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.
Scarps and other landscape features
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.
Determining fault activity
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.
When is a fault active?
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.
- 1888: North Canterbury (Hope Fault)
- 1929: Arthur’s Pass (Poulter Fault)
- 1929: Murchison (White Creek Fault)
- 1987: Edgecumbe (Edgecumbe Fault)
- 2016: Kaikoura (multiple faults including the Kekerengu, Papatea, Jordan Thrust, and Hundalee Faults).
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.
Offshore 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.