Scientists get clearer view of Hikurangi's quake-making potential

The Hikurangi Subduction Zone is a major offshore fault is where the Pacific Plate dives - or subducts - westward beneath the North Island. Image / GNS Science

Secrets buried in rocks amid New Zealand's greatest geological threat could reveal what triggers "mega-thrust" earthquakes and devastating tsunamis.

The Hikurangi Subduction Zone is a major offshore fault where the Pacific Plate dives – or subducts – westward beneath the North Island.

Scientists believe the subduction zone has the potential to unleash "megathrust" earthquakes larger than magnitude 8, such as those which created tsunamis that devastated Indonesia in 2004 and Japan in 2011.

Worst case scenarios of a major Hikurangi event could include thousands of fatalities and injuries, and billions of dollars worth of property loss.

In a new study, UK and New Zealand scientists combined traditional acoustic mapping of the subduction zone, with a newer method called full waveform inversion.

They found their new method gave them a clearer view of rocks along the fault.

"We can now scan underwater rocks to see their properties in greater detail," said the study's lead author, Melissa Gray, of Imperial College London.

"Hopefully this will help us to better work out how earthquakes and tsunamis happen."

In the process of subduction, two tectonic plates moved against each other, building pressure that eventually triggered one plate suddenly "slipping" beneath the other.

This sudden slipping could cause earthquakes, which in turn trigger tsunamis if they happen underwater.

But subduction could also cause silent quakes known as "slow slip" events.

Only discovered in recent times, these slow-burning earthquakes could displace faults over days or months.

By contrast, in a typical earthquake, fault movement occurs over a matter of seconds, unleashing an instant surge of energy.

Slow quakes have been found to unfold particularly around the subduction zone.

Scientists have watched them play out every one to two years near Gisborne, at a relatively shallow depth beneath the seabed, and usually driving a spate of localised quake activity.

The most recent one, which began around early April and last several weeks, triggered a swarm of East Coast quakes - among them a magnitude 5.1 jolt that struck near Mahia in May.

In nearly two decades of observations, that event was considered one of the biggest on record in New Zealand.

The authors of this new report said studying slow-slip quakes could unlock a treasure trove of information.

"Our new way of studying slow slip events could unveil a treasure trove of clues about how larger, more devastating quakes happen," Gray said.

Current rock mapping techniques use sound waves to build pictures of what rocks look like many kilometres below ground, as well as revealing how porous and hard they are and how much fluid and gas they are likely to contain.

This information helped scientists assess how rocks might behave when stress builds up, and how much shaking there would be in an earthquake.

In the new study, which also involved Gray's Imperial College colleagues Dr Rebecca Bell and Professor Joanna Morgan, along with GNS Science geoscientist Dr Stuart Henrys, the sound wave information was plugged into an imaging technique called full waveform inversion.

This method helped them paint a picture of the Hikurangi fault zone in unprecedented detail.

They also captured the shallow faults which were responsible for the large Gisborne tsunami in 1947 - an example of a large tsunami caused by a relatively small slow slip earthquake.

The method built on the concept of acoustic mapping where sound waves are sent from a boat on the ocean surface down to the seabed and kilometres into the Earth's crust.

The amount of time taken for the waves to bounce off different rock layers and back up to the boat - as recorded by underwater microphones being towed behind the boat - told scientists the distance to the seabed and rock layers, as well as the likely composition of the rocks.

The researchers combined data from acoustic mapping with the full waveform inversion technique.

This converted the sound waves into higher resolution, more intricately detailed maps of the seabed and rock beneath.

To check their data were accurate, the authors compared their models of the rock properties mapped by inversion with samples collected from recent drilling by the International Ocean Discovery Programme.

They found that the models and real data matched, indicating the technique is accurate and reliable, and can provide more information than current drilling methods.

"We can use this to study earthquake and tsunami-prone areas around New Zealand and the rest of the world," said Bell, a former GNS scientist.

Next, they plan to map the very point at which two edges of tectonic plates touched down to depths of 10 to 15 kilometres.

Henrys described the new approach as a "step-change".

"It's a powerful new technique that we can use to extract more out of the data than we have in the past."

Bell added: "Although nobody's seen fault lines like this at such scale before, we still don't know the properties of the Hikurangi plate boundary at the depth where slow slips occur.

"Ultimately, we want to understand why some slips cause devastating earthquakes, while others do not."