A new study has found that an underwater volcano atop a sinking tectonic plate off Japan's coasts plowing into the mantle of the Earth could be the reason behind the several strong earthquakes that shook Japan in the last 40 years.
Underwater Volcano Riding on Sinking Tectonic Plate
The prehistoric submarine volcano, named Daiichi-Kashima Seamount, rides on the Pacific tectonic plate, which is situated around 40 kilometers off the east coast of Japan. In this area, three tectonic plates intersect with each other, with the Philippine plate to the south and the Pacific plate to the east, both slipping under the Okhotsk plate to the north.
According to associate professor Eunseo Choi from the Center for Earthquake Research and Information of the University of Memphis, who is a co-author of the study, the seamount is situated on a plate section that started descending into the mantle of the Earth around 150,000 to 250,000 years ago.
Strong Earthquakes in Japan
However, the volcano is still sufficiently close to the surface for it to cause earthquakes. Postdoctoral researcher Sungho Lee, the study's lead author from the university, explains that the volcano sits less than 50 kilometers deep.
Though most seismic activity gets reflected as minor tremors, there have been several major earthquakes that have taken place over the years, with magnitudes ranging from 7 to 7.8 These were in 2011 (7.8), 2008 (7), and 1982 (7). Earlier research failed to explain such occurrences.
While an earlier study predicted that the friction that took place during the slipping or subducting of a tectonic plate was too weak to lead to earthquakes, newer data show that seamounts face huge resistance when they ride through a subducting plate and sometimes end up stuck. Lee explains that the seamount is close to being stationary as it has remarkably strong friction.
With the seamount digging into the plate, the stress builds up on its leading edge. The surrounding region ends up locking and grinding to a halt, while the remainder of the plate goes on with its descent into the mantle.
Lee explains that stress goes up at the seamount's edge, and as time passes, it propagates and then moves inward. He adds that the accumulation will not endlessly continue and that the stress eventually gets released when the seamount spontaneously frees itself from the plate and jerks forward.
As the seamount jerks forward, the plate jolts in opposite directions. This leads to a new earthquake that has been called a "hang-up earthquake" by the researchers in the study "Do Subducted Seamounts Act as Weak Asperities?"
Lee notes that these hang-up quakes could have led to tsunamis in the past. The researchers note that the subducted seamount's rupture offers the most possible source of massive tsunami earthquakes. He adds that the seamount is a prime example of a subducting seamount that leads to massive friction.
Whether the Daiichi-Kashima volcano will trigger an earthquake soon remains unclear. Choi notes that magnitude 7 quakes have been quite regularly occurring. When it comes to other seamounts on the east coast of Japan, Choi notes that it will take at least a million years for them to arrive at the subduction zone and another 2 million years for earthquakes to get triggered.
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