Jan. 17 marks the 10th anniversary of the Great Hanshin-Awaji Earthquake, which took the lives of more than 6,400 people. In the past decade, Japan’s earthquake countermeasures have changed enormously. Its earthquake observation system has become more sophisticated. Together with general observation and research, studies have made progress in understanding the active faults that cause inland underground earthquakes.
The vicious power of an inland underground earthquake demonstrated itself once again in October’s Niigata-Chuetsu Earthquake. Even when the total energy released is not so large, an earthquake of this type can cause violent jolting in a limited area. Since 2000 the government’s Earthquake Research Committee has been calculating the long-term probability of an earthquake occurring in the 98 inland active fault zones of Japan. By the end of 2004 it had completed evaluation of 73 locations. At the same time, the committee has evaluated the probability of an undersea earthquake occurring in places such as off the Sanriku coast, the Boso Peninsula, and the Tonankai and Nankai sea areas.
On the basis of this evaluation, the committee by March will make public a nationwide seismic activity forecast map showing the probability, in colored stages, of a region being struck by a strong tremor within a certain period.
For example, a strong earthquake has hit the area off Miyagi Prefecture about every 40 years, so the probability of a strong quake there within the next 30 years is high. Thus the area is shaded darkly on the map.
The seismic activity forecast map, which is the result of earthquake research, as well as hazard maps compiled by local governments, serve as the foundation of disaster-prevention plans. Hazard maps indicate the scale of damage that might be caused by a predicted earthquake based on estimates from specific data for each district in an area, such as the strength or weakness of the ground, the concentration of buildings, the deterioration of housing, and so on. Earlier this month the Cabinet Office released as models hazard maps drawn up for nine municipalities around the country.
Although the seismic activity forecast map gives a kind of bird’s-eye view of the whole, it is the hazard map that is important in actually helping to minimize damage and casualties. Local governments must draw up hazard maps, inform residents of them and encourage communitywide efforts to increase the earthquake resistance of buildings and formulate evacuation plans. In particular, it is essential that local governments prepare evacuation plans based on tsunami hazard maps.
The Meteorological Agency is developing a system that would reduce the time taken to issue tsunami forecasts from the current three minutes after an earthquake to between 30 seconds and one minute. Yet the extra warning time will not have the desired effect of reducing damage and casualties unless there is a prompt and orderly evacuation.
The earthquake observation network has taken shape rapidly over the last decade. There are seismometers belonging to the Meteorological Agency, universities and research institutes at about 3,800 places throughout the country plus about 1,200 GPS (global positioning system) observation points.
It has become possible to keep constant watch on the movements of the Japanese archipelago and to immediately analyze what changes have occurred in the Earth’s crust following an earthquake. In the Niigata-Chuetsu Earthquake, it is now known that a 20-km-long fault shifted as much as 1.8 meters.
The integration of observation data has also revealed the mechanism of earthquakes. Japanese research has attracted worldwide attention by throwing light on the slow-slip phenomenon — whereby the plate on the land side of an undersea quake moves slowly — and by revealing that the asperity, or roughness, along a plate’s boundary becomes the focus of a quake.
The vicinity of the Japanese archipelago is a region of frequent earthquakes because many plates are piled together in a complex manner. Since the 1946 Nankai Earthquake, though, the Great Hanshin-Awaji Earthquake has been the only large damaging temblor to strike a major urban area between Tokyo and the Kinki region.
In the last half century, Japanese society has achieved a high level of industrialization, with much of the population concentrated in urban areas. If adequate earthquake countermeasures are not taken for regions such as Tokai and Tonankai-Nankai, a strong quake could be devastating. The results of earthquake research, which Japan has been conducting with perhaps the world’s most dense observation network, must be put to use in reducing the scale of earthquake disasters.
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