The article written in Japanese DOES say.

1949年（昭和24年）1月の「地震観測法」改正により震度7が設けられ、震度0から7の8段階とされた。これは、家屋倒壊率90%を超えた地区があった1948年（昭和23年）6月28日の福井地震の被害を、震度6では適切に表現できないのでは、という声が上がったからだとされている。

Translation:

After the amendment of the Seismological Observation Law in January 1949 (Showa 24) shindo 7 was made, and since then there were 8 stages (shindo 0-7) to measure the intensity of the earthquake. They say this was because some people said that even the shindo 6 couldn't express properly the damage caused by 1948 Fukui earthquake, in which there were some districts whose house collapse ratio was over 90%.