The moist water oceanic crust supports the movement of the innocuous plates in subduction zones

New research indicates that water-rich rocks along the boundaries of the subduction zone may cause these regions to “crawl” without generating earthquakes.

Posted by Melanie Chan @ Melilo Gilochan

Quotation: Chan, M. 2020, Aquatic ocean crust supports the movement of benign plates in subduction zones, Tumblur, http://doi.org/10.32858/temblor.105

This SEM image shows a moist oceanic crust that has been deformed at about 370 ° C with 6 kilobars of pressure in the shear region of the plate front. Credit: Tulley et al., Science Advances (2020).

At around 14:46 Japanese time, on Friday 11 March 2011, Japanese communities living in the Tohoku region were informed by the country’s seismic early warning system that they would soon experience strong vibration. What people did not know at the time was that this intense vibration was the result of a massive earthquake, measuring 9.0-9.1 in size. They were unaware of the death and destruction awaiting them, caused by the earthquake and the resulting tsunami, which claimed tens of thousands of lives and laid waste on the Pacific coast in northeastern Japan.

Not all earthquakes reach such a large scale and do not create such disasters in their wake. Understanding the conditions that lead to benign geological events or catastrophic events is undoubtedly an important question in geological research. A recent study explores the conditions near Kyushu, Japan, that dictate whether tectonic plates in the subduction zone will either infiltrate each other in a benign manner or stumble and unleash any pent-up stress as a potentially catastrophic earthquake. Specifically, the research team is looking at whether water can moisten and weaken the ocean plate enough to accommodate the slow slipping of panels that cross each other.

How the plates slip

Tectonic plates move at centimeters per year over the top layer of the mantle. In the subduction zone, oceanic plates consist of a dense basalt slice below the smaller and more prosperous oceanic plates or less dense continental plates. The sediments that cover the ocean floor often cause it to ride to the ground, and because it deforms more easily than the basalt oceanic crust, it “forms a lubricant layer between the oceanic crust and the top cover. [tectonic] Chris Tolley, PhD student at Cardiff University and lead author of the new study, published in Science Advances.

Geologists usually assume that this lubricating layer of sediment allows the plates to slide across one another without producing earthquakes in a process called seismic crawl. However, Tolle says, “at the boundary of the plates where there is a relatively thin sediment cover and where the rough basalt seamounts stick above the sediments [one] You might expect more earthquakes “because the lubricant layer – oceanic sediments – is simply too thin. However, even here, some plates creep into each other without producing large earthquakes, indicating that ocean deposits may not be the only material to allow seismic crawl.

Sampling the deformed cortex

Researchers from Cardiff University and the University of Tsukuba, Japan, addressed this problem by taking samples of deformed oceanic crust from three locations in Kyushu. These three sites revealed rocks that had been deformed in the subduction zone in the late Cretaceous period, believed to have similar characteristics to the modern subduction zone off the southwest coast of Japan.

The researchers collected samples of deformed oceanic crust from three locations along the coast of Kyushu, Japan (Nagasaki Pana photos here). Credit: BirdsEyeLV, CC BY-SA 3.0

By using energy dispersive spectroscopy and electronic background scatter diffraction to analyze samples of aqueous oceanic crust in an electronic scanning microscope, researchers were able to understand the mechanical and chemical processes responsible for controlling how the oceanic oceanic crust was distorted. The researchers concluded that “if water is added to the crust of the basaltic oceans, then it has two [processes occur]First, water allows for weakness [clay] Minerals to form. Second, water facilitates dissolving and depositing metals in response to stress – a mechanism that allows deformation at low pressures. In other words, hard basalt experienced significant mechanical weakness with the addition of water. Their results showed that with weakening of the water of the upper basalt layer of a rolling slab, the slab could more easily deform by crawling – instead of the facade panels locking together would potentially cause a massive earthquake.

In this form of Science Advances, you can see the distorted rocks researchers took. In (a), metabasalts are detected as layers with a red mud stone that fills the space between them. In (B), easily soluble metal residues with metamorphic rocks reveal a lack of numbering. In (C), you can see a decrease in the silicon concentration inside the solution layers, due to the dissolution of the byte. In (D), you can see very fine grains of chlorite, pre and magnetite. In (E), asymmetric stress shadows around titanite refer to non-axial shear within dissolved minerals. Image rights (A): Å. Fagereng, University of Cardiff; Rest – Tulley et al., Science Advances (2020)

More research is needed

“This study will motivate people to take a closer look at margins that lack sediment,” says Michael Brown, a professor who specializes in this study, and forces researchers to deal with the idea that “metabasalts can be weak like sedimentary rocks at certain temperatures.” Transforming Geology at the University of Maryland. However, whether basalt moist water oceans are the main ingredients that dictate whether the subduction zone is moving through a catastrophic earthquake or a slow creep is still under discussion, Brown says.

Tolley says this research highlights the need for a better understanding of the environment and the specific geological processes responsible for a wide range of slip events (i.e. crawling versus earthquakes). Nevertheless, there is more work to be done, such as site observation “with shear regions of metasediment and strong and dry oceanic crust and hydrophilic crust combined” and what roles these groups play in different conditions in the behavior of complex sheet slip.

Check the risk of earthquake in Tempelor.

In-depth reading

Tulley, CJ, et al. Oceanic crust hosts massive mammoth crawl at low shear stress. Advancing Science, Vol. 6 (22), May 27, 2020. doi: 10.1126 / sciadv.aba1529.

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