北部フオッサマグナとその周辺地域の地質区と地震活動

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Earthquakes of three different natures are recognized among the quakes that occurred in north central Japan (137°-139. 5°E, 36°-39°N). The Niigata type, represented by the Niigata Earthquake (1964, M:7.5), is a main shock type and is associated with active tilting of crustal blocks. The Nagaoka type, represented by the Nagaoka Earthquake (1961, M:5.0), is also a main shock type and is characterized by uplift of a small area near the epicenter by a small amount. Progress of an "active fold" is sometimes observed. The Matsushiro type, represented by the recent Matsushiro Earthquakes, is a swarm type of earthquakes associated with broad uplift of the epicentral area. Five geologic provinces are recognized in north central Japan. Seismicity in each province seems to have had distinct features during recent 1,100 years. Among the correlations between geologic province and seismicity, the followings are remarkable; 1. The North Alps area (1), west of the Itoigawa-Shizuoka line (west border of the Fossa Magna) consolidated before the Early Cenozoic, is most inactive in seismicity compared with the Fossa Magna, especially with the Shin-Etsu folded belt (2). 2. In the area (2), which is characterized by thick Neogene deposits and with active tilting of the crustal blocks and active fold, there are many earthquakes of the main-shock type. On the other hand, swarm type earthquakes characterize the neighbouring area, the median belt of uplift (3) which is distinguished by the intrusion of Middle and Late Miocene granitic rocks and by subsequent stable and uplifting tendency. Interpretation of these correlations is attempted in relation to the heterogeneity of the mechanical structure of the crust expected by the history of the crustal movements in the late Cenozoic time.

北部フオッサマグナとその周辺地域の地質区と地震活動

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