東海沖でのフィリピン海プレートの詳細な沈み込み形態の解明

In 1992, the eastern part of the Nankai Trough off Tokai area, Japan, is seismically surveyed by the dense refraction profiling using an ocean bottom seismograph (OBS) array and airguns. In this area, the Philippine Sea plate is subducting under the Japan Arc along the Nankai Trough, and a remarkable topographic high, the Zenisu Ridge, exists along the southern side of the trough. The aim of this experiment was to obtain detailed seismic velocity structures of the crust at the plate boundary and to reveal the subducting pattern of the Philippine Sea plate at its north margin. We could obtain detailed velocity structure models because of the dense profiling, the good data quality and precise analysis by a ray-tracing method using both the travel-time data and the amplitude data. Results of this study are summarized as follows:<br>1) The eastern Nankai Trough, similar to the southwestern Nankai Trough off Shikoku, has a typical oceanic crust with the thickness of 8-10km, which varies along this profile. Igneous layers are composed of the oceanic layer 2A (P-wave velocity of 3.6-4.3km/s), 2B (4.7-5.6km/s), 3A (6.3-6.4km/s) and 3B (6.7-6.9km/s). And the velocity of the uppermost mantle is 7.8-8.0km/s.<br>2) The Zenisu Ridge has a similar velocity structure to that of the Nankai Trough. It shows a difference with the crustal structure of the Izu-Ogasawara Arc which is characterized by existence of the 6km/s layer. This is clear evidence that the Zenisu Ridge is a topographic high originated from the oceanic crust. Beneath the ridge, the sediment thickness is thin (0.5-2km) and the igneous basement, the oceanic layer 2, shows severe undulations as compared with the structure of the Nankai Trough.<br>3) Existence of a subducting oceanic crust beneath the continental slope was imaged by clearly observed wide-angle reflection phases. Our data support that the subducting oceanic crust lacks the layer 2A and the upper part of the layer 2B. It indicates a remarkable contrast to crustal structure models obtained at the southernmost Kuril Trench and the Ryukyu Trench where the oceanic crust subducts with the whole of the oceanic layer 2.<br>4) The velocity of the upper crust on the subducting oceanic crust was determined as 5.3-5.8km/s, which is relatively low as compared with that of the Japan Arc. The continental slope is characterized by an accretional prism composed of a thick sediment layer of 3.0-3.5km/s and a wedge shaped layer of 3.8-4.2km/s. The wedge seems to contain material of the upper part of the oceanic layer 2 which was scraped off during the subduction because of the similarity of velocities between them.<br>5) Beneath the south margin of the Zenisu Ridge, the oceanic layer 3 was found to have a discontinuity in layering. Furthermore, the layer and the Moho show a sudden dip to the north in the south side of the discontinuity. Although the feature of the structure indicates a possible subduction of the oceanic crust beneath the ridge, the extent of the dipping layer just reaches beneath the south margin of the ridge. This result may suggest that a new subduction of the Philippine Sea plate has started there.