Mysterious 'slow-motion' earthquakes that have shaken New Zealand could be due to a hidden 'ocean' buried two miles under the sea floor near the island nation.
The 'hidden ocean' was revealed by a 3D seismic image captured by sensors towed behind a research vessel.
The site where the researchers found the water is part of a vast volcanic province that formed when a plume of lava the size of the US breached the Earth's surface in the Pacific Ocean 125 million years ago.
The event was one of the planet's largest known volcanic eruptions and rumbled on for several million years.
The team used seismic scans to build a 3D picture of the ancient volcanic plateau in which they saw thick, layered sediments surrounding buried volcanoes.
Lab experiments on drill core samples of the volcanic rock and found that water made up nearly half of its volume.
Lead author Andrew Gase, who carried out the research as a postdoctoral fellow at the University of Texas Institute for Geophysics, said: “Normal ocean crust, once it gets to be about seven or 10 million years old should contain much less water."
The ocean crust in the seismic scans was 10 times as old, but it had remained much wetter.
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What are slow earthquakes?
The fault is known for producing slow-motion earthquakes, called slow slip events, where the energy from an earthquake is released over days or months, rather than minutes or seconds as in a normal earthquake.
These can release pent-up tectonic pressure harmlessly over days and weeks.
Scientists want to know why they happen more often at some faults than others.
Many slow slip earthquakes are thought to be linked to buried water.
But until now there was no direct geologic evidence to suggest such a large water reservoir existed at this particular New Zealand fault.
Gase said: "We can't yet see deep enough to know exactly the effect on the fault, but we can see that the amount of water that's going down here is actually much higher than normal."
Why is the finding important?
Understanding how the 'hidden ocean' dampens the earthquakes could help researchers to understand large earthquakes better, Gase said.
Gase speculated that the shallow seas where the eruptions took place eroded some of the volcanoes into a porous, broken-up rock that stored water like an aquifer as it was buried.
Over time, the rock and rock fragments transformed into clay, locking in even more water.
Gase is now calling for deeper drilling to find where the water ends up so that researchers can determine whether it affects pressure around the fault – an important piece of information that could lead to more precise understanding of large earthquakes, he said.
The finding is important because scientists think that underground water pressure may be a key ingredient in creating conditions that release tectonic stress via slow slip earthquakes.