There’s also Marlborough’s large strike-slip faults – the Hope, Clarence, Awatere and Wairau faults – as well as more than 20 set off in 2016′s extraordinarily complex magnitude 7.8 Kaikōura quake.
Data within the new fault model – outlined in a study just published in the New Zealand Journal of Geology and Geophysics - has already helped scientists better understand our earthquake risk.
It informed the recently updated National Seismic Hazard Model which, compared with previous estimates of seismic hazard, showed an increased risk of ground-shaking from future quakes in places such as Blenheim, Wellington, Napier and Gisborne.
Seismic risk is much higher in areas along the plate boundary, marked off the North Island’s East Coast by the Hikurangi Subduction Zone, and in the South Island by the Alpine Fault.
GNS Science geoscientist Dr Hannu Seebeck said it was also important to consider the quake-making potential of poorly understood faults.
After all, around half of the large, shallow earthquakes catalogued in New Zealand in the last 170 years occurred on faults that wouldn’t have been identified as active beforehand.
Faults with the potential for future quakes in the model include those at the base or within mountain ranges, but which scientists hadn’t been able to prove earthquake activity over the last 125,000 years.
“These types of faults that aren’t yet proven active generally have similar orientations to recognised active faults and are in locations where earthquake activity is expected,” Seebeck said.
“The inclusion of faults that have the potential to generate earthquakes is particularly important for lower seismicity regions such as the Waikato and South Auckland, Otago and the southern South Island - where average earthquake recurrence intervals on any single fault may be on the order of tens to hundreds of thousands of years.”
The model also included a comprehensive catalogue of offshore faults capable of generating a tsunami – something critical for hazard and risk analysis.
At the same time, Seebeck said there were likely to be many more “missing” faults that scientists didn’t yet have evidence for, and therefore weren’t factored into the model.
While it isn’t intended to be used for one-off hazard assessments, the model provided a valuable resource for scientists trying to simulate shaking in future events – even hundreds of thousands of years from now.
Researchers will also use it to untangle complex “multi-fault” ruptures like the Kaikōura event, which were increasingly considered to be common features of our earthquakes.
“At present, the NZCFM is one of the largest and most complex fault models developed globally due to [New Zealand’s] large number of faults that range in type, size and interaction with one another,” Seebeck said.
“Japan and China are also developing similar national fault models following this type of scientific, community-driven approach.”
Seebeck said as new information about our quake-making faults came to light, it’d be fed into the model over time to help improve our resilience to seismic hazards.
Shaky isles
20,000: The number of earthquakes recorded each year in New Zealand by GNS Science. About 100 to 150 of these quakes are large enough to be felt; scientists only know about the others because they were recorded by seismographs.
1000km: Faults can be as short as a few metres, and as long as 1000km. The fault rupture from an earthquake isn’t always a straight or continuous line.
125,000 years: If a fault shows evidence of having moved at least once in the past 125,000 years, geologists regard the fault as a potential source of earthquakes. If it has moved at least once in the past 5000 years, then it is considered a potential source of damaging earthquakes to any settlement within a radius of 50km.
