2007 Volume 116 Issue 3-4 Pages 313-324
Earthquakes can be classified into three groups geophysically : (a) Interplate earthquakes, which occur at the boundary between two plates ; (b) intraslab earthquakes, which occur within the subducting plate ; and, (c) intraplate earthquakes, which occur within the overriding plate. Directly beneath earthquakes are defined as earthquakes that occur just beneath people or artificial structures, causing casualties and damage. All three types of earthquake can be directly beneath earthquakes if they occur beneath or close to a land area. Earthquakes occurring beneath the Tokyo Metropolitan Area are those within : (1) the Okhotsk plate (type c); (2) at the interface between the Philippine Sea and Okhotsk plates (type a); (3) within the Philippine Sea slab (type b); (4) at the interface between the Philippine Sea and Pacific slabs (type a); and, (5) within the Pacific slab (type b). Although all of these types of event could be directly beneath earthquakes, damage caused by (1) or (2) seems to be larger than that caused by the others. There does not seem to be a consensus yet on rating the probabilities of the future occurrence of these events. It is important to conduct future studies on what types of earthquake pose a future threat beneath the Tokyo Metropolitan Area.
An earthquake as a geophysical phenomena is not a disaster. When an earthquake causes various human casualties, it is called a disaster. If a huge scale of damage is caused by an intermediate scale of earthquake, even if the earthquake is not so severe the result might be a largescale disaster. When the next earthquake of M7 class hits central Tokyo, it is imaginable that it will cause a large-scale disaster. It is called the next Tokyo Earthquake which is an earthquake occurring directly beneath the Tokyo Metropolitan Area, in this volume. We have to prepare various measures against the next Tokyo Earthquake, which require us to understand the general nature of the disaster caused, and to estimate the extent of damage. In 2005, the Cabinet Office, Government of Japan conducted damage estimation research on earthquakes occurring below the Tokyo Capital Region. Eighteen types of earthquake were evaluated under various conditions including season, time of occurrence, and wind speed. In 2006, Tokyo Metropolitan Government (TMG) also conducted damage estimation research on the next Tokyo Earthquake to understand the damage that would occur under the jurisdiction of each government- Special ward. of central Tokyo, City, Town, and Village- because each local government has to revise its earthquake disaster management plan.
According to these damage estimations, the Northern Tokyo Bay Earthquake of M7.3 causes most damage under conditions of : occurring on a weekday evening in winter with strong winds of 15m/s. In the case of Cabinet Office, Government of Japan research, 850, 000 houses collapse or are destroyed by fire, and there are 11, 000 fatalities. This scale of damage to housing is eight times that of the Hanshin-awaji Earthquake in 1995. As a result, this must be called a super urban disaster, compared to the Hanshin-awaji Earthquake, which was an urban disaster. Cabinet Office, Government of Japan published “General Policy Principles Relating to Countermeasures Policies for Tokyo Earthquakes” in 2005 and the “Tokyo Earthquake Disaster Mitigation Strategy” in 2006. TMG revised the “Earthquake Disaster Management Local Plan” in 2007. TMG plans to reduce damage by half over the next decade.
