著者

岡田 正実

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.35, no.1, pp.53-64, 1982-03-25 (Released:2010-03-11)

参考文献数

26

被引用文献数

1 2

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Monthly distribution of major shallow earthquakes (M≥7.5) in and near Japan is investigated using earthquake catalogs including pre-instrumental data. Major shallow earthquakes off Hokkaido and Sanriku districts are concentrated in spring, and those along the coast from Miyagi prefecture through Shikoku district occurred more frequently from August to December, as Mogi has pointed out. We suggest with some consideration that these variations are statistically significant at a 95% confidence level. In the other region the records on major earthquakes are not enough to discuss the seasonal variation. But it is able to show that large earthquakes with magnitude 7.0 or over occurred more frequently from March to September in the inland regions where no effect of the major earthquakes on the Pacific coast is expected. It seems that the decrease in the weight of water on land and lowering of the mean sea level trigger the occurrence of major earthquake between the oceanic and continental plates, when the strain accumulates closely to the limit. It is also considered that the increase of ground water in the fault zone has an influence on the occurrence of large shocks inland in their critical states.