New study helps explain ‘silent earthquakes’ along New Zealand’s North Island
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 in the journal Nature offers a new perspective and possible answers.
Scientists knew that the ocean floor in 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 the Lamont-Doherty Earth Observatory at Columbia University 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 offshore cruise to collect data. Similar to taking 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 known what these mountains can do when they plunge (plunge into deep land) and how these dynamics affect the interaction between the two plates.” , said Chesley.
It turns out that seamounts contain much more water than geophysicists had 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 that can cause a great earthquake. This helps explain the tendency for slow and quiet earthquakes at the northern end of the margin.
Using this data, Chesley and his colleagues were also able to take a close look at what is happening as seamount sub-conduits. 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.
âThis suggests that the seamount breaks the top plate, making it weaker, which helps explain the unusual pattern of silent earthquakes there,â 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. âThe more we study earthquakes, the more it seems that water plays a leading role in modulating slip on faults,â Chesley said. âUnderstanding when and where water enters the system can only improve natural hazard assessment efforts. ”
Borehole data in plate boundaries could explain slow earthquakes
The subducting topography rich in fluids generates an abnormal porosity of the fore-arch, Nature (2021). DOI: 10.1038 / s41586-021-03619-8, www.nature.com/articles/s41586-021-03619-8
Provided by Earth Institute at Columbia University
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