New Zealand has had dozens of 'slow' earthquakes that can last weeks. A hidden underwater sea may be the cause.

• Slow-slip events or silent earthquakes take place over weeks or months.
• Researchers have speculated that water may help dampen the effects of an earthquake.

Since 2002, dozens of slow-motion earthquakes have rippled beneath New Zealand. Researchers think a reservoir holding enough water to fill a sea may be the cause.

"Instead of an earthquake happening over the course of a couple seconds or minutes, these earthquakes can take up to a month to occur," Andrew Gase, a marine geophysicist, told Insider.

Researchers have simulated slow earthquakes in laboratories. The experiments showed that lots of fluid under high pressure can cause slower earthquakes. The water may help dampen the earthquake, making the shaking less intense.

But that was all theoretical until Gase and his colleagues drilled to gather rock samples and took seismic images of a New Zealand fault, the Hikurangi subduction zone.

That's how they discovered a hidden reservoir of water. The finding was published in the peer-reviewed journal Science Advances in August.

"Finding this much water in the fault kind of makes us think that this could be one of the reasons why New Zealand is so well known for hosting these slow earthquakes," Gase said.

Volcanic clay

About 125 million years ago, a gigantic volcanic eruption created a mass of igneous rocks known as a volcanic province. It was about a third the size of the contiguous US and eventually broke apart.

One chunk, the Hikurangi Plateau, started sinking below New Zealand. It now lies about 2 miles below sea level and is being pushed into the Hikurangi subduction zone.

When the researchers drilled into the plateau, they found volcanic rocks. But they weren't like the hard basalt found on land volcanoes. They seemed to hold a lot of water like clay.

"That was unusual," Gase said. "They're extremely soft."

The clay-like volcanic rocks meant there was a lot of water in the plateau. It could be a real-world example of the simulated slow-slip events where fluids under pressure caused slower earthquakes.

"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's co-author Demian Saffer said in a statement.

Slow-slip earthquakes

Part of the Hikurangi subduction zone runs along the eastern coast of New Zealand's North Island. It's where two tectonic plates meet, and the Pacific plate is sinking under the Australian.

"It's kind of like a conveyor belt that's pushing this rock underneath New Zealand," Gase said.

Usually, the two plates are locked together. When they detach, it can cause a slow-slip event, releasing a large amount of energy over several weeks, the New Zealand Herald reported.

These slow slips can be the equivalent of a magnitude 6 or 7 earthquake. "It would be a really big earthquake, but it's just moving really slowly for about a month," Gase said. Some can last years.

For regular earthquakes, scientists measure the magnitude with a seismometer. "They shake the ground," Gase said. "They send out seismic waves."

Slow-slip or silent earthquakes often don't produce a signal a seismometer can measure. Instead, researchers rely on GPS to track movement.

"You wouldn't be able to perceive it unless you had a really high precision instrument," Gase said. GPS allows them to see the ground move, even if it's just a couple of centimeters.

While these quiet quakes happen in many parts of the world without incident, researchers want to know if they could be a kind of early warning for bigger events. Slow slips have occurred before some devastating earthquakes, like the one in Japan in 2011.

"Even though it doesn't seem like a very dangerous phenomenon, it's really important for us to understand so that we can anticipate other types of earthquakes that could be more damaging," Gase said.