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Structure and evolution of the backstop in the eastern Nankai Trough area (Japan): Implications for the soon‐to‐come Tokai earthquake
Author(s) -
Le Pichon X,
Wmant S.,
Tokuyama H.,
Thoué F.,
Huchon P.,
Henry P.
Publication year - 1996
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/j.1440-1738.1996.tb00164.x
Subject(s) - subduction , geology , seismology , ridge , accretionary wedge , trough (economics) , décollement , wedge (geometry) , plate tectonics , tectonics , paleontology , geometry , mathematics , macroeconomics , economics
We present data showing that the intra‐oceanic shortening now occurring south of the eastern Nankai Trough that has produced the Zenisu Ridge has also been responsible for the formation of a previous ridge now buried below the continental margin. This ridge, that we refer to as Paleo‐Zenisu, is presently adjacent to the backstop and its location coincides with the outer limit of the seismogenic decollement. The subduction of the paleo‐Zenisu ridge below the wedge has led to its complete reorganization and has given its identity to the Great Tokai earthquake rupture zone. The formation of paleo‐Zenisu and its consequent subduction has induced the tilting of the backstop toward the northwest since ca 2 Ma. This model suggests that the backstop and possibly the wedge are dextrally sheared because they are extruded southwestward in relation to the collision of the Izu‐Bonin Ridge with Japan. We use the finite motion from Zenisu to paleo‐Zenisu to derive both the subduction vectors along the Nankai Trough and the shortening vectors within Zenisu‐Izu. The amount of shortening absorbed within Zenisu‐Izu increases toward the northeast. The corresponding subduction vectors of the Zenisu platelet below the wedge decrease accordingly to the northeast from 50 to less than 20 mm/year and the Zenisu body rotates clockwise with a pole near 36° North, 139° East. This might explain the apparent longer repetition time of great earthquakes in the Tokai area. On the other hand, the 25‐35 mm/year obtained for the rate of shortening along the Zenisu thrust indicates a high seismic potential there.