A research group from Kobe University has shed light on how low-frequency tectonic oscillations occur; these findings will contribute to better predictions of future megathrust earthquakes.
In addition to the subducting Pacific plate, the Alaska subduction zone is also characterized by a subductive oceanic plateau called the Yakutat terrane. Low-frequency tectonic tremors, which are a type of slow earthquake, have only been detected in the submerged Yakutat terrane. However, the mechanism by which these events occur is not well understood.
Researchers from Kobe University performed a 3D numerical thermomechanical simulation of thermal convection in Alaska’s subduction zone with the aim of revealing the mechanism behind these low-frequency vibrations. Based on the 3D thermal structure obtained from the simulation and the indications of hydrous minerals in the slab, the researchers calculated the distribution of the water content and compared the results of these calculations in the area where the vibrations occur.
The results revealed high levels of desiccation in the marine sediment layers and ocean crust in the earthquake region. The researchers believe that the reason the tremors only occur in the Yakutat terrane is because the marine sediment layers and ocean crust are thicker there, meaning the level of desiccation is higher than in the western adjacent Pacific plate (where tectonic tremors not prevent). to prevent).
The Kobe University research group consisted of 2nd year master student IWAMOTO Kaya (Department of Planetology, Graduate School of Science), scientific researcher SUENAGA Nobuaki and professor YOSHIDA Shoichi (both from the Research Center for Urban Safety and Security).
These results have been published in the British online scientific journal ‘Scientific Reports‘ (Nature Publishing Group) on April 14, 2022.
- Elucidating the mechanism by which low-frequency tremors occur is important for understanding the plate subduction process. It is believed this will also help alleviate how shallower megathrust earthquakes occur.
- In this study, the research group constructed a 3D thermomechanical model of the Alaska subduction zone and calculated the maximum water content and degree of desiccation of the subduction plate.
- The dehydration levels of the marine sediment layers and ocean 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 sunken plate contributes to the occurrence of these tectonic oscillations.
An oceanic plateau called the Yakutat terrane is sinking in the Alaska subduction zone. Low-frequency tectonic oscillations occur on this subduction plateau. The area where slow earthquakes (such as low-frequency tectonics) occur is deeper and adjacent to the area where megathrust earthquakes occur, suggesting a link between the two. Revealing the mechanism behind how low-frequency tectonic tremors occur is therefore important for understanding the occurrence of various earthquakes in subduction zones. This research group constructed a 3D thermomechanical model of Alaska’s subduction zone so that they could examine the temperature and level of desiccation in the areas near low-frequency tremors.
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 thought that as the Pacific plate subducts, it moves the hydrous minerals in the plate to the deep high temperature and high pressure regions, and these conditions trigger a desiccation reaction that expels water from the hydrous minerals. Based on the 3D thermal structure obtained from the numerical simulation, the researchers determined the dehydration levels of the hydrous minerals in the slab. From these results, it was understood that in the region where low-frequency vibrations occur, a large amount of water is expelled because of the high temperature and high pressure that cause the dehydration-degradation reactions. Low-frequency earthquakes are thought not to occur on the Pacific plate because it has thin layers and therefore experiences little desiccation. On the other hand, the ocean crust and marine sediment layers of the Yakutat terrane are relatively thicker, meaning it experiences a high degree of desiccation. The researchers concluded that this is why low-frequency tectonic tremors only occur in the Yakutat terrane.
In 1964, a megathrust earthquake occurred in Alaska. This is the largest earthquake to have occurred in the Alaska subduction zone and the second most powerful earthquake in world history. The low-frequency tectonic tremors that were the subject of this study occur near the epicenter of the 1964 earthquake, at the downward dip of the plate interface. Next, the research group will continue to create thermomechanical models of different subduction zones to look for universal and regional features of the causal mechanisms behind submarine megathrust earthquakes and slow earthquakes. This research will contribute to a better understanding of how earthquakes occur and our ability to predict future megathrust earthquakes.
1. Low Frequency Tectonic Vibrations: A seismic event characterized by seismic waves of a lower frequency than an ordinary earthquake.
2. Slow Earthquake: A phenomenon in which a fracture slides more slowly than in an ordinary earthquake.
3. Oceanic Plateau: A relatively flat part of the seabed.
4. Plate: refers to the subduction plate.
5. Hydrous 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 hydrous minerals and they expel water.
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