According to the census in 2005, there were 2,628,811 residents in Osaka, an increase of 30,037 or 1.2% from 2000. There were 1,280,325 households with approximately 2.1 persons per household. The population density was 11,836 persons per km². The 1923 Great Kantō earthquake caused a mass migration to Osaka between 1920 and 1930, and the city became Japans largest city in 1930 with 2,453,573 people, outnumbering even Tokyo, which had a population of 2,070,913. The population peaked at 3,252,340 in 1940, and had a post-war peak of 3,156,222 in 1965, but continued to decrease since, as the residents moved out to the suburbs.. How many years passed between 1920 to 1930  during the mass migration of the 1923 Great Kanto earthquake?