Is accelerated subduction before the great earthquakes?

Photo: GNSS stations are heavily populated in Japan. In Chile they are the most sporadic location. In both cases, they provided accurate data to measure the direction and speed of ground motion.
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Credit: Bedford et al. 2020, Nature: DOI 10.1038 / s41586-020-2212-1

A strange reflection of the Earth’s movement preceded two of the largest earthquakes in history. This is the result of a new study led by Jonathan Bedford of the GFZ’s German Center for Earth Sciences. In cooperation with a diverse team of geologists from GFZ, FU Berlin, Chile, and the United States, it investigated the signals recorded in Chile and Japan that capture the movement of GNSS stations before the 2010 Muli earthquake (magnitude 8.8) and the Tohoku Aoki earthquake in 2011 (magnitude 9.0) which It led to a devastating tsunami and Fukushima nuclear collapse. Scientists publish their findings in the latest issue of the scientific journal Nature.

Using the latest geodetic analyzes, the team reported a large area of ​​1,000 km from the surface of the earth near the boundaries of the plate, which alters the sense of movement with it over several months leading directly to earthquakes. Both events occurred at the edge of the Pacific Ocean where oceanic plates dive under the continental crust in a process called subduction. In Japan, a dense network of permanent stations is tracked by Global Navigation Satellite Systems (GNSS) with high accuracy so researchers can observe how fast and moving the Earth is. In Chile, the web is not dense but still follows most of the deformed continental plate. Usually, the terminals on the ground are very far away from the subduction trench where the continental crust is compressed and thus shortened. However, when studying the time series of GNSS signals, the researchers found the opposite trend: Suddenly the stations moved toward the subduction trench, i.e. towards the open ocean, then reversed their direction back to their normal motion (see moving images). Soon after this second reversal, the underground rupture and massive earthquakes occurred.

With the help of simple models and more well-known geological limitations, the authors suggest that these reflections capture periods of improved clouds caused by rapid and intense structural changes in the ocean plate when indulging. Accordingly, it is suggested that these periods of improved clouds have accelerated the inevitable failure of the shallow parts of contact with the subduction zone.

Jonathan Bedford explains: “It is a common assumption that deeper subduction progresses at a fairly constant speed between large earthquakes. And our study shows that this assumption is excessively simplistic. Indeed, its change may be a major factor in understanding how the largest nucleus earthquake occurs.”

Whether or not such strong repercussions will occur before the next major earthquake still has to be seen, but what is clear from this study is that the subduction zones are more dynamic over the observed date range than previously thought.

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Original study: Jonathan Bedford, Marcus Moreno, Ziguo Ding, Uno Unken, Bernd Shore, Tim John, Juan Carlos Pais, Michael Beavis: fluctuation over thousands of kilometers before sunken earthquakes. In Nature 2020. DOI: https://doi.org/10.1038/s41586-020-2212-1

Link: https://www.nature.com/articles/s41586-020-2212-1 (This link will be published after the ban ends)

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