Study on the Mantle-Upwelling Structure of Non Island-Arc Type West off Kyushu

冲绳海槽位于琉球-九州弧弧后地区，被认为是弧后扩张的早期阶段。为了了解九州弧后扩张和弧后火山活动的机制，我们于1999年秋季利用海底地震仪（OBSs）在九州以西的东北冲绳海槽进行了一次爆发地震。在沿着海槽中心的300 km长的NNE-SSW地震剖面上，每隔15 km布设了20个OBS。我们用气枪和76个炸药作为控制源。在气枪射击过程中，反射波记录从射击船拖曳的24通道水听器拖缆。所获得的地震反射剖面显示，尽管测区海底地形平坦，但上地壳的横向不均匀性很大。在OBS的距离-时间剖面上，初至视速度由于非均匀性而发生变化。沉积剖面由两层组成。上部沉积层 ...更多信息 厚度为200 - 500米，纵波速度（Vp）为1.7 - 1.9 km/s。上层的垂直速度梯度很小。下部沉积层Vp为2.0~3.5km/s，垂向速度梯度较大。下层厚度变化大，为800~3500 m。上地壳横向不均匀性大，也可分为两层。上地壳顶部的Vp变化范围为3.0 - 4.9km/s。上地壳下层的Vp为5.6 - 6.0 km/s。上地壳的厚度从3公里到9公里不等。上地壳和下地壳之间的边界在海面以下约10公里深。下地壳的Vp为6.5 - 6.7 km/s。与上地壳相比，下地壳相对均一。下地壳的厚度约为15公里。地壳总厚度约为25km，Pn速度为7.7 - 7.8km/s。Vp和地壳厚度的分布表明，东北部冲绳海槽的地壳与岛弧的地壳相似，而与大洋的地壳相似。地震构造表明，冲绳海槽东北部正处于弧后裂谷作用的初始阶段。少

The Okinawa trough which is considered to be in an early stage of backarc spreading locates in the backarc region of Ryukyu-Kyushu arc. Understanding mechanism of backarc spreading and backarc volcanism of Kyushu, we conducted an explosion seismology in the northeastern Okinawa trough west off Kyushu by using ocean bottom seismographs (OBSs) in fall of 1999. Twenty OBSs were deployed at intervals of 15 km in the NNE-SSW seismic profile of 300 km in length along the trough center. We used airguns and 76 explosives as controlled sources. During shooting of the airguns, reflection waves were recorded by 24-ch hydrophone streamer towed from the shooting ship. An obtained seismic reflection profile shows large lateral heterogeneity in the upper crust in spite of smooth seafloor topography of the surveyed area. On the distance-time sections of OBSs, apparent velocities of first arrivals vary due to the heterogeneity. The sedimentary section consists of two layers. The upper sedimentary layer … More has a thickness of 200-500m with P-wave velocity (Vp) of 1.7 - 1.9 km/s. A vertical velocity gradient of the upper layer is small. The lower sedimentary layer has Vp of 2.0 - 3.5 km/s with relatively large vertical velocity gradient. The thickness of the lower layer has large variety from 800 m to 3500m. The upper crust has large lateral heterogeneity and is also divided into two layers. Vp at the top of the uper crust vary in 3.0 - 4.9 km/s. The lower layer of the upper crust has Vp of 5.6 - 6.0 km/s. The thickness of the upper crust changes from 3 km to 9 km. The boundary between the upper and lower crust is about 10 km in depth below the sea surface. The lower crust has Vp of 6.5 - 6.7 km/s. Compared with the upper crust, the lower crust is relatively homogeneous. A thickness of the lower crust is about 15 km. The crust has a thickness of about 25 km in total and the Pn velocity is estimated to be 7.7 -7.8 km/s. The distribution of Vp and the thickness of the crust indicated that the crust of the northeastern Okinawa trough is similar to that of island arc than to that of ocean. It is suggested from the obtained seismic structure that the northeastern Okinawa trough is in the incipient stage of the back-arc rifting. Less