A Disaster to Dwarf 3/11? The Predicted Nankai Quake

Society Disaster

Experts believe there is a 70% to 80% probability of a severe Nankai Trough earthquake within 30 years. More than 70 years have passed since the previous such quake in this region, which sees a major shake every 100 to 150 years.

A so-called Nankai Trough megaquake is predicted to occur, with 70% to 80% likelihood, within the next 30 years. In the worst-case scenario, deaths from such a disaster could exceed 230,000. But what exactly is a Nankai Trough megaquake?

The Nankai Trough is an ocean-floor trench which runs underneath the Pacific Ocean off the coast of Japan from Shizuoka to Kyūshū, where the Philippine Sea Plate sinks below the Eurasian Plate. A “Nankai Trough quake” is a large-scale earthquake with its epicenter at the meeting point of these plates. These quakes, which happen periodically, feature large tsunamis, and a quake of the largest scale could see locations from Shizuoka Prefecture to Miyazaki Prefecture experience shaking as strong as 7 on Japan’s seismic intensity scale, with tsunamis of heights exceeding 10 meters striking a wide area of the Pacific coast from the Kantō region, home to Tokyo, down to Kyūshū.

In the worst case, deaths are forecast to exceed 230,000, which would be more than 10 times greater than the toll from the Great East Japan Earthquake of March 2011.

Predicted maximum seismic intensities from a Nankai Trough megaquake from 7, the strongest shaking on the scale, down to 3 and below.
Nankai Trough megaquake predicted maximum tsunami heights. From left: Miyazaki, Ōita, Ehime, Kōchi, Tokushima, Wakayama, Mie, Aichi, Shizuoka, Kanagawa, and Chiba Prefectures.

Highly Likely in the Decades to Come

In the past, the Nankai Trough has experienced repeated quakes at regular 100- to 150-year intervals. Over 70 years have passed since the most recent shakes, the 1944 Tōnankai earthquake and the 1946 Nankai earthquake. Thus, another quake is considered imminent and, in accordance with the typical frequency, has a 70% to 80% probability of occurring within 30 years.

Another feature of a Nankai Trough quake is that, should a magnitude 8 temblor occur, it could trigger successive quakes across the remainder of the region. In the Ansei Tōkai quake in 1854, another huge quake struck just 32 hours later, while the 1946 Nankai earthquake is thought to have been primed in part by the 1944 Tōnankai earthquake, all resulting in great destruction.

The Japan Meteorological Agency has contingency plans whereby, if a quake measuring 6.8 or higher in magnitude occurs along the Nankai Trough, or if abnormal changes are observed along the plate boundary, it will issue an extraordinary announcement and establish an evaluation committee to assess the likelihood of a subsequent megaquake.

If the evaluation committee determines that the initial quake has triggered a slip along half of the fault where a major follow-up quake is expected—a so-called han-ware event—government disaster guidelines will call for a cautionary response, including evacuation for one week beyond the lifting of tsunami warnings for residents of coastal and riverside areas where it is deemed there would be insufficient time to flee a tsunami caused by a subsequent disaster.

(Originally published in Japanese on FNN’s Prime Online on May 24, 2019. Translated and edited by Nippon.com.)


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