[3.11 Earthquake: Rebuilding] Major earthquake hazards are all over the islands of Japan

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It will soon mark ten years since the Great East Japan Earthquake. Various areas on the Japanese islands are fraught with large earthquake hazards.

The government periodically makes earthquake predictions, and if we can understand the system and use the information, it can contribute to disaster mitigation.

Megaquake’s influence on the coast of northern Japan was measured in a recent Cabinet Office study

The Earth’s surface is covered with dozens of huge rock panels. Because the mantle beneath it moves high heat into the planetary body, the plates move in all directions as they push and pull together.

The Pacific Plate, which arose from the rise of the mantle from the submarine mountain range off the coast of South America, is slowly moving towards the west. When it reaches Japan, it sinks under the top plate of the Japan Trench off the coast of Tohoku.

Here, a large amount of energy builds up from stress at the junction as the two plates are pushed against each other. When the energy reaches a critical limit, the shift of the junction causes a massive earthquake. This was what triggered the Great East Japan Earthquake of 9.0 magnitude.

Various earthquakes occur along the Pacific Trench. The February 13 earthquake occurred off the coast of Fukushima Prefecture in the interior of a submerged plate. There is also concern about “external earthquakes” occurring outside the ocean trench, which can generate massive tsunamis.

Source: Japanese seismic risk information station operators for various types of earthquakes

Trench earthquakes will occur under similar mechanisms on the Chishima Trench off the eastern coast of Hokkaido, the Sagami Basin (shallow trench) where the Philippine Sea Plate descends under the Kanto region, and the Nankai Trew, which extends from central Japan southwest to the coast of Kyushu. When the epicenter is shallow, tsunamis will occur, and when large earthquake hits M8 ~ 9 level, it will cause heavy damage.

In different conditions, on the side of the earth, when the plate sinks, the compressive forces are generated and the weaker parts of the bedrock frequently shift causing an earthquake. This is considered earthquakes on active faults.

Although earthquakes are of smaller magnitude compared to trench earthquakes, earthquakes on active faults can cause massive damage if they occur directly beneath cities. This was the case with the Great Hanshin earthquake of 1995 and the Kumamoto earthquake in 2016.

The State Seismological Research Committee periodically publishes long-term assessments of magnitude and probability predictions of earthquakes on active trenches and faults. Probabilities are calculated based on previous occurrence periods and the number of years since the last earthquake, and the information is updated annually. Since the plates are moving at nearly constant velocities, the frequency of each earthquake can be estimated at regular intervals.

According to a long-term assessment reviewed after the Great East Japan Earthquake, within the next 30 years, there is an 80% probability of an M8 earthquake in the Chishima Trench off the coast of Nemuro, and at an altitude of up to 40% the probability of a massive M8.8 earthquake or greater .

The probability of an event of the Great East Japan Earthquake occurring in the Japan Trench is nearly zero. However, given that there is a very high 90% chance of an M7 class occurring in areas such as the coast of Miyagi Prefecture, we need to remain alert.

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In the Sagami Basin, there is a 70% high probability of a M7 earthquake caused by plate subduction inducing tremors just below the Tokyo metropolitan area. In addition, there is an imminent 70 ~ 80% chance of M8 ~ 9 occurring in Nankai Basin.

Pay attention even if the likelihood seems low

In comparison, active errors appear less likely. While trench earthquakes have periods of tens to hundreds of years, active earthquakes have periods longer than thousands to tens of thousands of years, making the probability within about 30 years a small value.

The research committee evaluates a probability of 3% or more as a high rate for active fault type events. The Great Hanshin earthquake occurred at a maximum rate of 8%. The Kumamoto earthquake occurred when its probability rate was only 0.9%. Therefore, even if the numbers are low, we must always be on guard. It should also be noted that the probability numbers are uncertain for errors in previously unknown activities that are not well known.

It is said that there are about 2,000 active faults in Japan, of which just over 110 have been evaluated by the research committee. Earthquakes can occur even from unknown faults and faults with unknown probability. Even with the types of trenches, the likelihood rate of the Nansei-Shotō Trench that extends into the Okinawa area is not yet known.

It is important to think about what type of fault lines are in your area, assume the level of damage an earthquake might cause, and use this information to make your routine preparations. Japan’s probabilistic seismic hazard maps, which show the probability of experiencing an M6 or higher seismic tremor for each region, can also be useful material.

University of Tokyo professor Shinichi Sakai (Department of Seismology Watch) stated, “If we can make good use of the information and each of us acts with some awareness, we will become a safer country.”

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(Read the article in Japanese at this link.)

Authors: Maki Matsuda, Yosuke Osanai, Team of Writers, The Sankei Shimbun

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