In this 3D simulation, the segments illustrate the network of faults – including the Hope Fault and Leader Fault in the south and the Kekerengu Fault and Needles Fault in the north - that ruptured in the Kaikoura Earthquake. Seismic waves, ground motions and rupture propagation along a complex network of faults are simultaneously captured. Source: Ulrich/Gabriel, LMU
Scientists have pieced together the most complex earthquake ever recorded - the 7.8 jolt that shook Kaikōura in 2016 – with some surprising results.
The November 14 quake is remembered for causing two deaths, thousands of landslides, widespread coastal uplift and about $1.8 billion in insurance claims.
But to scientists, the midnight quake, which released the equivalent energy of 400 atomic bombs, remains one of the most intriguing geophysical events ever observed.
New Zealand studies into the quake have highlighted the sheer number of faults that ruptured in a seismic cascade that spread across the country.
More than 20 were activated, 14 of which ruptured violently enough to displace land by more than a metre.
One of the most dramatic examples was along the Kekerengu Fault in Marlborough, where the land offset was as much as 12m, and in some places created walls of raised-up earth across the countryside.
A new analysis, by international researchers, dissected the complex mechanisms at play using simulations carried out on a supercomputer in Germany.
Their work turned up many reasons for the many fault segments that were set off.
"Looking at the pattern of surface faults affected by the quake, one finds large gaps of more than 15km in between them," said study co-author Dr Alice-Agnes Gabriel, of Ludwig Maximilian University in Munich, Germany.
"Up to now, analyses of seismic hazard have been based on the assumption that faults that are more than 5km apart will not be broken in a single event."
A second unusual observation was that, although the earthquake began on land, it also resulted in the largest tsunami recorded in the region since 1947.
This indicated that the subsurface ruptures ultimately triggered local displacements of the seafloor.
The insights provided by the simulations had yielded a better understanding of the causes of the sequence of fault ruptures that characterised the earthquake.
"This was made possible by the realistic nature of our model, which incorporates the essential geophysical characteristics of fault failure, and realistically reproduces how subsurface rocks fracture and generate seismic waves," Gabriel said.
The model confirmed that the earthquake involved a complex cascade of fault ruptures, which propagated in a zig-zag fashion.
This model shows the propagation of seismic waves across the South Island, one minute after the onset of the 2016 Kaikōura earthquake. Image / Ulrich/Gabriel, LMU
The researchers put this down to the intricate geometry of the fault network, along with delays that occurred at transitions between the fault segments.
And contrary to the idea that decades of built-up tectonic forces would have been needed to push the pulse through this dense web, they found the amount of forcing required was actually quite weak.
Gabriel explained the rupture of such a weakly-loaded fault was boosted by gradual slippage below the faults, where the crust was more ductile, along with over-pressurised fluids and low levels of frictional resistance in the ground.
The researchers say their new model could now be used to improve hazard assessments in quake-prone areas elsewhere in the world.
Current hazard assessments required careful mapping of the fault systems in the region concerned, and their susceptibility to rupture under seismic stress was then estimated.
"Earthquake modelling is now becoming an important part of the rapid earthquake response toolset and for improving long-term building codes in earthquake prone areas," said the study's first author, Thomas Ulrich.
GeoNet recorded more than 20,000 aftershocks in the year following the earthquake, with several thousand more recorded since.
This 2017 map shows where ruptures were observed. Source / GeoNet
GeoNet's most recent earthquake shaking forecast – a statistics-based measure that calculated probability – gave a 44 per cent chance of one or more magnitude 6.0-6.9 earthquakes at some point within the next 12-months, with the likelihood falling over time.
The new study, just published in major scientific journal Nature Communications, also involved scientists from the University of Côte d'Azur in France and Hong Kong Polytechnic University.
