首都園地域における地殻活動観測体制  [in Japanese] Observation system of crustal activity of the Metropolitan area, Japan  [in Japanese]

The capital of Japan repeatedly suffered from earthquakes, of which more than half were generated just beneath the metropolitan area including Tokyo. Every important central function of the country is concentraed in Tokyo, and one quarter of the national population is populated in the metropolitan area. It, therefore, is very important to make possible earthquake prediction in the metropolitan area. However, the high sensitivity observations relevant to earthquake prediction is impossible by conventional methods, bccause the metropolitan area is covered with thick and soft Quarternary and Tertiary formations, and exists high background noises caused by havy traffics. Considering such social and natural situations, the National Research Center for Disaster Prevention launched a research project to construct three deep borehole observatories. The observatories realised which for the first time downhole observation whith highly sensitive seismometers and tiltmeters in the Pre-Tertiary basement beneath the metropolitan area, were completed in 1980 overcoming a number of technical difficulties. By these observatories earthquake detection and location capability are remarkably improved, and earthquake activities and tectonics in the metropolitan and surrounding area have been drasticslly revealed. However, observatories which are only three are surely insufficient to promote the earthquake prediction study aimed practical application in the metropolitan area. The dense and highly sensitive observation network of crustal activities over the metropolitan area is discussed.