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Rethink of hazards after Kaikoura earthquake – Expert reaction

Posted in Expert Reactions on March 24th, 2017.

Last November’s Kaikoura earthquake will likely change the way scientists think about earthquake hazards in plate boundary zones, according to the first study to come out of the data collected during and immediately after the earthquake.

Dr Ian Hamling, GNS Science.

The study, led by GNS Science researchers, will be published on Friday in Science. A range of data – including satellite radar imagery – shows that parts of the South Island moved more than five metres closer to the North Island, in addition to uplift of up to eight metres in some places.

GNS Science said the quake was a record-setter for its complexity and was one of the best recorded large earthquakes anywhere in the world.

Lead author, geodesy specialist Dr Ian Hamling, said the complex and lengthy nature of the rupture hampered early attempts to determine an accurate magnitude, which could pose issues for earthquake early warning systems.

He said the earthquake had underlined that conventional seismic hazard models were too simple and restrictive. “The message from Kaikoura is that earthquake science should be more open to a wider range of possibilities when rupture propagation models are being developed.”

The research was covered by local and international media, including:

Stuff.co.nz: Kaikoura earthquake moved the South Island, new research shows
Radio NZ: Kaikōura quake moved South Island 5 metres
NZ Herald: True damage of 7.8 Kaikoura quake revealed, and could change earthquake research worldwide
TVNZ: Kaikoura quake jolted South Island more than five metres towards the North
BBC: Kaikoura: ‘Most complex quake ever studied’
International Business Times: 
New Zealand’s 2016 Kaikoura earthquake was one of the most complex ever recorded
Science: Strange behavior of New Zealand quake suggests higher chances of ‘Big Ones’ elsewhere

The SMC gathered expert reaction on the study, please feel free to use these comments in your reporting. More information about the study is available on scimex.org.

Dr Mark Quigley, Associate Professor in Earthquake Science, University of Melbourne, comments:

“This is an excellent study that documents the complexity of the Kaikoura earthquake using a suite of different methods and datasets. The use of satellite radar data, GPS, seismology, and field geology is now standard practice after major earthquakes.

“In this case, the large international and multidisciplinary author list has worked well to combine these techniques into a rupture model that demonstrates just how complex earthquake ruptures can be. It is fantastic to have a domestically-led collaborative effort that includes three NZ CRIs and five different NZ universities in such a high-profile journal.

“The concurrent rupture of at least 12 major faults in this region, including faults with very different prehistoric earthquake chronologies and slip rates, is probably the most novel aspect of the study. The possible involvement of the Hikurangi subduction zone interface is particularly intriguing.

“When we characterise the seismic hazard of an area, we typically consider the magnitude potential of mapped faults independently and consider the potential for large cascading earthquakes like this one based largely on the geometry and distances between mapped faults. Recently we have begun to consider the statistical probability of earthquake ruptures to ‘jump’ across from one fault to the next with more sophistication.

“However, it is really challenging to input a rupture scenario like this one directly into a hazard model for an area. The relatively short time period over which we have been able to collect data with the precision that the authors have done in this study, and similarly short time period for which we have been able to instrumentally record earthquakes at all, means we don’t really know how common these types of earthquakes are.

“This study will become a classical example of a complex interplate earthquake and will stimulate earthquake research for decades; the authors should be commended.”

Professor Kevin Furlong, Professor of GeoSciences, Pennsylvania State University, comments:

“Key Points:

– The location of the event(s) is at a complex transition in the Australia-Pacific plate boundary including both subduction and near-surface strike-slip faulting
– Seismological studies of the energy radiated from this event indicate that the surface faulting studied in this paper is part of a larger system that included slip on the subjacent subduction megathrust.
– The occurrence of this faulting onshore provides an opportunity to examine the details of such deformation not normally available.

“The 2016 Mw 7.8 Kaikoura earthquake definitely represents an important earthquake for our understanding of plate boundary associated earthquakes. The earthquake (or multiple earthquakes) occurred in a complex region of the Australia – Pacific plate boundary.

“This location (Kaikoura and Marlborough) is both the southern terminus of the Hikurangi subduction zone and the northern initiation of the Alpine Fault plate boundary system. As a result, the nature of crustal deformation and associated earthquakes is expected to be complicated and perhaps provide us with new and unexpected observations.

“This earthquake certainly succeeded at showing new and unexpected behaviour. This paper focuses on the observations of surface faulting associated with the event. The complex nature of the faults that slipped at the surface during the event is certainly pretty complicated. It is important to note that in addition to the work presented here, there have been numerous studies that have utilised tele-seismic observations (that is seismological observations made at a distance from the earthquake), and virtually all of those studies identify the important role of the subduction zone in this earthquake.

“Because of the length and area of ruptures that can occur in large earthquakes, these tele-seismic data are the preferred method to study the overall seismological behaviour of a big earthquake. What these other studies have found is that 50% or more of the earthquake energy (to be more precise the earthquake moment) was produced on a shallowly dipping interface, most likely part of the southern end of the subducting Pacific plate.

“These tele-seismic studies see evidence of the shallow crustal strike-slip faulting, but the nature of that data does not allow the individual faults to be imaged. So the results of these other studies seem to indicate that the complex surface faulting represents at most about 50% (or less) of the earthquake energy/moment release, with the subduction megathrust providing the other half.

“This does not reduce the importance of this event. Rather it provides a view into nearsurface deformational processes that may occur in conjunction with subduction earthquakes. The geography of this region of the Australia-Pacific plate boundary means that the area of the subduction interface that ruptured is beneath land – more typically the rupture areas are offshore.

“As a result, the surface faulting that is able to be so well documented in the Kaikoura region is usually invisible (if it occurs) in other large megathrust earthquakes. This and some other studies focused on the rupture behaviour of the upper crust provide for the first time direct observations of such deformational behaviour.

“One exciting result is that although there is good evidence of complex patterns of faulting in these ‘upper-plate’ regions of subduction zones; this is perhaps the first time that it is clear that such complexity can occur within a single event rather than simply the superposition of multiple events over time.”

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