Research shows fault failed twice

New research shows the Ke¯kerengu¯ Fault - the fault that moved most during the Kaiko¯ ura earthquake - failed twice during the quake.

By using all available seismic and GPS data, and developing high resolution models of the quake, researchers have been able to show how the quake spread through the complex network of faults. That includes a detailed timeline.

Their research, published just three weeks before the first anniversary of the magnitude-7.8 Kaiko¯ ura earthquake, shows a separation of about 11 seconds between the start of the original failure on the Ke¯kerengu¯ Fault and the start of the second failure.

‘‘This is the first time rupture reactivation on a crustal fault has been identified,’’ GNS Science seismologist Dr Caroline Holden said.

Holden is the corresponding author of a research letter outlining the findings published in Geophysical Research Letters.

Scientists have been astounded by the complexity of the earthquake, with earlier research showing that after starting in North Canterbury it moved northward for more than 170 kilometres, rupturing at least 12 major crustal faults and nine lesser faults. It moved parts of the South Island more than five metres closer to the North Island, and uplifted parts of the South Island by up to 8m.

The Ke¯kerengu¯ Fault had the largest movement during the quake, with pieces of the Earth’s crust displaced relative to each other by up to 25m at a depth of about 15km. At the surface, the biggest horizontal movement measured was 12m.

According to the latest research, it took 60 seconds for the rupture, moving from south to north, to get to the Ke¯ kerengu¯ Fault.

The propagation of the rupture was slow compared with other strike-slip earthquakes, Holden said. That was despite large, concentrated slip on some faults. A strike-slip earthquake is one in which the

"This is the first time rupture reactivation on a crustal fault has been identified."

GNS Science seismologist Dr Caroline Holden

blocks on either side of a more-orless vertical fault move mostly horizontally.

Evidence for two failures on the Ke¯kerengu¯ Fault was strong. It included the two independent modelling approaches used, which provided robust constraints, Holden said.

Also, researchers used all available datasets from seismic and GPS stations very close to the faults ruptured during the quake, including a seismic station 2.7km from the Kekerengu Fault. The work found little or no slip on the Hope Fault, which is the source of the highest seismic hazard in the region.

It also found little or no slip on the Hikurangi subduction zone fault, contrary to some opinions about the quake. ‘‘Whether there was significant slip on the subduction zone fault is still debated,’’ Holden said.

Some studies had shown seismic data from a global network, with seismic stations more than 1000km away, indicated evidence for large slip on the subduction interface.

‘‘However, our results - using all the available near source data - show there was no or little slip on the subduction zone fault during the Kaiko¯ ura earthquake.’’

Holden said the large amount of slip on the Kekerengu Fault was no surprise. It was one of the fastest slipping faults in New Zealand, and had been identified as having the potential for a large release of seismic energy before the Kaiko¯ura earthquake.

The rupture of the Kekerengu Fault certainly contributed to the large amounts of shaking felt in Wellington, about 100km away.