Imperial geologists have improved the mapping of underwater rocks, which could lead to better understanding of earthquakes and tsunami hazards.
Their technique combines traditional acoustic mapping with a newer method called 2D waveform inversion. This enhanced their view of rocks along a fault line – a break in the Earth's crust – off the east coast of New Zealand's North Island.
The researchers hope that with a clearer view of the state of rocks around these fault lines – which can cause earthquakes and subsequent tsunamis – they better understand why such events happen.
The results are published in JGR Solid Earth.
Lead author Melissa Gray, from Imperial College London's Department of Earth Science and Engineering, said: "We can now scan underwater rocks to see their properties in greater detail."
Treasure Trove
Our new technique could unveil a treasure trove of information to help us better understand…earthquakes and subsequent tsunamis. Melissa Gray Department of Earth Science and Engineering
Just off the North Island coast of New Zealand, the edge of the Pacific tectonic plate ducks underneath the edge of the Australian plate – an area known as the Hikurangi subduction zone. Subduction refers to two plates moving against each other, causing pressure to build and eventually trigger one plate to suddenly 'slip' beneath the other, resulting in earthquakes.
These earthquakes can cause tsunamis if they happen underwater. However, subduction can also cause silent quakes known as 'slow slip' events, which release the same amount of energy as a typical earthquake, but over a much longer amount of time. They often go unnoticed and cause no damage, but the authors of this new report say studying them could be key to understanding how larger, more devastating quakes happen.
Melissa added: "Our new technique could unveil a treasure trove of information to help us better understand rocks, and therefore earthquakes and subsequent tsunamis, at subduction zones."
Quake quandary
We can use this to study earthquake and tsunami-prone areas around New Zealand and the rest of the world. Dr Rebecca Bell Department of Earth Science and Engineering
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 helps scientists assess how rocks might behave when stress builds up, and how much shaking there would be in an earthquake.
Now Melissa, together with Imperial's Dr Rebecca Bell and Professor Joanna Morgan, have plugged this sound wave information into an imaging technique called 2D 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 builds 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 – tells 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 2D waveform inversion technique. This converted the sound waves into higher resolution, more intricately detailed maps of the seabed.
To check their data were accurate, the authors compared their models of the rock properties mapped by inversion with samples collected from drilling by the International Ocean Discovery Program. The models and real data matched, indicating the technique is accurate and reliable, and can provide information much deeper than what current drilling can reach.
The researchers say this combination of techniques could help governments to produce more accurate hazard maps for earthquakes and tsunamis.
Study co-author Dr Bell said: "We can use this to study earthquake and tsunami-prone areas around New Zealand and the rest of the world."
Next, they will work to map the very point at which two edges of tectonic plates touch down to depths of 10-15 kilometres. Dr Bell added: "Although nobody's seen faults lines like this at this kind of scale before, we still don't know the properties of the Hikurangi plate boundary at the depth where slow slip occurs.
"Ultimately, we want to understand why some slips cause devastating earthquakes, while others do not."
This research was funded by the Natural Environment Research Council (NERC).
"Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2D Full Waveform Inversion" by Melissa Gray, Rebecca E. Bell, Joanna V. Morgan , Stuart Henrys, Daniel H.N. Barker, and the IODP Expedition 372 and 375 science parties, published online on 13 August in JGR Solid Earth.
Image credits: Gray et al., Imperial College London
