1950: The introduction of the Building Standards Act and what is now known as “kyu-taishin”.
The change was brought about after the 1948 Fukui earthquake which had a magnitude of 7.1. Earthquake resistant design was enforced nationwide and no longer limited to just town areas. A certain amount of earthquake load bearing walls were required. Extra framework was required for wooden structures.
A building is designed to withstand and not collapse in an earthquake of magnitude 5~7 and with a seismic intensity scale (Shindo-scale) of upper 5, although some damage may be sustained.
However, no standards were enforced in the event of a larger earthquake that produced a seismic intensity of 6 or higher.
- See more at: http://japanpropertycentral.com/real-estate-faq/earthquake-building-codes-in-japan/#sthash.4XjqDL19.dpuf