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タイトル: 24. Spatial Variation of Earthquake-Generating Stress in and around the Kanto District, Central Japan
その他のタイトル: 24. 関東地方における起震歪力の分布
著者: Maki, Tadashi
著者(別言語): 牧, 正
発行日: 1984年1月14日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第58冊第3号, 1984.1.14, pp. 591-646
抄録: A number of fault-plane solutions of earthquakes (n=454) in and around the Kanto District were determined by a numerical method, and the spatial variation of earthquake-generating stress is derived by using individual and average focal mechanisms. Averaging of direction cosines is made for both axes of maximum pressure and tension, and other axes are located by computation. The structure of earthquake-generating stress is derived from the spatial variation of fault types which are systematically classified into six types based on dip angles and azimuths of pressure and tension axes and null vector. The :spatial variations of focal mechanisms are derived by summarizing pressure and tension axes and comparing them with the distribution of earthquake hypocenters previously determined by correcting the Pn station biases. The northern focal hemispheres of fault-plane solutions are used for comparison with the vertical distributions of the relocated hypocenters. A reverse fault with the horizontal E-W compression is predominant for crustal and subcrustal earthquakes on the Pacific side, and it is directly related to the westward convergence of the Pacific Plate underneath the Eurasian Plate. Axes of maximum pressure show some variations for individual earthquakes and hypocenter locations. An eastward increase of dip angle of T axis is observed along the E-W section. The E-W compression appears also in the inland region of the northern Kanto District. In the southwestern part of the Kanto District a variety of focal mechanisms are observed. For very shallow earthquakes near Izu-Oshima and the Izu Peninsula a strike-slip with an N-S compression is predominant. Another type is the normal faulting for earthquakes located south off the Kanto District. These two types of faulting commonly have the horizontal E-W extension. Besides the down-dip compression observed for mantle earthquakes, the down-dip extension along the inclined seismic zone is observed for intermediate-depth earthquakes located on the Pacific side. But the tensional stresses along the lower plane of the double seismic zone show a variety of azimuths and dip angles. A significant contrast is observed in fault types between the IzuBonin region and the Pacific coast of the Kanto District and Northeast Japan. The thrust faulting is predominant for earthquakes with depths down to 60 km along the plate boundary below the Pacific coast, but in the Izu-Bonin region various types of faulting are observed without showing an obvious inclined seismic zone. Some source regions with similar focal mechanisms are observed; the reverse fault with the NW-SE compression in the sequence of the 1964 Niigata and 1972 Hachijojima Earthquakes, and the strike-slips in the Matsushiro Swarm and in the Izu Peninsula and Izu Islands region.
URI: http://hdl.handle.net/2261/12889
ISSN: 00408972


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