The link between silent earthquakes and the sliding of mountains under the sea, under the earth’s crust

Scientists using state-of-the-art imaging tools off the coast of Gisborne have discovered a link between undersea mountains forced under the Earth’s crust and slow-slip earthquakes.

As part of the project, the researchers were essentially able to take an MRI image of a submerged mountain under the sea, called the seamount, kilometers below the sea floor.

The team found that seamounts in the area can store large amounts of water that they carry back to Earth in the subduction zone of the Hikurangi margin. The area on the eastern coast of the lower half of the North Island is where the Pacific tectonic plate sinks beneath the Australian plate.

GNS

The researchers launched one of the tools used to collect imaging data from the rocks under the sea floor off the coast of Gisborne.

The study involved a new application of electromagnetic methods that enabled researchers to analyze the electrical conductivity of rocks, at depths they would not otherwise be able to reach.

READ MORE: * Instrumentation in the depths: Evidence for a 7.3 earthquake off the Eastern Cape downloaded to the sea floor in ROV data retrieval mission * Slow seismic studies reveal influence of undersea volcanoes * Motoica earthquake came from within the Earth’s crust * GNS using robotic science installed under the Water to look for slow-slipping earthquakes

With the collected data, they were able to compose a geophysical picture of the materials under the sea floor.

A research paper was published Thursday in the journal Nature.

The offshore area of ​​Gisborne had the most slow-slipping surface event on Earth, and it was thought that fluids coming into the subduction zone might play an important role in what was happening, according to GNS marine geophysicist Dr. Dan Bassett, one of the paper’s authors said.

Slow slip events, also known as silent earthquakes, can release the same amount of energy as a large earthquake, but the movement can take weeks or months and be too slow to be felt.

Bassett said the research showed that in places where seamounts are merging, they do a lot of damage to the forearm — the piece of crust above the subducting plate.

GNS

Slow slip events are explained in the Hikurangi subduction zone.

When a 1-2km-high seamount began to descend, Bassett said, the structure was “incompatible with two plates sliding over each other.”

“Either this seamount will break into a lot of small pieces, so it doesn’t provide that much barrier to subduction. Or you can damage the overthrust plate with so many small flaws and cracks that it makes it weak enough to push this seamount underneath.”

The imaging showed that the forearm at the Hikurangi margin had very high porosity, indicating that there was a lot of fluid in it, and indicating damage to the upper plate when the seamount sank beneath it.

“If large amounts of water enter the subduction zone, the fluid can be subjected to excessive pressure,” Bassett said.

The Hikurangi subduction zone can produce a devastating tsunami and earthquake.

“When that happens, it takes some of the weight of the upper shaft from the rock above it, so the stresses on the fault are actually less if you have large amounts of fluid. This can make it easier for the fault to slide freely.”

This would make it less likely that the two pieces of crust on either side of the fault would mesh together and accumulate large amounts of pressure that would be released in a major earthquake.

Profiles of two seamounts were collected. One of them was Tūranganui Knoll, which lies on the sea floor about 110 kilometers southeast of Gisborne.

The other, to the west, was going down under the forearm, Bassett said.

East Coast Lab

A look at the Hikurangi subduction zone, the Great Fault off the east coast of the North Island.

The upper part of this seamount was located about 5-6 kilometers below the sea floor, its edge probably began to decline about a million years ago. It will move about 50 km west every million years.

He showed a structure similar to Tūranganui Knoll, so he seemed to be battling around for the time being.

Frequent small earthquakes associated with a slow slip event in 2014 were grouped around the forearm-damaged region over the descending seamount.

“We think the damage area is a lot of very small faults, which produce these little earthquakes,” Bassett said.

“This is the first time we have had this type of image of an actively submerged seamount. We still need more work to figure out exactly what the link between slow earthquakes and seamount subduction is.”

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