A huge water reservoir has been discovered beneath the seafloor near New Zealand that may help shed light on the mechanics of slow-slip earthquakes and tectonic activity. Researchers have discovered an ocean's worth of water buried deep in the sediments and rocks of a lost volcanic plateau deep in the Earth's crust. Three-dimensional seismic images show that the water, located two miles under the sea off the coast of New Zealand, may be suppressing a major seismic fault in New Zealand's North Island.
A seismic imaging instrument is towed behind a research vessel during a survey of the Hikurangji subduction zone in New Zealand. Led by the University of Texas Geophysical Institute, the survey uncovered a vast, ancient water reservoir buried miles beneath the ocean floor. Image credit: University of Texas Geophysical Institute/Adrien Arnulf
Slow-slip earthquakes and water
The fault is known for producing slow-motion earthquakes, known as slow-slip earthquakes. These earthquakes release pent-up tectonic stress over days or weeks and are environmentally friendly. Scientists want to know why certain faults experience such earthquakes more often than others.
Many slow-slip earthquakes are thought to be related to groundwater. However, until now, there has been no direct geological evidence for the existence of such a large reservoir in this particular fault in New Zealand.
The Hikurangi Plateau is the remnant of an epic series of volcanic eruptions in the Pacific 125 million years ago. A recent seismic survey (red rectangle) conducted by the University of Texas Institute for Geophysics imaged the plateau's descent into New Zealand's Hikurangji subduction zone (red line). Photo credit: Andrew Gass
"We can't see deep enough yet to know exactly what the impact is on the fault, but we can see that the amount of water sinking here is actually much higher than normal," said the study's lead author Andrew Gase, a postdoctoral researcher at the University of Texas Institute of Geophysics (UTIG).
The study, recently published in the journal Science Advances, was completed on the basis of a seismic cruise and scientific ocean drilling led by UTIG researchers.
Seeking a deeper understanding
Gass, now a postdoctoral researcher at Western Washington University, called for deeper drilling to find where the water ended up so researchers can determine whether it affects pressure around the fault -- important information, he said, that could help lead to a more precise understanding of large earthquakes.
The origin of the reservoir
The location where the researchers found the water is part of a massive volcanic field that was formed 125 million years ago when a lava flow the size of the United States broke through the Earth's surface in the Pacific Ocean. This event was one of the largest known volcanic eruptions on Earth and lasted for millions of years.
Gass used seismic scans to build a three-dimensional image of the ancient volcanic plateau, in which he saw thick layered sediments surrounding buried volcanoes. His UTIG collaborators conducted laboratory experiments on volcanic rock drill core samples and found that water makes up nearly half of the volcanic rock's volume.
Seismic images of the Hikurangi Plateau reveal details of the Earth's interior and its composition. The blue-green layer below the yellow line shows water buried in the rock. Researchers at the University of Texas Geophysical Institute believe this water may dampen earthquakes in the nearby Higurangi subduction zone. Source: Andrew Gase
"Normal oceanic crust should contain much less water once it reaches about 7 million or 10 million years of age. The oceanic crust in the seismic scan is ten times older than normal, but it is still much wetter," he said.
Gass speculates that shallow seas from volcanic eruptions eroded some volcanoes into porous, fragmented rock that acted as aquifers to store water as they were buried. Over time, the rocks and rock fragments turned into clay, locking in more water.
Impact on understanding earthquakes
The discovery is important because scientists believe groundwater pressure may be a key factor in creating conditions that release tectonic stress through slow-slip earthquakes. This usually occurs when water-rich sediments are buried by faults, trapping water underground. However, there are few such typical marine sediments in the New Zealand fault. Instead, the researchers believe that ancient volcanoes and transformed rocks - now clays - carried large amounts of water as they were swallowed by faults.
UTIG Director Demian Saffer is one of the co-authors of the study and the co-chief scientist of the scientific drilling mission.
"This illustrates very clearly the correlation between fluids and the way tectonic faults move, including seismic behavior," he said. "This is what we have inferred from laboratory experiments and what some computer simulations predict, but there are few explicit field experiments that test this at the scale of tectonic plates."