About 38 million people, or about 30 percent of the Japanese population, live on weak foundations vulnerable to increased shaking and liquefaction in an earthquake, according to a new study by the National Research Institute for Earth Science and Disaster Prevention.
About 38 million people, or about 30 percent of the Japanese population, live on weak foundations vulnerable to increased shaking and liquefaction in an earthquake, according to a new study by the National Research Institute for Earth Science and Disaster Prevention.
Because land with a weak foundation is prevalent in the major metropolitan areas of Tokyo and Osaka, there is the possibility of serious damage should a huge quake strike those areas.
Weak foundations lead to an increase in shaking during earthquakes and could also be vulnerable to liquefaction of the soil that can lead to the tilting of homes.
The results of the study will be announced at the annual conference of the Japan Association for Earthquake Engineering scheduled for November in Tokyo.
The research team made the calculations by combining the hardness of foundations determined through the manner in which seismic waves are transmitted with population distribution based on the national census.
The researchers used the site amplification factor to determine which foundations were weak and vulnerable to shaking.
Any area with a factor of 1.6 or greater was determined to have a weak foundation.
The analysis by the research institute found that about 22 million people live in areas with an amplification factor of 2 or greater, which means the land is especially vulnerable to shaking. About 17 million people live in areas where the factor is between 1.6 and under 2, meaning the land could shake easily. Another 22 million people live in areas where the factor is between 1.4 and under 1.6, meaning shaking was possible depending on the location.
Although the land where the factor is 1.4 or greater only represents 9 percent of the total area of Japan and land where the factor is 1.6 or greater only accounts for 6 percent of the total area, areas of weak foundation are spread out along the major plain regions of Japan where population density is high.
In areas where population density is 15,000 or more people in an area of one kilometer square, more than half of the residents in such areas live on land with weak foundations.
The research group also utilized a seismic hazard map that combined the probability of earthquakes of trench types and active fault types with how vulnerable land foundation was to shaking. The analysis found that 53 million people, or about 40 percent of the total population, live in areas where there is at least a 26 percent chance within the next 30 years of a quake with an intensity of at least a lower 6 striking.
If the probability of the quake striking was lowered to at least 3 percent, about 100 million people, or about 80 percent of the population, face the possibility of being hit by a quake with an intensity of at least lower 6.