10.大地震の余震域の拡大

Spatial distributions of the aftershock energy of several great shallow earthquakes which occurred in the circum-Pacific seismic belt are discussed. In the interval of one year following the main shock, the active area clearly expanded outward in most cases. The pattern of the expansion, however, is different for different areas, as are the size and the shape of the aftershock area. A sharp contrast in these features of aftershock sequences between Japan and Aleutian-Alaska suggests a difference of the mechanism of aftershock occurrence in both areas. Moreover, a rapid systematic propagation of the aftershock activity during several hours following the main shock was found for the Aleutian Earthquake of 1957 and the Alaska Earthquake of 1964. From the standpoint of the fracture theory of earthquakes, these features in the development of the aftershock region are attributed to the nature of brittle fracture.|大地震の発生機構を研究する一つの方法として,その余震の発生特性を研究することが考えられる.個々の大地震については,かなり詳しい研究が行なわれてぎたのに対して,これらの大地震に共通な特性の系統的な研究は必ずしも十分ではない.本論文では,浅発注の大地震の余震活動の空間的な分布,とくにその時間的な変化を出来るだけ多くの例についてしらべ,大地震の余震の起こり方を明らかにすることを試みた.次にその結果を要約する