Giant Ancient Underwater Water Reservoir Found Beneath Pacific Ocean

Seismic imaging instrument surveying the subduction zone
Seismic imaging instrument surveying the subduction zone. Image credit: University of Texas Institute for Geophysics/Adrien Arnulf

Researchers have discovered a giant reservoir of water, hidden beneath the Pacific Ocean. The find may explain why nearby New Zealand experiences “slow-motion” earthquakes, which can last up to months, while causing relatively little or no damage to the surrounding area.

New Zealand’s Hikurangi Subduction Zone, where the Pacific tectonic plate falls underneath the Australian tectonic plate, is responsible for a lot of “slow-slip” quakes off the North Island’s east coast. Due to some quakes taking place in shallow waters off the coast of Gisborne, the area is particularly useful for studying the quakes, which were only discovered around 20 years ago.

One team, attempting to find out why slip events take place at some faults more than others, discovered through seismic imaging that a massive underwater reservoir was trapped in the rocks. Imaging the area, researchers found that thick, layered sediment surrounded ancient buried volcanoes.

The rocks beneath the volcanoes are fluid-rich.
The rocks beneath the volcanoes are fluid-rich. Image credit: Andrew Gase

Looking at drill core samples of volcanic rock from the area, the team found that water made up nearly half of its volume.

“Normal ocean crust, once it gets to be about 7 or 10 million years old should contain much less water,” postdoctoral fellow at the University of Texas Institute for Geophysics (UTIG), and author on the paper, Andrew Gase said in a statement, adding, “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.”

It is believed that water pressures are key to slow-slip events. Normally, this would happen where water-rich sediment is buried by the fault as one plate sinks, but here Gase believes that eroded, porous volcanic rock containing large amounts of water got buried by the fault, before turning to clay.

The team is keen to drill more samples for study, but believe the fluid plays its part in dampening earthquakes from the fault.

“It’s a really clear illustration of the correlation between fluids and the style of tectonic fault movement – including earthquake behavior,” UTIG Director Demian Saffer added. “This is something that we’ve hypothesized from lab experiments, and is predicted by some computer simulations, but there are very few clear field experiments to test this at the scale of a tectonic plate.”

The study is published in Science Advances.

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