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 of most active faults is that they repeatedly break 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 Remutaka 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.
Movements often shatter the bedrock along a fault during repeated earthquakes. Streams or glaciers easily erode broken-up rock along fault lines, producing distinctive linear valleys. Fault movement can also offset landscape features, and streams may develop right-angle bends where they cross faults.
Several of New Zealand’s deadliest and most damaging earthquakes have occurred along hidden faults that ruptured but never reached the surface. Earthquake activity and the broad rise or fall of an area may, however, indicate their presence. For example, the devastating Hawke’s Bay (Napier) earthquake on 3 February 1931 killed 256 people. The coastal area around Napier was uplifted by about two metres, but the exact location of the fault that caused that quake is still unknown. Movement along hidden faults beneath part of the Port Hills and the southern edge of Christchurch caused the 22 February 2011 earthquake. Parts of eastern Christchurch subsided slightly, making the area more prone to flooding.
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. Fault movement may block streams, creating depressions and ponds where sediment and vegetation can build up. 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, along 140 kilometres of the Wairarapa Fault in 1855, and along 29 kilometres of the Greendale Fault across the Canterbury Plains during the 2010 earthquake.
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 greatest earthquake hazard is presented by those faults 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 sometimes when earthquakes unexpectedly occur in areas with no previously mapped active faults.
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 built long before the risk of fault movement was known. The Greater Wellington region, including Wellington and the cities of Lower Hutt and Upper Hutt, has many buildings 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 zone. 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. About three-quarters of the 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 less than 310 years ago. During the last four earthquakes, sections of land on opposite sides of the fault moved past each other by about 5 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 1,000 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, determined by tree-ring dating and radiocarbon dating of plant material in trenches dug across the fault, indicate earthquakes around 1460 AD, and around 1630 AD between the Paringa and Ahaura rivers (about 250 kilometres). The most recent earthquake was about 1717 AD, when over 300 kilometres of the fault ruptured, from Milford to the Haupiri River.
On these occasions, there was over seven metres of horizontal movement and one metre of uplift along the fault, producing earthquakes with magnitudes of about eight. The quakes have not all been on the same section of the fault.
Because of recent research at sites where sediment and vegetation have accumulated due to fault movement, however, the Alpine fault now has one of the longest continuous earthquake records of any plate boundary fault in the world. There is now evidence of 27 surface ruptures of the fault, dating back to 6,000 BC. The average interval between large earthquakes on the fault is 291 years. The longest interval was about 350 years and the shortest about 160 years.
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.
The north-eastern region of the South Island is traversed by a series of major active faults, including the Wairau, Awatere, Clarence and Hope faults. Along these faults sideways and upward movement have created mountains such as the Richmond, and Inland and Seaward Kaikoura ranges, and major river valleys such as the Wairau and Awatere. On 14 November 2016, a series of over 20 separate short faults ruptured in succession north-eastward along 180 kilometres of the Marlborough coast, producing the Magnitude 7.8 Kaikoura earthquake.
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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.
Rodgers, Anna. The Shaky Isles: New Zealand Earthquakes, Grantham House, 2013.