Municipal hazard maps

Municipal hazard maps are one of the first documents a buyer or developer should pull when researching Japanese property. Unlike a zoning map, which tells you what you are allowed to build, a hazard map tells you what the ground and water may do under extreme conditions. For anyone working through 不動産 (real estate) due diligence, 登記 (registry) checks, or 再開発 (redevelopment) planning, the map is not a single official document but a set of municipal layers with different update dates, legal limits, and evidentiary weight. Japanese hazard maps are legally bifurcated screening tools where the local-government version carries regulatory weight for land-use restrictions, while the MLIT national layers provide only planning-level context, and neither set replaces a site-specific geotechnical investigation.

What a municipal hazard map is

A municipal hazard map (ハザードマップ) is a local-government publication showing the areas that may be affected by natural hazards including flooding, landslides, tsunamis, and soil liquefaction. Under the Act on Promotion of Sediment Disaster Countermeasures for Sediment Disaster Prone Areas (土砂災害防止法) and the Flood Control Law, Japanese municipalities publish these maps to inform residents and property buyers of risk zones. The maps are not a single countrywide dataset; each municipality prepares its own hazard layers, often using the National Land Numerical Information (国土数値情報) published by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) as a base, but the local version is the legally relevant one.

A hazard map is a planning document, not a guarantee of safety. It shows the modelled extent of a hazard under specific assumptions. The map does not tell you whether a particular building is safe, whether a lease can be terminated, or whether a parcel can be rebuilt after a disaster. Those questions require the land register, building confirmation records, and the relevant municipal planning department.

What the municipal hazard map actually contains

Municipal hazard maps in Japan are issued by the local government (市区町村), usually the city or ward planning department. They are published under the Flood Control Act and the Sediment Disaster Prevention Act, as amended, and are updated when new flood, landslide, or tsunami modelling is released. The map is a set of thematic layers, each with its own source, update cycle, and legal meaning.

Flood hazard layer

The flood hazard layer (洪水浸水想定区域) shows the expected inundation area and depth for a designated rainfall event. The event is set by the river administrator (usually MLIT or the prefecture) under the Flood Control Act. The map shows the maximum expected depth, not the water level at a specific time. Check the “planning basis” (計画規模) in the map legend. A 1-in-30-year rainfall event differs from a 1-in-100-year event. The same parcel can fall inside the flood zone under one event and outside it under another. The map does not predict the timing or sequence of flooding; it shows a static water depth for a single modelled event.

Flood hazard zones are prepared by river administrators for each river under the Flood Control Act. The maps show assumed inundation depths for two rainfall events: the current level and the planned level. The current level is based on the largest rainfall observed in the past; the planned level assumes a much larger, once-in-a-hundred-to-thousand-year rainfall. Both events are modelled, not measured. The maps do not predict a specific flood date, and they do not show flooding from storm surge, sewer back-up, or small-stream overflow unless the map explicitly says so. A property can sit inside the mapped zone but still be dry in a minor event; a property outside the zone can still flood from local drainage failure. The official source for these layers is the local government’s Flood Hazard Map (洪水ハザードマップ), published under the Flood Control Act (水防法), and the MLIT water-management portal. Update cycles are not fixed by countrywide law; municipalities are expected to revise the maps after major flood events or when levees or land use change. The map date is printed on the PDF or web-GIS layer; always cite it in a due-diligence memo. For site-specific follow-up, request a 建築確認申請 (building confirmation application) record from the municipal building department, and ask whether the parcel sits inside a designated 都市計画区域 (urbanisation promotion area) or a 市街地再開発促進区域 (redevelopment promotion area). MLIT’s National Land Numerical Information (国土数値情報) provides the underlying digital layers for flood, landslide, and tsunami hazard zones, but the public map is a screening tool, not a site-specific engineering determination.

Landslide hazard zones

Landslide hazard zones (土砂災害警戒区域) are designated under the Sediment Disaster Prevention Act (土砂災害防止法) and are mapped by prefectures, not by the countrywide government. The map shows two tiers: 警戒区域 (warning zones) and 特別警戒区域 (special warning zones). The special designation applies where a landslide could cause severe damage to buildings or human life. Designation depends on slope angle, geology, and historical landslide records; a parcel outside the mapped zone can still be at risk if the slope above it is unstable. The map does not predict timing or magnitude; it only marks areas where the probability of a landslide is high enough to justify land-use restriction. MLIT publishes landslide hazard maps as part of the National Land Numerical Information (国土数値情報) dataset, updated irregularly as local governments revise their hazard assessments.

