Earthquake intensity 6-lower and Japan's building codes
Two numbers matter when you judge a home's earthquake risk: how hard the ground may shake, and what the building was designed to withstand. This guide explains both and shows what the 2016 Kumamoto earthquakes revealed.
Shindo is shaking at one spot, not the size of the quake
Magnitude measures the energy of an earthquake. The Japan Meteorological Agency (JMA) intensity scale, 震度 (shindo), measures how hard the ground shakes at a given place. It runs 0, 1, 2, 3, 4, 5-lower, 5-upper, 6-lower, 6-upper and 7.
The JMA stresses that its table describes what is typically seen, not a guarantee. Damage also depends on how long the shaking lasts, how fast it repeats, the state of the building and the ground. Upper floors of tall buildings generally shake harder than the ground, and not every effect listed for a level will occur.
What 6-lower (震度6弱) looks like
These are the JMA's descriptions for shindo 6-lower:
| People | It becomes hard to stand. |
|---|---|
| Indoors | Most furniture that is not fixed moves, and some falls over. Doors may stop opening. |
| Outdoors | Wall tiles and window glass may break and fall. |
| Wooden houses, high resistance | Slight cracks may appear in walls. |
| Wooden houses, low resistance | Wall cracks become more common and large cracks may appear. Roof tiles may fall and buildings may lean. Some collapse. |
| Concrete buildings | Cracks may appear in walls, beams and columns. They become more common in buildings with low resistance. |
| Ground and slopes | Ground cracks may open. Cliff failures and landslides may occur. |
At 6-upper the picture worsens: more low-resistance wooden houses lean or collapse, and in low-resistance concrete buildings the columns of the ground or middle floors may crumble.
The JMA divides buildings into high and low resistance and notes that, roughly, buildings from 1981 and earlier tend to have low resistance and later ones high. It adds that this is a tendency and not a rule, and that a seismic diagnosis (耐震診断) is the way to know a particular building.
What "chance of 6-lower in 30 years" means
Each Readout page shows the probability that shaking at that spot reaches 6-lower or higher at least once in the next 30 years. It comes from J-SHIS, the Japan Seismic Hazard Information Station, which is run by the National Research Institute for Earth Science and Disaster Resilience. It is a statement about shaking, not about damage. A high figure does not mean a building will fail, and a low one does not make a weak building safe.
The two code changes that matter
- June 1981: the new seismic standard (新耐震基準). The aim is that a building does not collapse in a major earthquake, so people can get out. It can still be badly damaged. For wooden houses the ministry says the new standard requires about 1.4 times as much wall as the old one.
- June 2000: the revision for wooden houses. It tightened the rules on foundations, on how frames are joined and on how walls are balanced. The ministry's Kumamoto report singles out the joint specifications.
The date that counts is the building permit, not the year the home was finished. A home completed in 1981 or in 2000 may fall on either side of the line. A concrete apartment is still mainly judged on the 1981 rule.
What happened in Kumamoto in 2016
The April 2016 Kumamoto earthquakes recorded shindo 7 twice in the centre of Mashiki town. A committee of the Ministry of Land, Infrastructure, Transport and Tourism analysed the damage. Its report of 30 September 2016 used a survey by the Architectural Institute of Japan of wooden buildings in the worst-hit part of central Mashiki, 1,955 buildings in all, and sorted them by when they were built:
| Built | Collapsed |
|---|---|
| Before June 1981 (old standard) | 28.2% (214 buildings) |
| June 1981 to May 2000 | 8.7% (76 buildings) |
| June 2000 or later | 2.2% (7 buildings) |
The report credits the gap mainly to wall quantity. Houses rated 耐震等級 3 under the housing performance labelling scheme have about 1.5 times the wall of the 1981 standard, and most of them were undamaged. Among the post-1981 houses that collapsed and could be analysed, 73 of 77 had joints that did not meet the current specification.
Read these numbers with care. They cover wooden houses in one badly shaken area, after two very strong quakes. They are not a prediction for a particular building, and they say little about a concrete apartment in Tokyo.
Apartments
For reinforced-concrete buildings the JMA gives a similar guide: roughly 1982 and later tend to be more resistant, and earlier ones less so, though structure type and wall layout make a big difference. At 6-upper, columns on the ground or middle floors of low-resistance concrete buildings may crumble. If you are buying an apartment from before the mid-1980s, ask about a seismic diagnosis and any strengthening work.
How to check a home
- Ask for the date of the building permit (建築確認), not just the year built.
- For an older building, ask whether there is a seismic diagnosis, and what it found.
- For a wooden house, ask about its seismic grade (耐震等級) and about the joints and ground work if it dates from around 2000.
- Check the ground type. Soft ground shakes harder.
- Fix your own furniture. The JMA describes unfixed furniture moving and falling from 5-lower upward.
On Readout, the Notes tab has a year-built box that shows which code applies. For ground and flood context, see how to read a Japanese hazard map.
More guides
- How to read a Japanese hazard map, in English
- 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.