A slow slip event off the east coast of the North Island, with an estimated equivalent impact of a M6.9 quake, was detected this week. GNS Science geophysicist Laura Wallace explains this important new area of seismic research, and what’s up with the Hikurangi subduction zone

Among the most fascinating discoveries in the days following the monster Kaikoura earthquake last Monday morning has been what scientists call “slow slip events”, recorded as far north as Hawkes Bay and Gisborne.

How did the faults that ruptured on the Canterbury coast lead to these so-called “silent earthquakes” hundreds of kilometres to the north-east, and what is the Hikurangi subduction zone, the source of much fascination and intrigue for earthquake scientists, playing at?

Earlier today I spoke to Laura Wallace from GNS Science.

The Spinoff: What is the Hikurangi subduction zone?

Laura Wallace: A subduction zone is basically where one tectonic plate dives down beneath another, in a process called “subduction”. Much of the Pacific Rim is ringed by subduction zones. Subduction zones produce the world’s biggest earthquakes and are also a primary reason for volcanic activity we see around the Pacific Rim – hence the name “Ring of Fire”. Some of the largest Pacific Rim subduction zones occur offshore Japan, South America, and parts of North and Central America.

Our subduction zone offshore the North Island, the Hikurangi subduction zone, is where the Pacific plate dives down beneath the eastern North Island. The Hikurangi subduction zone continues northward and connects up with the Kermadec trench and the Tonga trench, so it goes all the way up to north of Tonga. It’s quite a long subduction zone.

And the Hikurangi zone is active as part of the pattern we’re seeing now?

The Hikurangi subduction zone was probably not involved in last week’s magnitude-7.8 quake. But, it does appear that the 7.8 Kaikoura quake caused some small stress changes on the Hikurangi subduction zone further north that produced the slow slip event offshore the North Island’s east coast that we have observed over the last week.

These slow slip events, they’ve been called “silent earthquakes”?

They’re similar to earthquakes, as slow slip events accommodate more rapid than normal movement on the plate boundary fault, in this case the Hikurangi subduction zone. Unlike earthquakes (which involve slip along faults in a matter of seconds), slow slip events can take weeks or months to occur. Because slow slip events are slow and don’t occur suddenly enough to release seismic energy, we have to use GPS to detect them. Basically, we look for mm-level changes in the position of the land above the slow slip events. East coast North Island slow slip events typically show up as 2-3 cm eastward shifts of the land on the GPS.

The slow slip event we are observing off the North Island’s east coast right now appears to have been triggered by the 7.8 earthquake, probably due to very small stress changes induced by the passing seismic waves from the M7.8 earthquake. We’ve seen slow slip events triggered by earthquakes in New Zealand before. The Te Araroa Earthquake that happened in early September triggered a slow slip event off East Cape. There was also an earthquake near Gisborne in late 2007, that triggered a slow slip event near Gisborne. In this case the M7.8 earthquake appears to have triggered slow slip over a larger part of the Hikurangi plate boundary, going from about southern Hawkes Bay up to East Cape. We haven’t seen this happen simultaneously over such a large area before, and this makes it a really interesting event.

Can the strong Porangahau earthquake yesterday afternoon be linked to the slow slip?

Yes, the 5.4 quake near Porangahau probably is related to the slow slip event occurring offshore in that area. Previous slow slip events offshore in the Porangahau region, in 2006 and 2011, have also been accompanied by increased rates of seismicity there, with similar magnitudes to what has been observed in the past few days.

There is some evidence from Japanese scientists that the 2011 M9 Tohoku-oki earthquake was preceded by a slow slip event. However, there have also been hundreds of slow slip events observed at subduction zones around the world that have not triggered larger, damaging quakes. So, if slow slip events do trigger large damaging quakes, it is very rare indeed. In New Zealand we typically have at least two or three slow slip events each year. Scientists have only discovered in the last 15 years that slow slip events occur, so trying to understand their relationship to larger, damaging quakes is still in its very early stages.

And we’ve only been able to measure slow slip events since GPS equipment was introduced?

When we started rolling out the GeoNet and the PositioNZ (LINZ) networks, in about 2002, we saw the first slow slip event in New Zealand on a GPS site up in Gisborne, in October 2002. Since then, we’ve realised that they happen all the time on our subduction zone. There’s probably at least a couple every year, and they’re a very important mode of accommodating tectonic plate motion on the Hikurangi subduction zone. They relieve a lot of stress on large parts of the plate boundary. But they also can increase stress on other parts of the plate boundary. A sign of this is that we often see small swarms of earthquakes associated with slow slip events, like what’s going on offshore Porangahau right now.

So this is a relatively new area in terms of research?

It is. The first slow slip events were discovered in Cascadia, in the northwestern US and western Canada – slow slip events were first discovered at the subduction zone there in about 2001. Since then we’ve seen them at most subduction zones in the world where we have GPS instruments. They’re a very new thing. Before about 15 years ago, we really didn’t know that faults behaved in this way, and that this kind of slow slip behaviour on faults was so widespread. The discovery of slow slip at plate boundaries around the world has led to a revolution in our understanding of how plate boundary works – the recognition of slow slip is probably one of the most exciting discoveries in the field of seismology in the last 15 years. It’s created a huge field of inquiry, with many scientists trying to figure out why they occur, what their relationship is to seismicity, and even potentially to, larger, damaging earthquakes. We’re really in the early stages of trying to understand that.

If these recent slow slip events weren’t slow, if they were conventional earthquakes happening almost instantaneously, would they be violent?

The amount of slip in this particular event off the east coast, at least based on my preliminary models, suggest if that slip had all occurred suddenly in an earthquake, it would have been about a magnitude 6.9.

Wow.

So they are taking up a lot of the action on the subduction zone, in slow slip instead of having earthquakes. But, although they reduce the need for earthquakes in some locations, they also redistribute stresses on to other parts of the plate boundary, and can cause swarms of earthquakes. We also saw swarms of microseismicity offshore Porangahau during slow slip events in 2006 and 2011, so this isn’t necessarily unusual. We basically just need to keep an eye on it, and see how it evolves.

Is new data that has come off the NIWA research vessel the Tangaroa, revealed yesterday, helping understanding of what happened on Monday?

NIWA have done some impressive mapping of the offshore extent of some of the faults that ruptured in the M 7.8 earthquake, particularly the Needles fault, offshore of the northeast of the South Island.

The Niwa team were already out as part of a big project we have funded by MBIE to study earthquake and tsunami potential at the Hikurangi subduction zone – they were collecting cores of turbidites to try to look at past Hikurangi subduction earthquakes, and then the earthquake happened and they diverted.

At the last count there were six or more faults identified as having ruptured. Is that still where we are? Is the Needles fault the same fault as the Kekerengu Fault? revealed yesterday

The Needles fault is basically an offshore continuation of of the Kekerengu Fault as it moves north towards Cook Strait. They’re all part of the same system but essentially different faults. It looks like many faults have ruptured in this earthquake – it will probably take a while for researchers to pin-down exactly how many.