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Q&A – Complex multi-fault rupture during the 2016 M7.8 Kaikoura earthquake, New Zealand

Questions and answers

1.Did the 6m of vertical and 8m of horizontal movement occur instantaneously during the earthquake or within minutes, hours or days of the earthquake?

Most of the observed surface displacements would have occurred simultaneously with the earthquake, or within minutes of it.

2. How does this study mesh with the studies of surface rupture undertaken by geologists and seismologists?

This study fits nicely with geological and seismological observations, although different science disciplines focus on different aspects of the earthquake. The geodetic data, in particular GPS and InSAR, precisely measure how the ground surface is displaced as the earthquake ruptured along the fault, or in this case faults. This enables scientists to build models to estimate how much slip occurred along the fault, not only at the surface, but also at depth. These models then help seismologists to run earthquake simulations to investigate how the rupture propagated through the fault network.

3. What is InSAR?

Interferometric synthetic aperture radar is a radar technique used in earth sciences and remote sensing. By comparing the phase information from radar signals produced by satellites orbiting 600-700km above the Earth, scientists can measure millimetre-scale surface displacements over thousands of square kilometres. Unlike optical satellites, which rely on the sun to illuminate the Earth’s surface and whose view can be obscured by clouds, radar systems are able to see through clouds and, with their own radiation source, can acquire images at any time of day or night. InSAR has applications for the monitoring natural hazards – particularly earthquakes, volcanoes and landslides. It is also used in structural engineering for monitoring surface subsidence.

4. Roughly how long did the rupture take to travel from its starting point in North Canterbury to its end point off the coast of Marlborough, a total distance of 170km to 180km?

This is hard to extract from the geodetic data as we only measure the final displacement field after everything has ruptured. From seismology data we know that the rupture took around two minutes to travel along its entire length.

5. How significant is the slip on the southern part of the Hikurangi subduction zone and how much of the quake’s total energy output can be attributed to the plate boundary movement?

Compared with slip on the crustal faults, which reaches around 25m at depth, any slip on the subduction interface was a fairly minor component. Modelling suggests it accounted for a relatively small amount (about 10-20%) of the total energy generated during the earthquake.

6. How common is it worldwide for crustal and plate boundary ruptures to occur simultaneously during a large earthquake?

To our knowledge, not very common. There are a few examples worldwide where it has been documented that a combination of slip along a subduction zone and in the crustal faults above occurred during an earthquake.

7. Is there something special or unusual about this part of New Zealand that means it produces complex earthquakes?

Scientists have known for a long time that this region – between North Canterbury and Cook Strait – is capable of producing complex earthquakes. Even so, the high level of complexity of the Kaikoura earthquake took many by surprise. It serves to underline why some scientists regard this region as the most complex plate boundary zone in the world. It is where the plate boundary transitions from subduction to the north to almost pure strike-slip along the Alpine Fault, which runs for about 600km along the spine of the South Island between Marlborough and Fiordland.

8. Is there further research planned using this, or similar, satellite data? If so what aspects will be investigated and what kind of outcomes could be expected?

The research is ongoing. While this slip model explains most of the large scale features, there are still areas where the complexity of the rupture cannot be captured. Scientists are also monitoring the post-seismic ground deformation as minor slip continues on the faults and the ground adjusts following the earthquake. We are fortunate that we are now able to access satellite imagery every 6-12 days to help us look at how the area is moving and we also have continuous GPS stations operated by the GeoNet project monitoring any changes. Outcomes from research on the Kaikoura earthquake will help to improve the understanding of similar plate boundary zones worldwide.

9. What are hazard models used for?

Their main use is to aid in decision making including things like building codes and as an input in the insurance sector.

10. How many other science papers are likely to be published this year on the Kaikoura earthquake and its impacts?

GNS Science is aware of at least 10 research papers at various stages of development on various topics associated with the Kaikoura earthquake and its impacts. GNS Science staff are either lead authors or contributors to these papers. There will be other research papers on the quake from national and international institutions that don’t involve GNS Science.

Contact: Dr Ian Hamling, Natural Hazards Geodesist, GNS Science, P: 04-570-4568, M: 027-950-0640