A group of island arcs related to various subduction zones constitutes Japan. These islands stretch from the Kurile Islands in the northeast to the Ryukyu chain in the south. Japan uses its own seismic scale, in units of Shindo, to measure the strength of earthquakes. The JMA scale differs from other seismic scales in the way that it describes the degree of shaking at a given point on the Earth's surface. The magnitude of JMA scale is measured between 0 and 7. The JMA reports of earthquake level are based on the peak ground acceleration (PGA).
After studying the two figures, it becomes clear that Japan is extremely prone to frequent, and very often, strong earthquakes. Japan is divided into northern, central and western regions; the possibility of seismic intensity equal to or larger than 6 Lower, within 30 years from the present is studied meticulously (Earthquake Research Committee).
Northern Japan Region
Areas with a high probability of hazard are noted on the Pacific coast of Hokkaido, the Pacific coast of Miyagi Prefecture, and the Pacific coast of Fukushima Prefecture, Yamagata Basin and the Hachiro-gata region of Akita Prefecture. Also, areas with fairly high probabilities extend across the inland areas and to the Japan Sea side. Sapporo City (Hokkaido) has a fairly high possibility of shaking equal to or larger than seismic intensity 6 Lower, within 30 years from the present; the influence is highest from the characteristic earthquakes of the 98 major active fault zones. This is caused by the Ishikari-teichi-toen fault zone which has a high occurrence probability.
These areas are largely influenced by earthquakes along the Nankai Trough, which have been evaluated with a high probability for the entire area of Shizuoka and Aichi Prefectures. Areas with high probability extend over the whole of Kanto Plain, where the Tokyo Metropolis, the prefectures of Kanagawa, Saitama and Chiba, and the southern part of Ibaraki Prefecture are located.
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