陸域制御震源地震探査から明らかになりつつある島弧地殻・上部マントル構造

抄録

Recent seismic expeditions with controlled sources in Japan provided important constraints on the deformation styles and physical properties of the island arc crust and uppermantle. The crustal structure in the Japanese Islands is characterized by an upper part with a large velocity variation (5.5-6.1 km/s), a middle part with a velocity of 6.2-6.5 km/s overlying a lower part whose velocity is 6.6-7.0 km/s. In many cases, most shallow microearthquakes are concentrated in the upper crust and upper half of middle crust while the lower half of middle crust and lower crust are reflective with very low seismicity. The uppermost mantle is characterized by a low P_n velocity of 7.5-7.9 km/s. Several observations on P_mP phase indicate that the Moho is not a sharp boundary with a distinct velocity jump, but forms a transition zone from the upper mantle to the lower crustal materials. A detailed structural section across the NE Japan Arc from intensive onshore-offshore profiling in 1997-1998 revealed crustal deformation associated with the Miocene backarc spreading of the Sea of Japan. The backbone range of this arc shows a pop-up structure formed by inversion tectonics due to the present compressive stress regime. Crustal thinning associated with the backarc spreading is very clear west of this pop-up structure where the crust deduces in thickness from 30 to 25 km. A section across the SW Japan arc elucidated the detailed subduction geometry of the Philippine Sea Plate and inland crustal evolution associated with processes of accretion and magmatic intrusion. The Outer Zone south of the Median Tectonic Line is characterized by northward dipping structures of accretionary complexes, while the lower part of the crust in the Inner Zone is quite reflective, probably modified and homogenized due to the magmatic intrusion at Cretaceous time. Clear structural images obtained for arc-arc collision zones in central Hokkaido and easternmost part of the SW Japan arc provides direct evidence of crustal delamination. The structure in Hokkaido strongly indicates that the delamination of the Kuril forearc occurs at its brittle-ductile transition zone.