A sunken seamount provides evidence of slow-moving earthquakes

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3D seismic image of Babaco seamount (grey bulge in the middle). The seamount is located near New Zealand’s Hikurangi subduction zone and lies more than 3 miles below the sea floor. Researchers at the University of Texas Institute of Geophysics found that the collision of seamounts with subduction zones may affect earthquake activity. Credit: University of Texas Geophysics Institute/Nathan Bangs

Scientists have long puzzled over what happens when seamounts — mountains and volcanoes on the seafloor — are pulled into subduction zones. Now, new research from the University of Texas at Austin shows that when seamounts sink, they leave behind a trail of soft sediment. Researchers believe that patches of sediment help tectonic pressure escape gradually in slow-slip earthquakes rather than violent tremors.

The findings, published June 7, 2023, in the journal Nature Geoscience, could be used to modify earthquake models and help scientists uncover the mechanisms that trigger earthquakes.

The research was led by Nathan Pang, senior research scientist at the University of Texas Geophysics Institute. In 2018, Bangs led an ocean seismic survey that resulted in the first-ever 3D survey of a large, tilting seamount. The long-extinct volcano known as Babaco Seamount lies three miles below the sea floor within the Hikurangi subduction zone off the coast of New Zealand.

Images from the survey show seamount impact at the subduction zone and the pattern of pressures, fluids, and sediments surrounding it. Previous models suggested that sediment is being pushed down a subduction zone by a seamount, but the survey revealed something different: a massive sediment trail in the wake of Pāpaku.

And in yet another surprise, scientists have discovered a faint trace of a much larger seamount that sank long ago under New Zealand’s North Island.

Pāpaku seamount is located east of New Zealand’s North Island, in the Hikurangi subduction zone. In 2018, scientists at the University of Texas Institute of Geophysics led the first 3D seismic survey of the seamount and surrounding areas (green shaded area). Credit: University of Texas Geophysics Institute

According to Bangs, the discovery indicates that sunken seamounts are pulling up enough water-rich sediment to create suitable conditions in the crust for slow-slide earthquakes, at least in New Zealand.

“This older type appears to be more closely associated with a high ridge that is already in the crosshairs where the recent slow-sliding activity was,” Bangs said. “There may be other places like Cascadia (in the Pacific Northwest, US) that have submerged seamounts and a lot of sediment, but because the subducting crust there usually has less water than Hikurangi, it might be less likely that they have the same shallow type. Slow sliding activity.

A 2018 seismic survey of the Hikurangi Subduction Zone, New Zealand. The survey involved an ultrasound scanner tracking behind the research vessel Marcus Langseth. Data collected during seismic ultrasound helped researchers at the University of Texas Institute of Geophysics produce the first-ever 3-D image of a large seamount as it collided with a subduction zone. Credit: University of Texas Geophysics Institute/Nathan Bangs

Slow earthquakes are slow-moving versions of large earthquakes, releasing similar levels of pent-up tectonic energy but in a creeping, harmless way that can take days or weeks to unfold. Scientists believe crust formation is a key factor in how tectonic energy is released, with softer, wetter rocks allowing plates to slide slowly, while brittle, dry rocks storing energy until it fails in violent and deadly earthquakes.

The new findings reveal how these conditions sometimes emerge and, more importantly, Bangs says, tell scientists what to look for in other subduction zones of the world.

Further information: Nathan L. Bangs et al, Slow sliding along the Hikurangi fringe associated with fluid-rich sediment undercutting intrusive seamounts, Nature Geoscience (2023). DOI: 10.1038/s41561-023-01186-3

Provided by the University of Texas at Austin

Citation: Sinking seamount provides evidence of slow-motion earthquakes (2023, June 22) Retrieved June 22, 2023 from https://phys.org/news/2023-06-seamount-clues-motion-earthquakes.html

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