The link between temperature, dehydration and tectonic tremors in Alaska

A research group from Kobe University has shed light on how low-frequency tectonic tremors occur; these findings will contribute to better predictions of future megathrust earthquakes.

In addition to the subducting Pacific plate, the Alaskan subduction zone is also characterized by a subducting oceanic plateau called the Yakutat terrane. Low-frequency tectonic tremors, which are a type of slow-moving earthquake, have only been detected in the subducted zone of the Yakutat terrane. However, the mechanism by which these events occur is not well understood.

Researchers at Kobe University performed a 3D numerical thermomechanical simulation of thermal convection in the Alaskan subduction zone in an effort to reveal the mechanism behind these low-frequency tremors. Based on the 3D thermal structure obtained from the simulation and the indications of hydrated minerals contained in the slab, the researchers calculated the water content distribution and compared the results of these calculations in the area where the jerks.

The results revealed high levels of dehydration in the layers of marine sediments and oceanic crust in the region of the earthquake. Researchers believe the reason tremors only occur in the Yakutat terrane is because the layers of marine sediment and oceanic crust are thicker there, meaning the level of dehydration is higher than in the plate. adjacent pacific to the west (where tectonic tremors do occur).

The Kobe University research group consisted of 2nd-year master’s student IWAMOTO Kaya (Department of Planetology, Graduate School of Science), academic researcher SUENAGA Nobuaki, and Professor YOSHIDA Shoichi (both from the Center for research on urban safety and security).

These results have been published in the UK online scientific journal ‘Scientific reports‘ (Nature Publishing Group) April 14, 2022.

main points

  • Elucidating the mechanism by which low-frequency tremors occur is important for understanding the process of plate subduction. It is also thought to help shed light on how shallower mega-thrust earthquakes occur.
  • In this study, the research group built a 3D thermomechanical model of the Alaska subduction zone and calculated the maximum water content and dehydration level of the subduction plate.
  • Dehydration levels of marine sediment layers and oceanic crust of the subduction plate were highest in the region where low-frequency tremors occur. Therefore, it is believed that the water expelled from the subducted plate contributes to the occurrence of these tectonic tremors.

Research fund

An oceanic shelf called the Yakutat Terrane is subducted into the Alaska Subduction Zone. Low frequency tectonic tremors occur on this subduction plateau. The region where slow earthquakes (such as low frequency tectonic earthquakes) occur is deeper and adjacent to the area where megathrust earthquakes occur, suggesting a connection between the two. Revealing the mechanism behind how low-frequency tectonic shaking occurs is therefore important for understanding the occurrence of various seismic events in subduction zones. This research group built a 3D thermomechanical model of the Alaskan subduction zone in order to be able to study the temperature and level of dehydration in areas close to where low-frequency tremors occur.

Research Methodology

The researchers performed a 3D numerical thermomechanical simulation consistent with the subduction of the Yakutat terrane and the Pacific plate in the Alaska subduction zone. It is believed that when the Pacific plate subducts, it brings the hydrated minerals from the slab into the deep regions at high temperature and high pressure, and these conditions cause a dehydration reaction where water is forced out of the hydrated minerals. Based on the 3D thermal structure obtained from the numerical simulation, the researchers determined the dehydration levels of the hydrated minerals in the slab. From these results, it was understood that in the region where the low frequency tremors occur, a large amount of water is expelled due to the high temperature and high pressure conditions that cause the dehydration degradation reactions. . It is believed that low frequency earthquakes do not occur in the Pacific plate because it has thin layers and therefore experiences little dehydration. On the other hand, the oceanic crust and marine sediment layers of the Yakutat terrane are comparatively thicker, which means that it experiences high levels of dehydration. The researchers concluded that this is why low-frequency tectonic tremors only occur in the Yakutat terrane.

Further research

In 1964, a mega-earthquake occurred in Alaska. It is the largest earthquake to occur in Alaska’s subduction zone and the second most powerful earthquake recorded in world history. The low-frequency tectonic tremors that have been the subject of this research occur near the epicenter of the 1964 earthquake, down-dip from the interface of the plates. Next, the research group will continue to make thermomechanical models of various subduction zones to research the universal and regional characteristics of the causal mechanisms behind submarine earthquakes and slow earthquakes. This research will help improve understanding of how earthquakes occur and our ability to predict future megathrust earthquakes.

Glossary

1. Low frequency tectonic earthquakes: Seismic event characterized by seismic waves of lower frequency than a normal earthquake.

2. Slow earthquake: A phenomenon where a fault slides at a slower speed than during a normal earthquake.

3. Ocean Shelf: A relatively flat area of ​​the seabed.

4. Slab: refers to the subducted plate.

5. Hydrated mineral: Minerals that contain OH groups in their crystal structure.

6. Dehydration degradation reaction: As the plate subducts, the resulting temperature and pressure cause phase transformations of the hydrated minerals and they expel water.

Source of the story:

Materials provided by Kobe University. Note: Content may be edited for style and length.

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