Emperor and Empress visit Minami Sanriku after the Great East Japan Earthquake in March 11, 2011.
Japan is stepping up disaster prevention initiatives after government scientists recently made a forecast that a Magnitude 9 earthquake, which carries a risk of 20-plus-meter-high tsunami, is highly probable in the Kuril Trench.
The government’s Earthquake Research Committee announced last December 19th the results of its long-term evaluation of the Kuril Trench, which stretches along the Pacific side of Hokkaido.
In the Kuril Trench, the Pacific Ocean plate is being subducted under the land-side plate and large Magnitude 8-level earthquakes occur repeatedly along this boundary section. After the Great East Japan Earthquake of 2011, the Earthquake Research Committee reevaluated seismic activity from the Tokachi Coast in East Hokkaido to Shikotan and Iturup Islands in the Northern Territories for the first time in 13 years.
From results of the investigation of coastal tsunami deposits, earthquakes of the largest scale were judged to reach a magnitude of around 8.8. Since the average recurrence interval is 340-380 years, and 400 years have already passed since the last one, recurrence is “highly imminent,” with a maximum 40% chance of one occurring in the next 30 years.
Committee Chairman Naoshi Hirata warned: “There is an extremely high probability. The chances of an earthquake like the East Japan Earthquake occurring on the Kuril Trench are high.”
The epicenter region is more than 300 kilometers from the Tokachi coast to Nemuro. According to previous predictions, the largest earthquake occurring in this vicinity was Magnitude 8.3.
East Hokkaido was hit by the tsunami after Magnitude 8.2 Tokachi coast earthquake in March 1952.
Compared to the Nankai Trough earthquakes occurring in western Japan, there is a paucity of East Hokkaido historical records. As such, drilling samples from coastal swamps and wetlands were used to investigate sand displaced by past tsunami. Maximum size and recurrence intervals have been estimated, and analysis indicates the region has been ravaged by 18 tsunami in the past 6,500 years, with one 17th-century tsunami surging to a maximum of 4 kilometers inland.
Moreover, the maximum size of the Tokachi coast earthquakes has been raised from Magnitude 8.1 to M8.6, with the chance of recurrence in the next 30 years evaluated as 7%.
Likewise, the maximum size of Nemuro coast earthquakes was raised from Magnitude 7.9 to M8.5, with approximately 70% recurrence probability.
The Cabinet Office is expected to collate megathrust earthquake-related tsunami height and arrival time by the end of March 2018. Large tsunami are thought to have a wider impact, so it appears new disaster prevention procedures will be needed not only in Hokkaido, but also in the Tohoku region.
Having reflected on the unexpected scale of the East Japan megathrust earthquake, the Earthquake Research Committee recommended that earthquake-related predictions be reassessed nationwide, and in 2013 the Nankai Trough megathrust earthquake was predicted to peak at Magnitude 9.1. There are plans to investigate the Japan Trench in the future.
24-meter-high Tsunami 400 Years Ago
The long-term evaluation released by the Earthquake Research Committee on December 19th did not reveal tsunami height for the extreme megathrust earthquake threatening Eastern Hokkaido. However, Hokkaido University earthquake researchers have calculated that, “approximately 400 years ago the largest tsunami was 24 meters in height, surging over East Hokkaido.”
In East Hokkaido, soil carried by the tsunami has been found some four kilometers inland. In the coastal town of Taiki, tsunami-displaced soil has even been found in a 18-meter high cliff.
Yuichiro Tanioka, professor of seismology at Hokkaido University, and his colleagues reverse-calculated the tsunami in order to explain the distribution of tsunami-displaced soil. It was found that a Magnitude 8.8 earthquake caused by a 25-meter shift in a fault located 300 kilometers offshore fit the data perfectly. It was an extreme megathrust earthquake occurring at the plate boundary in the Kuril Trench.
The tsunami is estimated to have reached 24 meters above sea level at Lake Oikamanai, in Taiki, and even exceeded 15 meters high in Kushiro City.