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Massive water reservoir may explain mysterious earthquakes in New Zealand

Researchers have discovered a vast reservoir of water deep within the sediment and rock of a buried volcanic plateau, equivalent to an entire sea. 

Located two miles beneath the ocean floor off the coast of New Zealand, this discovery may provide answers to the mysterious phenomenon of slow-motion earthquakes, known as slow slip events.

Unusual earthquakes

The colossal water reservoir was identified through 3D seismic imaging and is situated beneath New Zealand’s North Island, close to a major earthquake fault known for its slow slip events. 

Unlike typical earthquakes that release tectonic stress rapidly, these slow slip events release the pressure more gradually, spanning days or even weeks. 

Despite their less destructive nature, the causes behind the frequent occurrences of these slow-motion earthquakes at certain faults have remained elusive.

Massive water reservoir 

Many of these slow slip events, also known as “silent earthquakes,” were believed to be associated with buried water. Nevertheless, the scale of the water reservoir in this specific fault near New Zealand was unprecedented. 

“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,” explained Dr. Andrew Gase, the lead author of the study.

Ocean drilling

The research was based on seismic cruises and scientific ocean drilling led by the University of Texas Institute for Geophysics (UTIG) team. 

Dr. Gase, currently a postdoctoral fellow at Western Washington University, is calling for deeper drilling to find where the water ends up so that researchers can determine whether it affects pressure around the fault. This is an important piece of information that could lead to more precise understanding of large earthquakes, he said.

Ancient volcanic terrain

The reservoir’s site is a relic of a mammoth volcanic eruption that transpired around 125 million years ago when a colossal plume of lava, nearly the size of the United States, broke through the Pacific Ocean’s surface. 

This event ranks among the most significant volcanic eruptions in Earth’s history, lasting several million years. 

Dr. Gase utilized seismic scans to reconstruct a 3D portrayal of this ancient volcanic terrain, identifying thick sediment layers enveloping dormant volcanoes. Subsequent lab tests on volcanic rock samples revealed that almost 50% of the rock volume was water.

Subterranean water pressure

According to Dr. Gase, regular oceanic crust of around 7 to 10 million years should have substantially lesser water content. However, the crust identified in these scans was a century older, yet significantly more hydrated. 

The experts theorize that the initial shallow seas, where these eruptions occurred, weathered some of the volcanoes into porous formations capable of retaining water. Over millennia, these fragments morphed into clay, further encapsulating the water.

This insight is crucial, as it’s widely believed that subterranean water pressure plays a pivotal role in engendering conditions conducive to slow slip earthquakes. 

Saturated sediments

Typically, this transpires when sediments saturated with water are buried alongside the fault, confining the water below ground. However, the sediment composition of the New Zealand fault diverges from this norm. 

Instead, the researchers propose that the ancient volcanic remnants and their subsequent transformation into clay are ushering significant water volumes downwards, as they are subsumed by the fault.

Study implications 

Emphasizing the broader implications of this discovery, UTIG Director Demian Saffer suggested that earthquake faults worldwide might share similarities with this scenario. 

“It’s a really clear illustration of the correlation between fluids and the style of tectonic fault movement – including earthquake behavior,” said Saffer. 

“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 research was funded by the U.S. National Science Foundation and research agencies in New Zealand, Japan and the United Kingdom.

The research is published in the journal Science Advances.

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