Researchers have shown that slow-moving earthquakes are controlled by the permeability of rocks

This image shows one of the rocky outcrops collected by researchers in New Zealand in 2022. Source: Nicola Tessato / Jackson School of Geosciences

Earthquakes are the most dramatic and noteworthy results of the movement of tectonic plates. Often devastating and deadly, or at least physically felt, they are groundbreaking geological events in the literal sense. However, not all tectonic movements lead to effects that humans can perceive.

Slow-slip events occur when pent-up tectonic forces are released over the course of a few days or months, such as an earthquake that unfolds in slow motion. The more gradual movement means people won't feel the ground shaking under their feet and buildings won't collapse. But the lack of destruction does not make slow slide events any less scientifically important. In fact, its role in the earthquake cycle may help lead to a better model for predicting when earthquakes will strike.

In a paper recently published in Geophysical Research Letters, a Jackson School of Geosciences research group led by Harm van Avendonk, Nathan Bangs, and Nicola Tessato explores how the composition of rocks, specifically their permeability — or how easily fluids can flow through them — affects the frequency and intensity of slow-slip events.

In 2019 and 2022, the group traveled to New Zealand's North Island to collect rocks from several outcrops near the Hikurangi Margin. This is a subduction zone off the coast of New Zealand where slow slip events occur routinely, about once a year. The researchers brought a cache of rocks back to the University of Texas, where they tested their permeability and elastic properties.

Rock samples collected from outcrops in New Zealand in 2022 were taken to laboratories at the Jackson School of Geosciences at the University of Texas at Austin. Credit: Nicola Tessato/Jackson School of Geosciences

Their tests showed how pores in the rocks could control regular slow-slip events in this subduction zone. Previous studies have indicated that a layer of impermeable rock at the top of descending tectonic plates acts as a closed lid, trapping fluids in the pores of the underlying rock layers.

As fluid builds up under the seal, the pressure builds, eventually becoming high enough to cause a slow-slip event or earthquake. This event then breaks the impermeable seal, temporarily fracturing the rock, allowing it to absorb fluid. Within a few months, the rock heals back to its initial permeability, and the cycle begins again.

While studying this cycle, Tessato and other researchers tested rocks from nearby surface outcrops that were once part of an earthquake fault deep underground. Previous permeability studies have only been performed on bulk sediments that are embedded in solid rock.

“We are showing for the first time, using rocks representative of those at depth, that permeability is controlling (slow slip events),” he said.

Laura Wallace, a researcher at the Geophysics Institute at the University of Texas and GEOMAR in Germany, has been studying slow-slip events for more than 20 years, and was the first person to record slow-slip events occurring in the Hikurangi margin. This paper adds more data points to inform the time scales over which fault zone permeability changes can occur, potentially affecting the observed slow-slip event cycles, she said.

View of the ocean from the coast of New Zealand. Credit: Nicola Tessato/Jackson School of Geosciences

“It adds some additional data constraints on how the rift valve process works, how fluid circulation might work in a subduction zone — if that's actually what drives the periodicity of these things,” Wallace said.

The ultimate goal of this research, Tissato said, is to understand why earthquakes occur and eventually build a convincing model that can predict them, a code that scientists have not yet been able to solve.

He and graduate student Jacob Allen are currently analyzing rock samples from the center of the margin and testing for differences in permeability. The rocks at the northern end of this subduction zone are richer in clay than those at the southern end.

Because clay is flexible and can hold a lot of water and other liquids, it is ideally suited to trap, break up and channel those liquids. This could explain why slow-slip events at the northern end of the subduction zone occur more frequently, while they rarely occur at the southern end, Tessato said.

“We have to do an exercise to understand why we have slow slips north of the Hikurangi margin, and why we have fewer slow slips south of the Hikurangi margin,” Tissato said. “Because if we understand that, we have one more step forward toward prediction.”

Three graduate students from the Jackson School of Geosciences also contributed to this paper: Carolyn Bland, Kelly Olsen, and Andrew Gies.

More information: Nicola Tissato et al., Permeability and elastic properties of rocks from the northern Hikurangi margin: implications for slow-slip events, Geophysical Research Letters (2024). doi: 10.1029/2023GL103696

Provided by the University of Texas at Austin

Citation: Researchers show slow-moving earthquakes are controlled by rock permeability (2024, May 7) Retrieved May 7, 2024 from https://phys.org/news/2024-05-earthquakes-permeability.html

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