Frequently Asked Questions
Q: What does “AF8” stand for?
A: It stands for Alpine Fault magnitude 8 earthquake. Geological studies have shown that an earthquake of this magnitude occurs on the Alpine Fault at regular intervals. AF8 has been set up to assess the impacts that the next such earthquake will have on South Island communities and infrastructure, help people understand and prepare for this inevitability, and develop a planning framework for a coordinated response.
Q: Sometimes you talk about an earthquake and sometimes a rupture. What’s the difference?
A: Basically they are the same thing – a fault ruptures during an earthquake. When an earthquake happens the ground moves on one side of the fault relative to the other. One of the physical outcomes of an earthquake can be surface rupture. This is where the earthquake breaks the ground surface, and is a visible sign of the movement that has occurred along the fault, where the built environment can be significantly damaged because of movement across the fault.
Q: What’s the difference between the magnitude of an earthquake and its impact?
A: Magnitude relates to the amount of seismic energy that is released by an earthquake. The impact an earthquake has on a region or community will also be influenced by its proximity to the earthquake rupture, and its vulnerability to earthquake-induced hazards, such as ground shaking, landslides, liquefaction, and tsunami, for example.
Intensity is a metric commonly used to refer to the amount of damage an earthquake causes to the built environment. It’s measured using the Modified Mercalli Intensity Scale.
As an example: A large earthquake which occurs in a remote location, or is deep beneath the earth’s surface may cause less damage to the built environment than a smaller but shallower quake that is located near a built-up area. The two Canterbury earthquakes of September 2010 and February 2011 illustrate this: the 7.1 magnitude earthquake of 4 September was centred near Darfield, about 40km west of Christchurch and about 11km deep. Though it caused widespread damage across Canterbury and one death, its impact was less than the magnitude 6.3 earthquake which occurred on 22 February 2011, centred 10km south east of the city centre, 5km deep.
Q: Ok, that makes sense, but what is the Modified Mercalli Intensity Scale?
A: In New Zealand, where earthquakes occur from near the surface right down to a depth of over 600 km, the Modified Mercalli Intensity Scale is a better indicator of an earthquake’s effects on people and their environment. New Zealand scientists have modified it further to suit New Zealand conditions. It is used by many professionals who need to know the relationship between the strength of shaking at ground level and the degree of damage, which is why we often hear earthquake being referred to in terms of their intensity. You can find out more at GNS Science
Q: Could a magnitude 8 earthquake on the Alpine Fault cause a tsunami?
A: It’s possible that an earthquake on the Alpine Fault could generate a local tsunami. The Alpine Fault is more than 600km in its full extent, and extends from offshore in Fiordland, right up to southern Marlborough. The scenario that the AF8 scientists believe is the most probable focusses on a rupture of the fault from Milford Sound to Lake Kaniere. In this scenario, the southern parts of the fault could rupture across the seafloor and, depending on movement across the fault during the earthquake, could generate local tsunami. Shaking from such an earthquake will cause rockfalls and landslides. Where these occur into lakes, fiords, or other waterways, or offshore under the sea, they have the potential to generate significant tsunami waves and present a threat to people and the built environment around waterways and the coast.
Q: Could a big earthquake on the Alpine Fault rupture other faults in the South Island?
A: Earthquakes release seismic energy, and it is well understood that some of this energy can be redistributed onto other faults, which is often where aftershocks happen. We cannot discount that an Alpine Fault earthquake could trigger other seismic activity, in the same way as has occurred after the Canterbury earthquakes of 2010-2011 and the Kaikōura earthquake of 2016. However, the nature and extent of seismic loading onto other structures is very difficult to measure and impossible to predict with any degree of certainty.
Q: Could a big earthquake on the Alpine Fault cause a rupture the Hikurangi Subduction Zone?
A: The Hikurangi Trench and Alpine Fault form part of the Australian-Pacific plate boundary that transects New Zealand. In between the Hikurangi Subduction Zone and the Alpine Fault is a broad area of complex active faults, called the Marlborough Fault System.
It is very difficult to predict whether an Alpine Fault earthquake could trigger any activity on the Marlborough Fault System and the Hikurangi Subduction Zone, but it is highly unlikely that a single earthquake could rupture right through from the Alpine Fault to the Hikurangi Subduction Zone. In the past ~1000 years there have been four large earthquakes on the Alpine Fault and two large earthquakes at the plate boundary on southern part of the Hikurangi Subduction Zone. The earthquakes on the Alpine Fault occurred at different times to the earthquakes on the southern Hikurangi Trench, showing that they were not one continuous earthquake rupture.
It is possible that a large Alpine Fault earthquake could also rupture the western part of some faults of the Marlborough Fault System. We do know that the Kaikōura earthquake, which occurred within the Marlborough Fault System, triggered slow slip earthquakes on the southern and northern parts of the Hikurangi Subduction Zone, which illustrates the connections between faults across New Zealand.
Q: Will the impacts of a major earthquake on the Alpine Fault only affect the South Island?
A: No. Just as the Kaikōura earthquake in 2016 had a significant impact on Wellington, we expect that when the Alpine Fault ruptures in a large magnitude earthquake it will be widely felt across the lower North Island. AF8 is focused on planning for this event in the South Island, but some North Island CDEM groups are also taking the Alpine Fault into account in their planning. We hosted a national agencies AF8 workshop in Wellington during the development of the draft SAFER Framework. The Ministry of Civil Defence & Emergency Management is planning to run a national exercise based on the AF8 scenario in 2020.
Q: Is it likely that the Australian and Pacific Plates might stop pushing against each other and start moving away from each other, releasing the pressure?
A: It is not likely to do so anytime soon! The plate boundary that is located across New Zealand has existed for more than 20 million years. The relative motion between the Australian and Pacific plates today involves the Pacific plate moving south-west past the Australian plate, with some compression across the Southern Alps and this situation has been happening for around the last 5 million years. These massive tectonic forces change and evolve through geological time, and it is highly likely that in future these forces will continue to change. However, human timescales are much shorter, so we are not likely to see any significant change within the lifetimes of New Zealanders.