovered.
About a third of all New Zealand’s shallow earthquakes occur offshore. Lying east of the country is a region of continental shelf that is being deformed as the Pacific Plate descends beneath the Australian Plate. This zone is up to 150 kilometres wide and includes many active faults. A number of the country’s major faults also extend into Cook Strait, and the Alpine Fault has offshore segments. These submarine faults present a particular hazard for coastal cities and towns, because offshore earthquakes can cause tsunamis, either directly by the movement of land along the fault or by triggering underwater landslides.
During earthquakes, the land on one side of a fault may suddenly move horizontally or vertically up to several metres. When this occurs, any buildings or other structures that straddle the fault will be torn apart and severely damaged. Such ruptures are often not a narrow line, but may be a zone of up to tens of metres wide.
New Zealand has a number of cities and towns with buildings on or close to an active fault. Many of these buildings were constructed long before the risk of fault movement was known. The Greater Wellington region, including Wellington and the cities of Lower Hutt and Upper Hutt, is the largest urban area threatened by fault movement. Many buildings lie on or close to the Wellington Fault – it even runs through the grounds of the prime minister’s official residence. Other centres built on active faults include Franz Josef, Hanmer Springs, Blenheim, Nelson, Porirua, Waikanae, Waverley and Waipukurau.
While it is difficult to protect existing buildings, some local authorities are now controlling new development on known active faults, especially those that move at frequent intervals. In 2003 the Ministry for the Environment produced guidelines for the development of land close to active faults. It was recommended that any new building be set back 20 metres from a known active fault. Many local authorities have now adopted this.
To enforce such regulations the exact position of a fault must be known. Some faults are simple linear features, with a scarp that is only a few metres wide. Others have deformed broader zones of land, tens and even hundreds of metres wide.
Many active faults have scarps on the ground’s surface that can be mapped using ground studies and aerial photographs. However it is not straightforward to locate the fault line in some areas. For example, parts of the Wellington Fault are under water, covered by river sediment, or modified by urban development, and complex investigations are required to accurately locate it.
The Wellington Fault runs through the Upper Hutt suburb of Totara Park. This area has been planned with a number of special features to protect residents. One section that the fault traverses, California Park, has been set aside as a recreation reserve. Through the rest of the suburb, the fault line runs down the centre of California Drive. This street has two lanes separated by a wide grassed berm that covers the fault trace. No house is closer than 20 metres to the fault. Very few service lines, such as water, gas and sewer systems, cross the fault. Those that do cross it have flexible joints to withstand ground shaking.
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 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.
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.
Aitken, Jefley J. Rocked and ruptured: geological faults in New Zealand. Auckland: Reed, 1999.
Coates, Glen. The rise and fall of the Southern Alps. Christchurch: Canterbury University Press, 2002.
Cooper, A., and J. Aitken. New Zealand’s Alpine Fault. Alpha series 104. Wellington: Royal Society of New Zealand, 2000.
http://www.otago.ac.nz/geology/af/alpinefault.htm
Information on the Alpine Fault, with detailed maps showing the location of the fault at various localities. Produced by the Department of Geology, University of Otago.
http://www.pce.govt.nz/reports/allreports/0_908804_96_2.shtml
A report prepared by the Office of the Parliamentary Commissioner for the Environment on the problems of building on or near known active faults.
http://www.rsnz.org/publish/nzjgg/notable/Wellman1953.pdf
In 1951 Harold Wellman scanned all the available aerial photographs of the South Island and identified active fault traces. All his observations are listed in tables, and his map is the first to show New Zealand active faults. Published in the New Zealand Journal of Science & Technology in 1953, it was one of the first of such studies in the world (PDF, 1 MB).
http://data.gns.cri.nz/af/about.jsp
Information and data on all known active faults in New Zealand, produced by the Institute of Geological and Nuclear Sciences.
http://www.mfe.govt.nz/publications/rma/planning-development-active-faults-dec04/html
An interdisciplinary group prepared these guidelines for building on or near active faults, published by the Ministry for the Environment.