8. Possibility of Four Metre Deep Flooding in Densely Populated Low-Land Area of Tokyo Induced by a Mega-Earthquake

In this chapter, the authors describe the historical evidence of how the ground conditions in Tokyo have changed over the past 150 years and those changes would influence the area if a mega-earthquake hits Tokyo in the near future. In the recent 150 years, Tokyo was developed as a highly industrialized city protected from flooding by dykes, flood-gates and reclaimed land which have been built in progressive manners. Water needed in industrialized low-land area of Tokyo was partly obtained by deep-well pumping of the groundwater during a period of 100 years from 1873 to 1973. The excessive groundwater withdrawal resulted in lowered groundwater level down to a maximum depth of about 60 m below sea water level, which led to serious land subsidence in the low-land area particularly during a period of 1960–1973 when the withdrawal was extremely heavy. The latest big earthquake struck Tokyo area in 1923 when the altitude of Tokyo low-land area was still higher than the sea water level. Since then Tokyo low-land area has experienced the following three major changes: (i) land subsidence (max: about 4.5 m) due to the excessive groundwater pumping, (ii) construction of extremely extensive underground networks of lifelines, railways, roads and shopping areas, and (iii) increasing danger of failure of the riverine levees due to the recent increase in the torrential rainfalls typically accompanied by typhoons that strike Japan several times a year because of increasing typhoon-activation power supplied from the sea water 1 or 2° (Celsius) warmer than before. These changes made the low-land area of Tokyo much more susceptible to flooding than the time of 1923. In case that the sea water of Tokyo Bay flows onto the low-land area of Tokyo through possible breakage of some part of the existing seawalls and/or the flood gates, the depth of the water in major part of the area is expected to reach about 4 m at the deepest area and about 1.5 m on average. This may cause unacceptably serious situation in which huge number of people will suffer from the submersion of subways when the sea water flows into the underground facilities. About two million local residents in the low-land area submerged by flood water need to move to higher areas because almost all lifeline systems become out of service in the low-land area. Many of submerged underground facilities will not work anymore even after drying off and therefore will have to be replaced by new ones. About ten million people currently using subways in their daily commuting, business trips etc. every day will have practically no alternative means of transport. In the worst scenario, this will give extremely serious effects on enormous numbers of economic activities not only in Tokyo but also in the entire Japan.