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Tokyo map missing 35 areas of infill vulnerable to liquefaction in quake

Cracks are seen in a road in the Oyamadai district of Tokyo's Setagaya Ward in this Oct. 9, 2019 photo. (Mainichi/Tetsuro Hatakeyama)

TOKYO -- A Tokyo Metropolitan Government map leaves out 35 parts of the city totaling some 661,000 square meters built on infill, which is susceptible to liquefaction, landslides and other damage during a major earthquake, the Mainichi Shimbun has found through a freedom of information request.

During Japan's rapid urbanization before World War II, and in the postwar growth boom years, tens of thousands of hilly areas across the country were leveled and valleys were filled in with dirt to open them up to development. While the practice helped feed voracious demand for housing, the older fill gets, the more susceptible it is to landslides and liquefaction during a large temblor. Because of this, the land ministry in 2006 asked local governments to publish maps indicating these patches of land.

Metro Tokyo began its survey in 2007, and mapped 1,491 areas of infill across its jurisdiction including 108 in the city's central 23 wards. The 35 areas missing from the Metro Tokyo guide all date back before World War II. While the Tokyo government is considering tapping a specialist committee for its opinion on the matter before weighing up whether to release results, one expert has called the failure to reveal the "hidden infill" to residents "a real problem."

There are 10 unlisted infill areas in Setagaya Ward, six in Shibuya Ward, five in Nerima Ward, three each in Nakano and Suginami wards, two each in Shinagawa and Ota wards, and one in each in Minato, Meguro, Kita, and Itabashi wards. The sites range from about 3,000 to 75,900 square meters each. Some extend beneath residential areas, including parts of the upscale Denenchofu neighborhood of Ota Ward and the fashionable Ebisu district of Shibuya Ward.

Mainichi reporting has revealed that the 35 areas were not only left off the infill map, but that they were not even subjected to on-site surveys.

Explaining the omissions, the head of the metro government's land adjustment section told the Mainichi, "The contour lines on prewar topography maps were difficult to interpret, apparently making it impossible to judge the depth of the infill." The official in charge of the survey stated, "Right when we were dealing with the lack of topographic maps for areas outside Tokyo's wards, we found U.S. military aerial photographs for the whole region taken in around 1945. That's how we could identify infill areas outside the 23 wards."

However, Kyoto University professor Toshitaka Kamai, who has studied the infill in metro Tokyo's 23 wards, said that failing to reveal the areas missing from the maps sends out an erroneous message to residents. "The authorities should release the information forthwith," he said.

One of the 35 "hidden infill" spots is a Setagaya Ward neighborhood about 500 meters from Oyamadai Station on the Tokyu Oimachi Line. The area's stylish homes look like they've been glued carefully to the gently sloping landscape.

"I heard from my parents that water used to run through this area, which was a valley. I also knew that it was filled in," said an 88-year-old woman who has lived in the Oyamadai area since 1934.

The village of Tamagawa, which makes up part of this corner of present-day Oyamadai, was slated for residential development in 1924. Construction began in 1928 and was completed in 1931. The shape of the neighborhood has remained more or less the same ever since.

A 1926 map in Setagaya Ward's archives shows there was indeed a natural spring and a watercourse running through the district. Another historical map confirmed that the area was once a valley.

"This establishes without a doubt that the area was filled in," said Kamai.

The Kyoto University professor has a home in neighboring Meguro Ward, and his investigation of the infill issue was triggered in 2014 when he noticed that metro Tokyo's infill map showed just five such spots in all of Setagaya Ward -- a suspiciously low figure. Using historical maps and geological survey data, Kamai produced his own infill map to present at research conferences and other venues. The Oyamadai neighborhood was one of the areas missing from the metro government map but was included in Kamai's version.

A quick stroll around Oyamadai reveals numerous signs parts of the land are sinking. Cracks riddle the streets and even the walls of people's homes.

"The cracks in the roads have certain characteristics that suggest the shape of the ground is changing, if only a little," said Kamai. "There's a danger of houses tilting or even collapsing here if a big earthquake hits."

When told about the possible infill beneath her feet, one woman in her 70s who moved to the area with her husband six years ago told the Mainichi, "The real estate agent didn't say anything about that. If I'd known that, I might not have chosen to live here."

Meanwhile, few local governments have made the effort to identify prewar infill areas. The land ministry has directed municipalities to create infill maps based on the oldest information possible. However, without a specific timeframe to refer to, local officials have had to decide on their own how far to go back.

For example, the city of Osaka in western Japan has stated that there are no infill areas within its jurisdiction. Districts developed by 16th century warlord Toyotomi Hideyoshi have since been transformed into residential neighborhoods, but this "conforms to the natural topography, and there is thus unlikely to be much fill," officials said. The city thus only referred to data from 1960 to 1961 in its analysis.

On the other end of the spectrum, Yokohama searched back to 1922, as far as its records could take it with any accuracy, and mapped 3,271 spots that had been filled. The official in charge of surveying residential neighborhoods of the city south of Tokyo told the Mainichi, "There were old areas of infill in Yokohama, which developed as a port town from before the start of the Meiji period (in 1868). Even if including the (older) data might reduce the accuracy of the map somewhat, we decided to keep them in."

(Japanese original by Tetsuro Hatakeyama and Takashi Miyazaki, Special Reports Department)

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