Tsunami hazard map

The tsunami hazard map (津波ハザードマップ) shows predicted inundation depth, arrival time, and evacuation routes for a designated event. The source is the municipal hazard map, and sometimes the prefectural or countrywide tsunami inundation assessment. The scale is usually 1:2,500 to 1:10,000 for municipal planning. Update cadence occurs after major seismic or tsunami events and after revised countrywide tsunami source models; there is no fixed cycle. The map shows modelled events, not a maximum possible event; a larger earthquake or a different fault rupture can produce inundation beyond the mapped zone. For site-specific follow-up, compare the municipal event with the prefectural and countrywide tsunami hazard assessments; check the evacuation building designation and the local tsunami evacuation plan. MLIT published the National Land Numerical Information hazard layers in 2023; the layers are GIS files, not parcel-level certificates.

Liquefaction

Liquefaction is the loss of soil strength caused by saturated, loose sand temporarily behaving like a liquid during strong shaking. Liquefaction zones are mapped by local governments, not by the countrywide government; each municipality publishes its own hazard map with different update cycles. Liquefaction maps are screening tools, not parcel-level proof; they indicate susceptibility, not the actual behaviour of a specific site during a given earthquake. A site-specific geotechnical investigation is required before construction.

How to use the map in due diligence

Start by downloading the most recent hazard map from the municipal website or the city planning counter. Note the publication date on each layer. A flood layer from 2018 may not reflect a levee rebuilt in 2021. A landslide layer from 2015 may miss a slope failure that occurred the following year. If the map is a web-GIS service, take a dated screenshot of the parcel and the legend.

Next, overlay the parcel’s registry boundaries on the hazard layers. The 登記 (registry) shows the official lot lines; the hazard map shows risk zones at a planning scale, usually 1:2,500. A parcel that appears outside a flood zone at that scale may still sit in a local depression that collects water. A parcel marked inside a landslide warning zone may include a stable section of flat ground at the toe of the slope. The map alone cannot resolve that ambiguity.

Then check the planning basis in the legend for each layer. A flood map based on a 1-in-30-year rainfall event tells you about frequent, moderate flooding. A map based on a planned-level event tells you about rare, catastrophic flooding. A parcel outside both zones carries a different risk profile from a parcel inside the planned-level zone but outside the current-level zone. Record the planning basis in your due-diligence memo so a future reader understands what the map does and does not show.

Finally, contact the municipal planning department and the river or prefectural engineering office that produced the layer. Ask whether a more detailed hazard assessment exists for the parcel, whether any disaster recovery or land-use restriction has been applied to the area, and whether the parcel falls inside a 市街地再開発促進区域 (redevelopment promotion area) that may affect rebuilding rights. The hazard map is the starting point for those conversations, not the conclusion.

What the map cannot do

A hazard map does not certify that a building is safe. It does not replace a geotechnical survey, a structural assessment, or a flood-level certificate from the river administrator. It does not predict when a disaster will occur. It does not guarantee that a lender will approve a loan or that an insurer will write a policy. It does not tell you whether a lease can be terminated under Japanese law or whether a seller has a duty to disclose the hazard designation. Those questions sit outside the map’s scope and require separate legal and engineering advice.

Where to get the maps

Municipal hazard maps are available at the city or ward planning department counter and on the municipal website. Many municipalities publish the maps as PDF files or web-GIS viewers. MLIT’s National Land Numerical Information download service provides the digital hazard layers for flood, landslide, and tsunami zones as GIS shapefiles, but those layers are planning-level data and do not carry the same legal weight as the local-government publication. Prefectural governments sometimes publish consolidated hazard map portals that cover multiple municipalities; check the prefecture’s disaster prevention bureau website. The Geospatial Information Authority of Japan (国土地理院) hazard map portal aggregates links to municipal maps but does not replace the local source. Always confirm that the map you are using is the version currently published by the municipality with jurisdiction over the parcel.