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New 3D images give never-seen-before views inside New Zealand’s largest fault
New research has revealed the first ever 3D images of the Northern Hikurangi margin - and how New Zealand's largest fault moves.
Subduction zones produce the largest earthquakes in the world
Aotearoa New Zealand's largest fault, the Hikurangi Subduction Zone (HSZ), is where the Pacific tectonic plate dives west beneath the Australian plate and underneath the east coast of the North Island.
In some parts of the subduction zone, GPS instruments are showing the plates slowly move by a few millimetres a year. This behaviour is called a "slow slip" and occurs over periods of weeks or months. However, in other parts the plates are stuck, locked together, and building up pressure.
By understanding the structural factors that create the smoother slipping and stuck zones, scientists are seeking to better diagnose what areas could generate potential future earthquakes and tsunami. As Aotearoa's largest source of potential earthquakes and tsunami, its critical to be able understand the HSZ in high-resolution detail.
New 3D images reveal hidden structures in the Hikurangi Subduction Zone
In 2018 a collaboration of researchers from USA, Japan, UK, and GNS Science used the RV Marcus Langseth to record numerous overlapping race-track "seismic reflection data" lines. The data were gathered together alongside deployments of ocean bottom seismographs and onshore seismometer in a effort called the "NZ3D" survey.
In an international collaborative effort spanning three recent high-profile publications, the first ever spectacular 3D seismic images of the northern part of Hikurangi margin have now documented new insights for understanding the structural, stratigraphic and hydrogeologic characteristics of the HSZ. Understanding these qualities, specifically how they transport fluids, are key to knowing the conditions that lead to generation of subduction earthquakes.
Model of Magnitude 8.9 Hikurangi Earthquake and Tsunami
This scenario helps emergency responders to plan and prepare for a magnitude 8.9 earthquake and tsunami on the Hikurangi Subduction Zone.
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How the 3D images were created
Seismic reflection data are typically how geophysicists visualize the crust. To capture this data a specialist vessel, in this case the R/V Marcus Langseth, tows an array of individual sound sources that are tuned and combined to radiate a sound wave downward to the seafloor. The echoes that bounce back from layers in the earth are recorded on a streamer towed behind the vessel and on sensitive seismographs located onshore and on the seabed.
While a grid of 2D profiles is good enough to identify major plate boundary structures, this high-resolution 3D data are needed to visualize details within subduction zones to improve understanding of fault geometry and slip behaviour. The 3D data are combined in a CAT scan image of the subduction zone that shows the architecture and properties of the boundary between tectonic plates can contribute to variability in the location of strong and seismogenic versus weak slipping segments.
The 3D data provides new constraints on the physical conditions and rock properties to inform computer simulations and forecasts of earthquake ground shaking and tsunami inundation that greatly help improved hazard preparedness and response.
How fluids and underwater volcanoes influence how New Zealand's largest fault moves
In June a Nature Geoscience paper reports how the NZ3D data capture a seamount (underwater volcano) caught in the act of subducting beneath the shallow part of the Hikurangi margin and forms sediment lenses in its wake that appear to enhance slow slip.
Further, in a Geology paper the NZ3D data reveal a detailed map of the deeper parts plate interface that shows that it has kilometer-high hill and valleys. The new NZ3D data show that the plate interface may strongly govern the nature of how the margin deforms, including the localization of both slow slip and hazardous fast-slip earthquakes.
Most recently, a Science Advances paper revealed a previously hidden water reservoir within the layers of the Pacific plate being swallowed up in the subduction process. The new finding suggests that subducting plate of volcanic rocks act as amplified source of water that influences the slip behaviour of the margin. The trapped water is under pressure and results in the plate boundary being weak and prone to unlocking and sliding in slow slip. The study highlights the presence of significant water delivery to slow slip source from the incoming Pacific, that were previously unknown.
Importantly, we are able to pinpoint the location of water rich layers, that allow smooth slipping, versus other water-poor segments that are stuck and will likely rupture in fast earthquakes.
Revealing the mysteries of the subduction process in ways never possible before
The hope is that these new generation 3D images will be able to identify areas of the plate boundary where water rich layers enable smooth slip and other areas that are locked and stuck.
By understanding how the slip behaviour varies along the subduction zone, it allows scientists to better diagnose and pinpoint areas that are more prone to generate large earthquakes.
Our 3D data also provides new constraints on the physical conditions and rock properties to inform simulations of earthquake ground shaking and tsunami inundation that greatly help improved hazard preparedness and response.
These results represent another piece in the puzzle that we can start using in our large-scale earthquake cycle simulations
This research was supported by New Zealand Ministry of Business Innovation and Employment, USA National Science Foundation, UK Natural Environment Research Council and Japan Agency for Marine-Earth Science and Technology.