Tokyo earthquake risk: what the numbers actually mean

Everyone knows Tokyo has earthquakes. The useful questions are more specific: how strong, how likely, and — most overlooked — how does the ground under your particular address respond. Here is how to read Japan's official numbers without reading Japanese.

Sources: J-SHIS (NIED, probabilistic seismic hazard maps, 2024 edition), Tokyo Metropolitan Government community risk survey. Reference information, not advice.

Shindo: Japan measures shaking, not magnitude

News reports use magnitude, but Japanese hazard data uses shindo (震度) — the intensity of shaking at a specific place, on a scale from 0 to 7. The same earthquake produces different shindo in different neighborhoods. The key threshold to remember is shindo 6-lower (6弱): shaking that makes it hard to stay standing, where unsecured furniture can topple and older buildings can be seriously damaged. That is the level Japan's official probability maps are usually quoted at.

What "80% in 30 years" actually means

Japan's official hazard model (J-SHIS, by the national research institute NIED) publishes, for every ~250m grid square, the probability of experiencing each shindo level within the next 30 years. As a verified example from our own pipeline: a point near Tokyo Skytree (Oshiage, Sumida ward) shows 83.2% for shindo 6-lower or stronger — with 99.7% for 5-upper and 31.5% for 6-upper.

The honest translation: over the life of a 35-year mortgage, plan as if it happens. This is not a prediction of one specific event — it is the aggregate of many possible earthquakes. The practical conclusion is the same either way: the question is not "will strong shaking come" but "how will my building and my ground behave when it does".

The variable people miss: the ground itself

Two addresses a kilometer apart can shake very differently. Soft ground — river deltas, former wetlands, reclaimed land — amplifies shaking; firm upland damps it. J-SHIS classifies the landform under every grid square and estimates this amplification. Our Oshiage example sits on reclaimed lowland (polder) with a low shear-wave velocity (AVS30 ≈ 150m/s) — soft ground that amplifies shaking and is also prone to liquefaction. Western Tokyo's Musashino upland is generally firmer. This is why address-level checking beats city-level reputation.

Tokyo's extra layer: block-by-block risk ranks

For Tokyo specifically, the metropolitan government publishes its own survey ranking all 5,192 town blocks (1–5 scale) for building-collapse risk and fire risk — finer than the national 250m mesh, based on actual building stock and ground conditions. It is published only in Japanese; our paid report overlays it automatically for Tokyo addresses.

One insurance note

In Japan, standard fire insurance does not cover earthquake damage — including fire caused by an earthquake and liquefaction damage. Coverage requires a separate earthquake insurance rider. If you are buying on soft ground, this is usually the single most consequential paperwork decision.

Check your address (free)

mamoie's free checker shows the 30-year shindo probabilities and landform class for any Japanese address, alongside five other hazards. Developers can get English JSON from our free API with ?lang=en.

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