New study helps explain ‘silent earthquakes’ along New Zealand’s North Island

An electromagnetic sensor is on its way to the ocean floor off New Zealand to collect data. Credit: Samer Naif/Lamont-Doherty Electromagnetic Geophysics Lab

Hikurangi Margin is located off the east coast of New Zealand’s North Island, where the Pacific tectonic plate sinks beneath the Australian tectonic plate, in what scientists call a subduction zone. This tectonic plate interface is partly responsible for the more than 15,000 earthquakes 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 major earthquakes occurred in the southern part of the rim before humans began keeping records.

Geophysicists, geologists, and geochemists from around the world are working together to understand why plate boundaries behave the same, producing both silent, imperceptible earthquakes, as well as potentially large earthquakes. A study published today in Nature offers a new perspective and possible answers.

Scientists have known that the ocean floor in the northern part of the island, where the plates slowly slide together, generates small, slow-moving earthquakes called slow-slide events – movements that take weeks and sometimes months to complete. But at the southern tip of the island, instead of slipping slowly as it does in the northern region, the tectonic plates are closing in. This lock creates the conditions for the sudden release of the panels, which could result in a major earthquake.

“It’s really intriguing and doesn’t get why, in a relatively small geographic area, you’d go from a lot of small, slow-moving earthquakes to a really big earthquake potential,” said Kristen Chesley, a marine electromagnetic geophysicist. Columbia University’s Lamont-Doherty Earth Observatory and lead author on the new paper. “That’s what we’ve been trying to understand, the difference in that margin.”

In December 2018, a research team began a 29-day deep-sea cruise to collect data. Similar to taking Earth’s MRI, the team used the energy of electromagnetic waves to measure how current moves through features on the ocean floor. From this data, the team was able to get a more accurate look at the role seamounts and large undersea mountains play in generating earthquakes.

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

“The northern part of the rim has really big seamounts,” Chesley said. “It wasn’t clear what those mountains could do as they sink[into the ground]and how this dynamic affects the interaction between the two plates.”

It turns out that seamounts contain a lot more water than geophysicists expected – about three to five times more than typical oceanic crust. Abundant water lubricates the panels where they join, helping smooth any slippage, and preventing the panels from sticking that can cause a major earthquake. This helps explain the tendency towards slow, silent earthquakes at the northern end of the margin.

Using this data, Chesley and her colleagues were also able to examine more closely what happens when the seamount descends. They discovered an area in the upper plate that appears to have been damaged by the slope of a seamount. This upper area of ​​the plate also appears to contain more water.

“This indicates that the seamount is breaking the upper plate, making it weaker, which helps explain the unusual pattern of silent earthquakes there,” Chesley said. The example provides another indication of how seamounts affect tectonic behavior and earthquake risk.

Lead author Kristen Chesley is a graduate student at Columbia University’s Lamont-Doherty Earth Observatory. Credit: Keri Kee

Conversely, the lack of lubricants and the effects of weak seamounts may make the southern part of the island more prone to adhesion and generate large earthquakes.

Chesley, who is on track to complete her Ph.D. In the fall, she hopes, these findings will encourage researchers to look at the way the water in these seamounts contributes to seismic behavior as they continue to work to understand slow-moving earthquakes. “The more we study earthquakes, the more it appears that water plays a major role in modulating slip on faults,” Chesley said. “Understanding when and where water is introduced into the system can only improve natural hazard assessment efforts.”

Data from wells at plate boundaries could explain slow earthquakes. More information: Fluid-rich sloping topography generates anomalous foot-porosity, Nature (2021). DOI: 10.1038/s41586-021-03619-8, www.nature.com/articles/s41586-021-03619-8 Submitted by Columbia University Earth Institute

Citation: New study helps explain ‘silent earthquakes’ along New Zealand’s North Island (2021, July 7), retrieved July 7, 2021 from https://phys.org/news/2021-07-silent-earthquakes-zealand- North Island. programming language

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