海洋地质与环境重点实验室知识库

菲律宾海板块地处欧亚板块、太平洋板块和印度洋-澳大利亚板块的交汇处，四周几乎全部被俯冲海沟所包围，研究表明，菲律宾海板块的形成过程十分的复杂。菲律宾海板块呈南北向狭长的菱形走向，东部边界为伊豆-博宁-马里亚纳海沟、雅浦海沟、九州帕劳海脊和阿玉海槽，西部边界为南海海槽、琉球海沟、马尼拉海沟和菲律宾海沟。以中央南北走向的九州-帕劳海脊将菲律宾海盆分为东西两个大型的海盆：西菲律宾海盆和四国-帕里西维拉海盆；其中四国-帕里西维拉海盆又以索夫干断裂分为北部的四国海盆和南部的帕里西维拉海盆。而在板块最东部的伊豆-博宁-马里亚纳块体则是以南北走向的两条岛弧和中间的马里亚纳海槽组成。 

海洋重力测量中，由于深海海底地形较为复杂，简单布格校正已不能满足精度需求，加入一项地形改正，以消除海底地形的起伏产生的重力效应，从而使得观测数据更加真实的反应地质构造特征。在完全布格异常的基础上进行解析延拓和导数运算，从而解决具体实际的地质问题。空间重力异常经过简单布格校正、地形改正、均衡改正之后得到的异常为均衡异常。均衡校正消除了地壳上表面的地形起伏引起的重力响应，同时消除了由于莫霍面起伏（即均衡补偿面）叠加的重力响应，常用来反应地壳内部结构的变化。 

本文通过对研究重力均衡理论，以Airy均衡学说为基本原理，使用了经过改良的USGS(美国地质调查局)算法,计算地形栅格图的Airy区域均衡重力异常和残余重力异常。模块用地形栅格图、区域密度、莫霍面密度对比和海平面补偿深度来计算莫霍面(即“根”)的深度。计算得到菲律宾海板块整体的地壳厚度与莫霍面深度。计算的结果与前人通过其他方法计算的结果比较一致，因此该方法可以较为简便且准确的计算大尺度的莫霍面埋深。同时给出了在菲律宾海板块附近使用airy均衡学说进行莫霍面深度的计算所使用的具体参数：海平面补偿深度35km；陆壳布格密度2.67g/cm3；洋壳布格密度为2.8 g/cm3；陆地莫霍面密度反差为0.63 g/cm3；海洋莫霍面密度反差为0.5 g/cm3。 

结合前人所做的工作，包括多道地震剖面、OBS数据、层析成像结果，在研究区选取一条横穿中国大陆、东海、冲绳海槽、琉球岛弧、西菲律宾海盆、九州帕劳海脊、帕里西维拉海盆、马里亚纳岛弧的长剖面，进行重震联合反演，得到长剖面的密度结构。重震联合反演结果表明：用airy均衡模式计算的补偿面深度与前人不同计算结果接近，符合真实的地质情况，拟合情况较好。

The Philippine Sea plate is in the intersection of Eurasian plate, the Pacific plate and the Indian - Australian plate, surrounded almost entirely by subduction zones, studies show that the formation of the Philippine Sea plate is complicated. Philippine Sea Plate shows a north-south strip of diamond, the eastern border is Izu - Bonin - Mariana Trench, the western boundary is the Nankai Trough, the Ryukyu Trench, Manila Trench and the Philippine Trench. The Philippine Sea Plate is divided by the central north-south Kyushu-Kyushu Palau ridge into two parts, the West Philippine Basin and Shikoku - Parisi Vera basin; the Shikoku - Parisi Vera basin is divided into the north Shikoku basin and the southern Parisi Vera basin by a big fracture. The eastern Izu - Bonin - Mariana block is based on two north-south arc of the Mariana island arc and intermediate west Mariana through. 

In the marine gravity measurements, due to the complex deep seabed topography, the simple bouguer gravity correction has been unable to meet the accuracy requirements, terrain correction helps to eliminate the effects of gravity generated undulating seabed topography making observation data fits the real tectonic features. Upward and downward continuation and derivative operations on the basis of complete Bouguer anomaly can solve more geological problems. After the simple Bouguer gravity anomaly correction, terrain correction and isostatic correction we get the isostatic gravity anomaly. The isostatic correction eliminate the gravity anomaly generated by the topography changings, while eliminating the gravity response of Moho depth changings (ie. the isostatic compensation surface) and the isostatic gravity anomaly is commonly used to reflect changes in the internal structure of the earth's crust. 

Based on the study of isostatic theory and on the principles of Airy theory, through using a modified USGS (US Geological Survey) algorithm to calculate the Airy raster terrain balanced regional gravity anomaly and residual gravity anomaly. Result of this method is consistent with the previous calculated by other methods, so this method can be convenient and accurate large-scale computing Moho depth. Also given the use of a balanced doctrine airy near the Philippine Sea plate specific parameters to calculate the depth of the Moho using sea level compensation depth of 35km; Bouguer density crust 2.67g/cm3; oceanic crust density 2.8 g/cm3; land Moho density contrast of 0.63 g/cm3; marine Moho density contrast of 0.5 g/cm3. 

Combined with the work done by our predecessors, including multi-channel seismic profiles, OBS data, tomography results, we select a profile across the study area from mainland China, East China Sea, Okinawa Trough, Ryukyu island arc, the West Philippine Basin, Kyushu Palau ridge, Parisi Vera basin, to Mariana island arc, to conduct gravity-seismic inversion to obtain density structure long the profile. Gravity- seismic inversion results show that: the moho depth calculated based on the isostatic theory works well with the real geological conditions.