Kaiko¯ura key to big quake risk
Scientists are looking at the surprising amount of seismic activity across the country following the Kaiko¯ura earthquake to try to work out the likelihood of future large earthquakes.
It’s another strand of the work that is making New Zealand a hotbed of earthquake research, giving scientists from around the world an insight into how and why large quakes often cluster relatively close together in time.
Among the more surprising observations from the November 2016 7.8-magnitude event was the increased number of earthquakes throughout New Zealand triggered by passing waves from the earthquake, GNS Science seismologist Dr Bill Fry said.
The general rule of thumb says aftershocks from a big earthquake are located within about one or two fault lengths away, Fry said. ‘‘That might have taken us to central Wairarapa. That’s what we could consider the normal aftershock zone of an earthquake this large.’’
While a huge number of aftershocks were generated in the northern South Island aftershock zone, locally generated seismic activity after Kaiko¯ura was also recorded on seismometers as far away as Fiordland and Northland.
In some cases there were earthquakes on local faults, while in others the activity was something called tremor.
Tremor is an emergent signal, a burst of seismic energy. Being emergent meant it wasn’t sharp like an earthquake, rather it ramped up and then ramped back down, Fry said.
‘‘If it’s happening on a fault, it provides some information about how close that fault is to failure,’’ Fry said.
Work was under way on designing software that would search through the continuous data recorded by GeoNet since about 2001, when the network of seismic recording equipment started ramping up.
‘‘You take the record after Kaiko¯ura that shows this tremor signal, then you look back in time and search for other times when ground motions look similar ... It’s like speech recognition. The same way computers know what you’re saying, we’re applying the same method to seismic data.’’
As well as looking at the tremor signals to try to see how close faults were to failure, work was also being done to try to understand the many fast, sharp earthquakes triggered by the Kaiko¯ura earthquake.
‘‘There was a pulse of stress as Kaiko¯ura earthquake waves were crossing the country,’’ Fry said.