Liquefaction in Sapporo caused by poor soil conditions

The September 2018 Hokkaido Iburi Earthquake resulted in severe liquefaction in part of Sapporo’s Kiyota Ward, leaving roads caved in and homes leaning at dangerous angles.As at October 3, as many as 1,452 homes had suffered damage in Kiyota Ward. This wasn’t the first instance for the neighborhood with liquefaction observed during the 2003 Hokkaido Earthquake which had an epicenter 300 kilometers away. The liquefaction hazard map issued by the city had previously designated this area as being of high risk for liquefaction damage.

Looking into the history of the neighborhood gives some indication as to why it is prone to liquefaction. The name Kiyota essentially means a beautiful water-filled paddy field. Aerial photos from over 40 years ago, before it was developed into a residential neighborhood, showed rice paddies. The Satozuka district, where much of the liquefaction occurred, was created over a former river with land built up from earth and sand excavated from the surroundings. Of the 539 homes in the district, 85 are currently at risk of collapse while another 88 are in need of supervision. Most of the damage was centered around a few blocks that were built over the old river bed.

Soil liquefaction can typically occur in low-lying areas or those with a high water table and with loose or sandy soil conditions. Vibrations from an earthquake push water to the surface, resulting in buildings and roads losing their support below. The sometimes drastic shift and change to the ground surface can leave structures leaning, cracks in the ground and serious damage to roads and buried infrastructure. Once the land has subsided, it does not return to its original level, leaving the ground uneven.  In areas with poor soil conditions, risks can be mitigated with soil compacting.

Most of Tokyo is low-risk, but some of the low-lying areas towards Tokyo Bay and the man-made islands are at high risk of liquefaction. These areas, in particular the entire area east of Shinagawa Station, were once part of Tokyo Bay and have been reclaimed over the years. In the 2011 Tohoku earthquake, some areas along the bay area in Tokyo suffered damage from liquefaction. Cars in the car park at Tokyo Disneyland were trapped in sand, while a nearby school building sunk by 50 cm. The Odaiba and Shinkiba islands in Tokyo’s bayside area saw about 30 cm of liquefied sand in some locations, along with dislodged manholes, cracked pavements and leaning fences and walls.

Homes in Urayasu City, Chiba, suffered from serious liquefaction in 2011 with many owners forced to pay between 1 ~ 4 million Yen in groundwork to repair the damage.

What can you do as a buyer?

  • Check for city-issued liquefaction hazard maps either on the local district’s website or at the local city hall. Some may also have prepared them in English. However, according to the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), only 20% of Japan’s municipalities have drawn up any kind of liquefaction hazard maps. The MLIT has a list of links to hazard maps issued by cities across Japan here: https://disaportal.gsi.go.jp/hazardmap/bousailist/tablelist.html?hazardcode=3 (Japanese language).
  • Check old maps at the library or city office or aerial photos to see if there were any old rivers, lakes or other bodies of water that were later filled in. Subdivisions developed in areas that were once old water-filled rice paddies are also prime culprits for liquefaction.
  • The name of a neighborhood can also be a sign of its past or geographical features. Place names that include the characters for river (川), valley (谷), field (田) or marsh (沼) can be giveaways.
  • Do a soil test before you build. This is usually a requirement in any case. While you may be able to take steps to mitigate the liquefaction risk within your property, there isn’t anything you can do for the surrounding homes, streets and infrastructure. If there is wide-scale liquefaction in the neighborhood, there may be little you can do to your land to avoid it. Buying an older, existing home may also carry some risk.

Sources:
The Yomiuri Shimbun, October 5, 2018.
FNN, September 7, 2018.
The Asahi Shimbun, September 14, 2018.