How to check a property's ground history before buying — spotting former riverbeds, fill, and cut

Confirming that a property sits outside the projected inundation zone does not, by itself, tell you whether the ground under it is sound. Liquefaction, differential settlement, and embankment slides are governed less by modeled disaster scenarios than by how the land itself was formed.

And ground history is, as a rule, not covered in the material-information disclosure. It is information that does not surface unless the buyer goes looking for it. This guide walks through how to identify former river courses, filled ponds, fill, and cut using only free data published by Japan's Geospatial Information Authority (GSI) and municipalities. It takes about an hour and costs nothing.

Ground data is not on the hazard map

Hazard maps show the modeled outcome of scenarios where water arrives from outside — river overflow, tsunami, storm surge. Ground-driven damage, by contrast, happens with nothing arriving at all.

• Liquefaction: sandy ground becomes water-saturated during an earthquake, and buildings settle or tilt
• Differential settlement: ground stiffness varies across the site, so the building sinks unevenly — and this progresses over years without any earthquake
• Embankment slide: fill placed over a valley or slope moves like a landslide during a major earthquake

Any of these can cost a sum comparable to the price of the building itself to remedy, and the economic damage frequently exceeds that of above-floor flooding. Yet nobody surfaces this information for you automatically.

The four GSI map layers to use

Everything happens on one site: GSI's map portal (maps.gsi.go.jp). It is free, requires no registration, and supports these four overlays.

The landform classification is served as vector tiles, so clicking anywhere on the map pops up that point's class name, description, and the natural-disaster risks associated with it. You never have to decode a legend, which makes it usable without specialist knowledge. Coverage keeps expanding but is not yet nationwide — where it is missing, fall back to historic aerial photography.

Telling fill from cut

In housing developments carved out of hill terrain, cut lots and fill lots sit side by side within the same subdivision. The artificial landform layer distinguishes them:

• Cut land: flat or sloped ground created by excavating mountain or plateau terrain
• Fill and reclaimed land: flat or sloped ground created by adding soil onto mountain, plateau, lowland, or water areas

Broadly, cut sides retain the original in-situ ground and are stable, while fill sides depend on the quality of compaction. The condition that deserves the most attention is a lot that straddles the cut–fill boundary: ground stiffness changes abruptly within a single site, which is exactly the setup for differential settlement.

The era of grading matters too. Regulation has tightened in stages, and the current framework is the Act on Regulation of Residential Land Development and Specified Embankments (commonly the "Embankment Regulation Law"), effective May 26, 2023, under which Residential Land Development Regulated Areas and Specific Embankment Regulated Areas are designated. The safe assumption is that the older the development, the looser the standards it was built to.

Checking the large-scale fill map

Separate from landform classification, municipalities publish a large-scale fill map identifying major embanked development sites. Nationwide publication was completed on March 30, 2020. Searching the municipality name together with the map name will find it.

Two types are covered:

• Valley-fill type: fill placed into a valley or ravine, where the filled area is 3,000 square meters or more
• Slope-fill type: fill added onto a slope, where the pre-fill ground angle is 20 degrees or more and the fill height is 5m or more

One point is easy to misread, so let us be explicit: this map does not mark sites that are dangerous right now. It identifies fill whose safety should be verified, based on how such sites actually behaved during past major earthquakes. A match is not a reason to walk away — it is a signal to additionally check the retaining walls, the grading year, and whether remedial work has since been done.

Finding former river courses and ponds

On flat terrain, the highest-value check is for former river courses. Old channels hold loosely deposited sand and gravel, sit slightly lower than their surroundings, and drain poorly. They add risk on both fronts: liquefaction during earthquakes and settlement in ordinary times.

Both the natural landform classification and the flood landform classification map show former river course classes. Overlaying historic aerial photography on top makes filled ponds and pre-consolidation canals visually obvious. 1960s imagery is especially useful, since it often predates the full wave of high-growth-era housing development.

A reliable visual tip: look for a single meandering street among otherwise straight blocks. When land consolidation culverted a waterway, the old channel frequently survives in the shape of the road.

Reading landform classes — lowland is not uniform

"Lowland means dangerous" is too coarse to be accurate. Within the same lowland, risk splits sharply by landform class, and the flood landform classification map lets you read that breakdown.

• Natural levee: a slight rise built up from sand deposited alongside a channel through repeated flooding. Higher than the surrounding lowland with comparatively good drainage — the better end of lowland conditions
• Back marsh: the wet lowland behind a natural levee. Water does not drain, and soft clay layers run thick. Prone to both flooding and settlement
• Former river course: an old channel. Sandy and loose, hence liquefaction-prone
• Flood plain: the general term for flat land formed by river flooding, mixing all the microtopography above

What matters is that these classes switch over distances of a few dozen meters. Within the same neighborhood and block number, one street can separate a natural levee from a back marsh. Do not judge by neighborhood name — click the actual parcel location.

Signs you can read on site

Once the map gives you a hypothesis, confirm it during the viewing. The key insight is that ground behavior shows up in exterior structures and roads before it shows up in the building.

It is worth looking at several neighboring properties together. A single tilted structure could be poor workmanship — but several leaning the same way points to the ground.

What this method cannot tell you

Maps and site observation get you as far as how the land was formed. The following require other means:

• Depth to bearing stratum and load capacity — requires a ground survey (such as a screw weight sounding test). For new or near-new construction, ask the seller to disclose the ground survey report
• Whether ground improvement or piling was done — check building confirmation filings or the contractor's records
• Compaction quality and grading year of the fill — development permit records may still be held by the municipality
• Groundwater level — directly governs whether liquefaction occurs, and cannot be read from public maps

Put differently, this walkthrough is a free screening step for deciding whether a ground survey report is worth chasing. A property that lands on a former river course or valley-fill embankment is a high priority for spending money on further investigation.

Note that ground history falls outside the material-information disclosure. For what is and is not disclosed, see "The disaster checklist for Japan's material-information disclosure". For the hazard-map side of the analysis, see "How to read Japanese hazard maps — 5 practical steps".

SafeLand's assessment automatically characterizes terrain from GSI elevation data and returns a per-address Grade A to D alongside hazard, vacancy, population, and convenience indicators. It is a useful way to get your bearings before reading the landform classification yourself.