February 28, 2017 3:10 pm JST

Unexpected phenomenon arising from 2011 quake

Parts of coastal northeastern Japan seem to be getting a push

TOKYO -- Land sunken by the magnitude 9.0 earthquake that devastated Japan's northeast region in 2011 is mysteriously rising.

The phenomenon is taking place along certain areas of Japan's Pacific coast, prompting experts to step up studies to unravel the mystery.

Conventional seismology fails to explain the land rise, leaving experts to speculate that the mantle, a layer between the crust and outer core of the Earth, is moving. According to this theory, the quake altered the pressure on the mantle, which is now slowly pushing up the ground.

When it comes right down to it, though, ground motions following a major earthquake leave many seismologists perplexed.

A sign of the rising seabed can be seen at a fishery port on the Oshika Peninsula, in the northeastern part of Miyagi Prefecture. The pier there is gradually getting higher.

The 2011 quake sank the peninsula by around 1 meter, according to the Geospatial Information Authority of Japan. As the fishery port thus fell below sea level and became inundated, a new pier was built.

But the ground level of the port has since risen by up to 40cm, and the pier has partially heaved so high that it has become difficult to embark onto or disembark from boats. Landing catches is more of a chore, too, local fishermen said.

Port officials are considering lowering the pier.

Before the quake, the land plate on which Tohoku stands was being dragged toward the oceanic plate, and the Pacific coast of the region was gradually sinking.

The land plate was expected to be heaved up in case of a big quake. But it actually subsided on March 11, 2011. Seismologists theorized that the subsidence occurred as the edge of the plate took a really big slide, leaving behind a deformed surface.

The new rising trend was initially thought to be "after effect slides." These occur at a plate's edge and are not accompanied by quakes people can feel. But as the land bulges have continued, they can no longer be explained by the after effect theory alone.

Seismologists pay attention to the mantle. The nucleus of the Earth is like the yolk of a boiled egg, while the mantle is the white. The eggshell is the planet's crust.

The upper part of the mantle and the crust are called a plate -- its surface is the ground.

Made of stones, the mantle slowly moves (think molasses). This is related to the movements of plates and continents. There is a phenomenon called "viscoelastic relaxation" in which the mantle moves in the direction of a continental plate's slide following an upheaval.

According to experts, the mantle under the Oshika Peninsula and other parts of Japan's northeastern Tohoku region is "viscoelasticating," pushing up the ground.

This is happening "earlier than expected," said Ryota Hino, a professor at Tohoku University's International Research Institute of Disaster Science.

Hino leads a group of researchers that has confirmed a close relationship between viscoelastic relaxation and motions in the ground after the great shaking of 2011 through a combination of seafloor observations and simulations. The effect of the mantle's inflow, Hino said, "is expected to last for a long time."

Monitoring mantle movements is important in trying to predict seismic activity. If a plate's edge finds a firm notch and can no longer slide, then seismologists say, an earthquake is waiting to happen.

But "it is difficult to grasp moves in the plate because they are concealed under the effect of viscoelastic relaxation," said Teruyuki Kato, professor at the Earthquake Research Institute of the University of Tokyo.

Earthquakes with magnitudes in the 7 range have occurred off Miyagi Prefecture every 30 to 40 years. It is unknown whether the cycle will change because of the 2011 quake.

Ryoko Nakata, a project research scientist at the Japan Agency for Marine-Earth Science and Technology, has run simulations on a supercomputer and says the next earthquake off Miyagi is highly likely to occur within less than half of the cycle.

Although Nakata stresses that this is "no more than a simulation," she also warns that observations need to be stepped up in areas around the plate.

(Nikkei)

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