How to read a Japanese hazard map, in English
Japan publishes hazard maps for almost every address, but the official ones are in Japanese. This guide explains what they show, how to read the colours, and where they stop being reliable.
The official map, and where it lives
The national hazard maps are on the Hazard Map Portal Site (ハザードマップポータルサイト), run by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) and the Geospatial Information Authority of Japan (GSI). We could not find an English version of it.
It has two tools:
- 重ねるハザードマップ layers flood and internal-water flooding, landslides, storm surge and tsunami risk, road disaster information and land formation onto one map.
- わがまちハザードマップ links to the hazard maps that each municipality publishes under legal requirements.
Readout reads the same GSI flood, tsunami and landslide tiles and labels them in English, so you can check a spot here first and then confirm it on the official sites.
Flood: what the colours mean
The flood layer shows how deep water could get at a spot. The portal opens on the assumed maximum scenario (想定最大規模), and you can switch to the planned scale (計画規模) instead. Readout uses the assumed maximum. The depth colours are:
| Colour | Water depth |
|---|---|
| under 0.5 m | |
| 0.5–3 m | |
| 3–5 m | |
| 5–10 m | |
| 10–20 m | |
| over 20 m |
As a rough guide, count a storey as about 3 m. Water of 0.5 to 3 m floods much of a ground floor, 3 to 5 m reaches the second floor, and more than 5 m can cover two storeys.
Assumed maximum versus planned scale. The assumed maximum is the heaviest rainfall the authorities consider plausible. It is a rare event and a worst case, not a forecast. The planned scale is the smaller rainfall that river improvement works are designed around, so its zones are smaller. Reading both tells you how much worse the rare case is.
"Not coloured" is not the same as "safe"
If the map shows no colour at a spot, the flood layer has no inundation shown there. That can mean the spot really is outside the modelled zones. It can also mean the nearby river is not in the dataset: some rivers managed by prefectures are missing from the national layer, and municipal maps may show more.
Two more limits to know:
- Readout's flood layer is the river-flood map at the assumed maximum. It does not show internal-water flooding, where heavy rain overwhelms the drains, or storm surge. The official portal has both categories.
- Municipal maps differ in style. In Tokyo, for example, you can be shown two flood maps, a 洪水浸水想定区域図 and a 浸水予想区域図. Always read each map's legend.
Landslides (土砂災害)
Landslide maps mark areas where a slope could fail and reach homes below. They cover three kinds of event: debris flows (土石流), failures of steep slopes (急傾斜地の崩壊) and landslides (地すべり).
The zones come in two grades: warning zones (土砂災害警戒区域) and special warning zones (土砂災害特別警戒区域), often called yellow and red zones. Special warning zones carry building restrictions. Readout's panel shows which grade, if any, applies at the spot, and says when a spot sits near a zone instead of inside it.
Tsunami (津波)
Tsunami maps show how deep water could become along the coast in the scenarios that prefectures assume. They matter on low ground near the sea or a river mouth, and mean little inland.
Ground: the part people skip
The 土地の特徴・成り立ち layer of the portal shows how the land was formed. Readout's Ground row uses a similar landform class from J-SHIS, which tells you whether a spot is, for example, a hill, a valley bottom, a former river channel or reclaimed land.
Soft ground shakes harder in an earthquake, and in loose sandy ground with a high water table, liquefaction can occur. The Japan Meteorological Agency notes that this can start from around shindo 5-lower. There is no national liquefaction map, so check the municipal map before you commit. Readout says so on every page.
A quick method for a property
- Check flood at the assumed maximum and note the depth band.
- If the property is on a slope or at the foot of one, check the landslide zones.
- Check the ground type.
- Open the municipal map for internal-water flooding and liquefaction.
- Ask your agent what they must show you. See what your agent must tell you about flood risk.
- Walk the area. Look for slopes, rivers and the low points where water would collect.
Try it on a real spot
Open the map at Yoyogi and click around. To compare towns, browse Koto, on Tokyo's low-lying east side, and Setagaya. Each town page lists the flood, tsunami, landslide and earthquake figures read at its centre.
For earthquakes, continue with earthquake intensity and Japan's building codes.
More guides
- Earthquake intensity 6-lower and Japan's building codes
- What your agent must tell you about flood risk in Japan
Sources
Readout repackages public Japanese government data for screening. It is not engineering, insurance or legal advice.