Studies by a project team of the science and technology ministry indicate that the Tokyo area could be hit by an earthquake with a maximum seismic intensity of 7 on the Japanese scale, instead of an upper 6 as previously believed, sources said.
Studies by a project team of the science and technology ministry indicate that the Tokyo area could be hit by an earthquake with a maximum seismic intensity of 7 on the Japanese scale, instead of an upper 6 as previously believed, sources said.The team of scientists, who have been investigating underground structures beneath the Tokyo metropolitan area, discovered that a plate boundary lay about 10 kilometers closer to the ground in some locations than traditionally believed. This means that an earthquake could strike at shallower depths than assumed, the sources said.While it has been thought that shocks of "upper 6" in the seismic intensity scale of the Japan Meteorological Agency could hit some areas if an earthquake struck beneath the northern part of Tokyo Bay, that estimate may have to be revised to 7, the maximum level on the JMA seismic intensity scale.However, the ground motion estimates contain uncertainties, because they are based on a number of assumptions including the extent of the seismic source, the rupture initiation point, locations of major fault motion and ground conditions, the sources added.Members of the project team installed about 300 seismometers in the Tokyo metropolitan area and analyzed their seismic records to investigate detailed underground structures. The analysis revealed that the boundary between the continental plate and the Philippine Sea plate that subducts beneath it, which has been thought to lie at depths of 30-40 km, was about 10 km closer to the surface in some locations.Because of the complicated geological features beneath the Tokyo metropolitan area, scientists have not managed to identify the sources of magnitude-7 earthquakes that struck in the past. The government's Central Disaster Management Council therefore hypothesized 18 seismic sources to forecast potential seismic damage to the Tokyo area.They included a hypothetical seismic source beneath the northern part of Tokyo Bay, which could cause especially large shocks to central parts of Tokyo. Such an earthquake could cause shocks of JMA seismic intensity "upper 6" in some areas, the Central Disaster Management Council had previously said.The council had also said that shocks of seismic intensity 7 could hit some areas if an earthquake struck along the Tachikawa fault in western Tokyo.