Trough - Torafu (English spelling)

Japanese: トラフ - とらふ（英語表記）trough

Among the long and somewhat wide trenches running on the ocean floor, the maximum depth does not exceed 6000m, and the origin is not important. The Nankai Trough, Suruga Trough, and Sagami Trough were all formed due to the subduction of oceanic plates, just like ocean trenches, and are characterized by the earthquake distribution common to ocean trenches, the formation of accretionary prisms on the landward slope, and prominent crustal movements. The Nankai Trough is a long and narrow trench 700km long that runs almost east-west on the ocean floor about 100km south off the coast from the Tokai region to the Kii Peninsula and Shikoku. It is the entrance where the ocean floor of the Shikoku Basin subducts under the Japanese archipelago, and connects with the Suruga Trough east of Omaezaki. The crustal strain that is the source of the Ansei earthquake (1854), the Tonankai earthquake (1944), the Nankaido earthquake (1946), and the predicted Tokai earthquake is caused by subduction in this trough. The deepest part is flat at a depth of less than 4,500 m, but the landward slope is lined with steep cliffs about 300 m above sea level, indicating intense fluctuations in the seafloor. The Mariana Trough and Okinawa Trough are back-arc basins, but because they have only recently begun to spread, they are long and narrow and are classified as troughs.

(Kazuo Kobayashi, Professor Emeritus, University of Tokyo / 2007)

Source : "Chiezo" published by Asahi Shimbun Publications Co., Ltd. About Chiezo

Japanese:

海底を走る長くやや幅の広い溝のうち、最大水深が6000mを超えないもので、成因は問わない。南海トラフ、駿河トラフ、相模トラフは、どれも海溝と同様、海洋プレートの沈み込みに伴って生じたもので、海溝に共通の地震分布や、陸側斜面での付加体の形成や地殻変動が顕著なのが特徴。南海トラフは、東海地方から紀伊半島、四国にかけての南方沖約100kmの海底をほぼ東西に走る、長さ700kmの細長い溝。四国海盆の海底が日本列島の下へ沈み込む入り口に当たり、御前崎東方で駿河トラフとつながる。安政地震(1854年)、東南海地震(1944年)、南海道地震(1946年)や予想される東海地震の源である地殻歪みは、このトラフでの沈み込みで生じる。最深部は水深4500m足らずで平坦だが、陸側斜面には標高約300mの急傾斜の崖が並び、海底の激しい変動を示す。マリアナトラフ、沖縄トラフなどは背弧海盆だが、拡大を始めて間もないため細長く、トラフに分類される。

(小林和男東京大学名誉教授／ 2007年)

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