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タイトル: Contemporary Stress Field in the Wadati-Benioff Zone at the Japan-Kurile Arc-arc Junction (North Honshu, the Hokkaido Corner and Hokkaido Island) by Inversion of Earthquake Focal Mechanisms
その他のタイトル: 応力テンソル逆解析による本州 : 千島弧会合部の沈み込む太平洋プレート内の応力場
著者: Cenka, Christova
Hirata, Naoshi
Kato, Aitaro
著者(別言語): クリストヴァ, チェンカ
平田, 直
加藤, 愛太郎
キーワード: Wadati-Benioff zone
North Honshu- Kurile junction
regional stress field
inverse method
発行日: 2006年
出版者: 東京大学地震研究所
掲載誌情報: 地震研究所彙報. 第81号第1冊, 2006, pp. 55-70
抄録: This study addresses the space distribution of the stress field in the Wadati-Benioff zone of Northeastern Japan and southernmost Kurile area based on homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth (1984). The data set used consists of 785 JMA focal mechanism solutions (FMS) and 97 FMS listed in Kosuga et al. (1996) for shallow and intermediate depth earthquakes. The detailed analysis of the space distribution of orientation of P (compression) and T (extension) axes of FMS allowed the outlining of the following WBZ subvolumes for which we applied the stress inversion: three planar structures in North Honshu (NH) and the Hokkaido corner (HC) WBZ (Plane1, Plane 2, Plane 3), and upper and lower subvolumes in the Hokkaido Island (HI) WBZ. The stress field parameters are evaluated along the northeastern Japan and southern Kurile arcs for these WBZ subvolumes. The stress field in Plane 1, mainly low-angle thrust faults, is characterized by shallow dipping and close to strike normal maximum compression s1 and down dipping minimum compression σ3. Plane 2, the upper surface of the WBZ below 60-70km in NH and HC, is under slab parallel s1 and close to slab normal s3 all along NH, while in HC the minimum compression rotates counterclockwise about 30° relative to the slab normal. Plane 3, the lower surface of the WBZ, is characterized by close to slab normal s1 and close to slab parallel σ3. The stress regime in Plane 1 is of general compression everywhere but in segment HT, beneath the Hokkaido corner, where it is of general extension (Guiraud, 1989). The stress regime in Plane 2 is of general compression, and in Plane 33 of pure extension. A characteristic feature of the two WBZ subvolumes outlined beneath Hokkaido is that the upper subvolume overlies the lower one everywhere but in the southern part of the island. The orientations of the maximum and minimum compressive stresses in the upper subvolume of the Hokkaido WBZ, considered relatively to the local slab geometry, are similar to these in Plane 1 of NH and HC WBZ, i.e. close to strike normal s1 and down dipping σ3. However, the dip of the principle stresses is different 3 here s1 is steeper and s3 is shallower. The stress field in the lower WBZ subvolume beneath Hokkaido is characterized by strike aligned s3 dipping north at about 50°, s1 trends SE being strike normal beneath the southern part of the island and slab normal beneath its northern part. The orientations of P and T in the upper WBZ subvolume in central Hokkaido differ significantly from these in the upper WBZ volumes to the south and to the north but are similar to those in the lower subvolume here. The stress inversion results indicate homogeneous stress field in the upper and lower WBZ subvolumes beneath central Hokkaido. The orientation of the minimum compression here (strike aligned, trending north) is close to the orientations of s3 in the southern and northern lower parts of the HI WBZ, while the s1 is dipping steeply WSW. These stress directions, if considered kinematically, indicate that the preferred faulting occurs at plane that is almost vertical and perpendicular to the strike of the slab (the strike of the trench) with the northern wall moving down and the southern one moving up. The stress regime is of general extension in all the considered subvolumes in the HI WBZ. The results of this study clearly indicate 3-planar distribution of stresses in the WBZ beneath North Honshu and the Hokkaido corner. We outlined two subvolumes (upper and lower) in the WBZ beneath Hokkaido, which are characterized by different orientations of the principle stresses. The stress field in the upper WBZ subvolume is perturbed by a deformation zone (DZ), located beneath central Hokkaido. This DZ is perpendicular to the slab’s strike and is cutting through the slab, the stresses in the upper and lower subvolumes of it are of similar orientation. The directions of the best-fit stress model in the DZ suggest that its northern wall moves down while its southern wall moves up. One plausible explanation is that this deformation zone represents a crack or a tear cutting through the entire slab.
URI: http://hdl.handle.net/2261/5778
ISSN: 00408972
出現カテゴリ:東京大学地震研究所彙報
東京大学地震研究所彙報

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