A slab fragment wedged under Tokyo and its tectonic and seismic implications |
|||
|
|||
Perspective views of the Kanto fragment with the Pacific and Philippine Sea plate slabs, volcanoes and seismicity. The left panel shows view of southwest Japan (the coastline is grey), together with the volcanic front (yellow pyramids), their presumed magma conduits (red vertical lines) tend to lie along the 120-140 km (75-85 mi) depth contour (blue dashed lines) of the Pacific plate. The blue ball is Tokyo. The right panel shows a close-up view of the proposed Kanto fragment with microseismicity colored by depth. Toda et al argue that the Kanto fragment broke off the Pacific plate 2-3 million years ago, and is being pushed northwestward under Tokyo. Note the seismicity streak on the upper surface of the proposed fragment, perhaps indicative of its motion with respect to the overlying crust. These are frames from the Movie. フィリッピン海プレートと太平洋プレートに挟み込まれる関東フラ グメントの透視図.北東から南西を望む.プレート(スラブ)正面 に引かれた線は等深線とその深さ(km). ▲は活火 山.火山は太平洋プレートの上面の深さ約120-140kmの位置と ほぼ一致する.右図は左図の拡大版で,点は個々の地震に対応し, 深さ毎に着色している.関東フラグメント上面では線状の地震の分布 (streak)が観測され,ユーラシアプレートとの擦り合う方 向(相対運動の方向)を示している. |
|||
Anonymous contemporary woodcuts of Tokyo (formerly, Edo) before and after the great 11 November 1855 magnitude~7.3 Ansei-Edo earthquake. Toda et al contend that this quake struck as a result of the movement of the Kanto fragment against the Pacific plate slab below, about 70 km (45 mi) deep. The Japanese government estimates that a repeat of this quake today would cost $1 trillion, destroy about 500,000 buildings and take up to 10,000 lives. Woodcut source, Documenting Disaster: Natural Disasters in Japanese History, 1703-2003, Foundation for Museums of Japanese History, Chiba, 2003. 1855年マグニチュード7.3の安政江戸地震の前(左図)と後 (右図)の江戸の様子(「 ドキュメント災害史 1703-2003地震・噴火・津波,そして復興」国立歴史民族博物館発行 より).安政江戸地震も関東フラグメントの動きに起因して発生し たと考えられる.政府の試算では,同規模の地震の被害総額は約 100兆円,全半壊50万棟,死者10万人と示されている. |
|||
Topography and bathymetry of the Kanto tectonic ‘triple junction’ viewed from the south. Topography and bathymetry are vertically exaggerated by a factor of four. The Kanto plain marks the land surface of the deep Kanto basin. The main Japanese island of Honshu is in the upper left. The Philippine Sea plate descends west-northwestward beneath Tokyo the along the Sagami and Suruga troughs. The broad Izu volcanic ridge and the young age of the Philippine Sea plate make the plate buoyant, uplifting of the peninsulas south of the Kanto Plain, which form the southern margin of the Kanto Basin. The much older Pacific plate descends to the northwest along the Japan and Izu trenches. A chain of Pacific seamounts is visible entering the Japan Trench on the right, forming the east margin of the Kanto basin. The authors argue that the buoyant seamounts interfere with the descent of the Pacific plate and so distort the trench into a broad arc. 3枚のプレートの会合点(三重点)周辺の地形.地形は鉛直に約4 倍誇張されている.フィリッピン海プレートは西北西に向かって移 動し,駿河トラフと相模トラフで本州の下に潜り込むとともに,関 東平野南部を幾分隆起させている. また,伊豆ー小笠原弧を のせている若くてあたたかい部分は幾分浮力があり,富士山の北で 本州と衝突し,周辺地域を大きく変形させている.一方,古くて冷 たい太平洋プレートは北西に移動し,日本海溝で本州の下に,伊豆 海溝沿いでフィリッピン海プレートの下に沈み込んでいる.図の右 側には太平洋プレート上に複数の海山がみられ,一部は現在も沈み 込んでいる.我々はこれらの海山列が太平洋プレート上部の破断や ユーラシアプレートの関東平野周辺の変形に寄与してきたと解釈した. |