The giant past tsunami presents a bleak picture of fate in the store: Asahi Shimbun

A shocking study suggests that a massive earthquake could trigger a 30-meter-high tsunami across coastal areas of the Pacific Ocean from Hokkaido in the far north to Chiba Prefecture in eastern Japan on deposits in the geological formations caused by such catastrophic events in the distant past.

A recent projection by a government expert panel was based on seismic sources along the Japan Trench to the north from the source area of ​​the 2011 Great East Japan Earthquake and along the Chishima (Corel) Trench, which extends north of it.

Team members looked at the result of a 9.1-magnitude earthquake that occurred along the Japan Trench and a 9.3-magnitude force that struck along the Shishima Trench.

The commission said on April 21 that the previous event could trigger a tsunami of up to 29.7 meters in Miyako, Iwate Prefecture, and the last of up to 27.9 meters in Eremo, Hokkaido.

Depending on the seismic source’s location, some regions are expected at some point in the future to cope with the possibility of a tsunami twice as high as the waves caused by the Great East Japan Earthquake that killed nearly 20,000 people.

“Our estimates are very likely, because they are based on the largest tsunami events that have occurred in the past,” said Kenji Sataki, a seismologist and director of the Seismological Research Institute at the University of Tokyo, who chairs the panel of experts. “We hope it will be used in developing measures to combat the tsunami, with an emphasis on the evacuation of the population.”

The most recent projections are drawn from sediment studies washed ashore in various regions due to the tsunami.

After the tsunami struck the interior and receded, it left behind a geological formation of clay, sand and other materials known as the tsunami deposits that were originally located on the sea floor.

Studies of sediments and geological formations above and below can tell, for example, roughly when a tsunami strikes and how widespread internal floods are.

A wealth of historical earthquake records exist along the Nankai Basin, which is low on the sea floor in southern Japan. By contrast, the seismic and tsunami events that struck Hokkaido and the Tohoku region in the north are not well documented in ancient literature and other materials.

In the latter areas, the vast plains remained intact without digging for agriculture and other purposes, which allowed the studies of the tsunami deposits. Geological formations in these areas also contain volcanic ash from previous volcanic eruptions, making it easy to determine when they occurred.

Studies of tsunami deposits started relatively early in Hokkaido and Miyagi Prefecture. Similar research was conducted in various other fields after the importance of science attracted new attention after the Great East Japan Earthquake.

“I have lost traces of tsunamis in long-inhabited areas, as the Earth has been dug over and over again,” said Kazumi Herakawa, emeritus professor of natural geography at Hokkaido University, who sits on the expert panel. “Tsunami deposits may not have been extensively studied in any other part of the world on this wide geographical scale.”

Research has found that a 17th-century seismic event led to a tsunami of up to 6 km inland in and around Tokachi in Hokkaido. Studies have also shown that the coastal areas of Noda, Iwate Prefecture, have been affected by the tsunami 18 to 20 times during the past 7,000 years.

Experts at the Governmental Committee studied available data on tsunami deposits from thousands of years past in Hokkaido and the Tohoku region. Then they calculated – with the exception of extreme cases – the magnitude and sources of earthquakes that could have left identical quantities of tsunami deposits in the regions concerned. This allowed them to rebuild the heights of the first-rate tsunami in the past.

Herakawa said: “Recent studies of tsunami deposits allow us to identify the behavior of the tsunami.” “Tsunami estimates were available earlier that were based on local deposit data, but our forecast is the first set of forecasts that have used similar data on this wide geographical range, so they are groundbreaking. I hope they will be used in developing disaster management plans.”

However, the challenge remains how to deal with earthquakes occurring along the Japan Trench south of the source area of ​​the Great East Japan Earthquake.

Experts were unable to perform calculations of such seismic events due to the advanced state of development in the southern Tohoku region and Kanto region, near the putative seismic source region. This means that there are few sites available to study tsunami deposits there.

The expert group said that the issue was still a task to be undertaken in the future.

* * *

Examples of previous seismic events that caused the tsunami deposits in Hokkaido and the Tohoku region

The third or fourth century: a huge earthquake

Fourth or fifth century: a huge earthquake

869: Gogan earthquake

Twelfth or thirteenth century: a huge earthquake

1611: The Kiko Sanriku earthquake

17th century: a massive earthquake (possibly identical to the Keicho Sanriku event)

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