TSUKUBA, IBARAKI PREF. – The surface level of the Pacific coastline in eastern Tohoku has been rising since sinking in the March 2011 giant earthquake, prompting action to lower the height of seawalls in some areas.
The changing surface level reflects continued active crustal movements six years after the magnitude-9.0 quake, which sent a large tsunami across a wide swath of the region.
The Geospatial Information Authority says not all areas have risen since the disaster, and that it is difficult to predict future trends given the complexity of the phenomenon.
According to the government agency based in Tsukuba, Ibaraki Prefecture, satellite observation shows that a spot on the Oshika Peninsula in Miyagi Prefecture has risen over 40 centimeters from the level measured a day after the March 11, 2011, temblor.
The surface sank more than 1 meter and shifted some 5 meters eastward in the quake.
Numerous spots used as height benchmarks for public works projects also showed major upward shifts in elevation in the Pacific coastal region, according to data released by the agency late last month.
Compared with the last data update in October 2011, the benchmarks rose 30 cm and 24 cm in the cities of Ishinomaki and Kesennuma, respectively, both of which are located in Miyagi Prefecture; 17 cm in Kamaishi, Iwate Prefecture; and 14 cm in Shinchi, Fukushima Prefecture.
In line with the changes, the height of seawalls at 89 locations in Miyagi that have yet to be completed will be lowered as much as 30 cm. At the port of Ayukawa on the Oshika Peninsula, work is due to begin in April or later to shave off the top 30 cm of the seawall there after its height was raised in a rebuilding project completed in October 2014.
In contrast to coastal areas on the Pacific side of Tohoku, the region’s Sea of Japan coast has steadily sunk, with the Akita Prefecture city of Yuzawa recording a drop of nearly 30 cm since before the 2011 quake, according to the agency.
“In the long term, there is a possibility that the Pacific coast could start sinking again, but it is difficult to predict that,” said Satoshi Fujiwara, a senior researcher at the Geospatial Information Authority.