2016's Kaikoura Earthquake moved mountains and wrecked highways - but, unlike other big quakes, it didn't come without precursory signals. Photo / Mike Scott
2016's Kaikoura Earthquake moved mountains and wrecked highways - but, unlike other big quakes, it didn't come without precursory signals. Photo / Mike Scott
They're as much a part of our natural environment as pōhutukawa and pīwakawaka - and they happen so often that sensors pick up around 15,000 of them each year.
Yet there's much that scientists have to understand about what causes our earthquakes – some of which strike with no detectableprecursory signals, and others, like 2016's 7.8 Kaikōura event, which follow foreshocks.
In a basic sense, we know earthquakes to be a sudden movement of the Earth's crust.
In New Zealand's case, quake activity can be linked back to a continual, geological scrum going on between the Pacific Plate and the 47-million square kilometre Australian Plate.
At the southern end of the South Island, the Australian Plate dives down, or subducts, below the Pacific Plate while beneath the North Island, the opposite situation occurs with the Pacific Plate being pushed under by the Australian Plate.
In between, through most of the South Island, the two plates grind past each other along the Alpine Fault that runs along the mountainous spine of the island.
Ultimately, this motion creates thousands of quakes each year, of which between 100 and 150 are big enough to be felt by people, and countless more are too small to be observed.
As the two plates push together at a steady rate, the rocks along the boundary become more and more stressed until eventually something has to give - and an earthquake occurs along a fault somewhere in the plate boundary zone.
Scientists often compare this to a bending stick: as it becomes more deformed, it breaks and each of the pieces spring back in a relatively straight but new position from each other.
Yet, there's a surprising amount of variation in the physical processes and properties that determine how our quakes kick off.
New Zealand straddles the boundary of two tectonic plates. Image / Supplied
Victoria University geophysics lecturer Dr Calum Chamberlain noted that, like Kaikōura, the 2016 Te Araroa earthquake near Gisborne, which triggered an early-morning tsunami warning, struck after scientists observed foreshocks and some "slow slip" activity in the plate boundary.
By contrast, a 5.8 quake that hit near Wanaka in May 2015, and caused local buildings to sway for several seconds, came out of the blue.
"Other earthquakes around the world have also had no detectable precursors despite detailed observations that should be able to detect them," Chamberlain explained.
"It also looks like some foreshocks occur driven by larger scale slow-slip or pre-slip processes, similar to those that we think we observed prior to the Te Araroa earthquake, but others appear to trigger each other without any larger-scale process and randomly cascade into larger earthquakes."
Unfortunately, however, scientists still have no way to tell whether such quiet and deep-seated activity could be setting up a main shock, or if they simply represent some background process that never eventuates in an earthquake.
Source / GNS Science
"I think that understanding why some earthquakes start in different ways might be the key to forecasting some earthquakes, although it looks like many, if not most, earthquakes will be completely unpredictable.
"The more observations and the greater our understanding of how earthquakes start, the better we will be able to forecast the likelihood of future earthquakes. But, much like weather forecasting, exact prediction is impossible," he said.
"Nevertheless, understanding how earthquakes start is key to understanding earthquake physics in general."
This month, the Royal Society Te Apārangi awarded Chamberlain a Rutherford Discovery Fellowship to explore the origins of our earthquakes more closely.
That will involve building a pair of large datasets from records continuously collected by GeoNet, and other information that have been gathered from Victoria University projects over the past two decades – including a new network Chamberlain and colleagues have been establishing along the Alpine Fault.
All of that data-wrangling will culminate in a major new earthquake resource for scientists to use – along with separate catalogues of "low-frequency" quakes that unfold silently and slowly around the country.
"These catalogues are being developed in collaboration with researchers around New Zealand, America and Japan and will use cutting-edge methods to provide the most complete records of earthquakes possible," he said.
"A lot of this work will involve machine-learning methods applied to large seismic datasets using supercomputing resources.
"This is a large undertaking, and is only possible with close collaboration with world-leaders in this type of analysis."
Once Chamberlain and his team have a clearer picture of the background variability behind our quakes, they hope to be able to reconstruct some of our largest recent events and reveal more about what started them.
A 7.8 earthquake in Fiordland's Dusky Sound left behind evidence of landslides and rockfall. The remote region is is New Zealand's most productive seismic zone. Photo / GNS Science
The project also aims to shed more light on the Puysegur subduction zone – a 150km-long system extending from the Alpine Fault's southern end.
Largely because of this region's remoteness – it runs offshore and deep beneath the wild, rugged ranges and forests Fiordland – there are few seismic stations in place to monitor local activity.
"However, the Puysegur subduction zone regularly produces large earthquakes, and is New Zealand's most productive seismic zone, which makes it a great place to study earthquake processes, providing we have some data," he said.
"So, we're planning to deploy new passive seismic and GNSS instrumentation to record earthquakes and slow earthquakes there, and use these new data to study how earthquakes and slow-earthquakes interact."
