Explain the “silent earthquakes” along the North Island of New Zealand

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An electromagnetic sensor en route to the ocean floor off New Zealand to collect data. Credit: Samer Naif / Lamont-Doherty Laboratory of Electromagnetic Geophysics

Seamounts offer clues to solving a tectonic puzzle.

The Hikurangi Margin, located off the east coast of the North Island of New Zealand, is where the Pacific tectonic plate plunges below the Australian tectonic plate, in what scientists call an area of subduction. This tectonic plate interface is partly responsible for the more than 15,000 earthquakes that the region experiences each year. Most are too small to be noticed, but between 150 and 200 are large enough to be felt. Geological evidence suggests that large earthquakes occurred in the southern part of the margin before human record keeping began.

Geophysicists, geologists, and geochemists around the world have worked together to understand why this plate boundary behaves the way it does, producing both imperceptible and potentially major silent earthquakes. A study published today (July 7, 2021) in the journal Nature offers a new perspective and possible answers.

Christine chesley

Principal author Christine Chesley is a graduate student at the Lamont-Doherty Earth Observatory at Columbia University. Credit: Kerry Key

Scientists knew that the ocean floor on the northern part of the island, where the plates slowly slide together, generate small, slow earthquakes called slow-slide events – movements that take weeks, sometimes months. But at the southern end of the island, instead of sliding slowly like in the northern area, the tectonic plates get stuck. This locking sets up the conditions for a sudden release of the plates, which can trigger a large earthquake.

“This is really curious and it is not clear why in a relatively small geographic area you would go from many small, slow earthquakes to the potential for a very large earthquake,” said marine electromagnetic geophysicist Christine Chesley, student. graduated from Columbia university‘s Lamont-Doherty Earth Observatory and lead author of the new article. “That’s what we tried to figure out, the difference in that margin.”

In December 2018, a research team began a 29-day high seas cruise to collect data. Similar to performing an MRI of the Earth, the team used the energy of electromagnetic waves to measure how current moves through features of the ocean floor. From this data, the team was able to gain a more precise insight into the role that seamounts, large seamounts, play in generating earthquakes.

“The northern part of the margin has very large seamounts. It was not clear what these mountains can do when they plunge (dive into the deep earth) and how this dynamic affects the interaction between the two plates, ”said Chesley.

It turns out that seamounts contain much more water than geophysicists predicted – about three to five times more than the typical oceanic crust. Plenty of water lubricates the plates where they meet, helping to smooth any slippage and preventing the plates from sticking which can cause a great earthquake. This helps explain the tendency for slow and quiet earthquakes at the northern end of the margin.

Hikurangi Subduction Zone Map

Map of the Hikurangi subduction zone and locations where electromagnetic receivers have been deployed to collect data. Credit: Christine Chesley, using GeoMapApp and data from William Ryan et al., Geochemistry, Geophysics, Geosystems (2009)

Using this data, Chesley and his colleagues were also able to take a close look at what is going on as sub-conduits of a seamount. They discovered an area in the upper plate that appears to be damaged by a subductive seamount. This top plate area also seemed to hold more water.

“This suggests that the seamount breaks the top plate, making it weaker, which helps explain the unusual pattern of silent earthquakes there,” Chesley said. The example provides another indication of how seamounts influence tectonic behavior and seismic risk.

Conversely, the lack of lubrication and the weakening effects of seamounts can make the southern part of the island more likely to stick and generate large earthquakes.

Chesley, who is on track to complete his doctorate. in the fall, hopes these findings will encourage researchers to consider how the water in these seamounts contributes to seismic behavior as they continue to work to understand slow earthquakes. “The more we study earthquakes, the more it seems that water plays a leading role in modulating slip on faults,” Chesley said. “Understanding when and where water enters the system can only improve natural hazard assessment efforts. “

Reference: “Fluid-rich subducting topography generates anomalous forearc porosity” by Christine Chesley, Samer Naif, Kerry Key and Dan Bassett, July 7, 2021, Nature.
DOI: 10.1038 / s41586-021-03619-8

Samer Naif, former Lamont Research Assistant Professor, now Assistant Professor at Georgia Tech; Kerry Key, Associate Professor at the Lamont-Doherty Earth Observatory; and Dan Bassett, researcher at GNS Science, collaborated on this research. This project was funded by the National Science Foundation.

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