Inversion Of Gravity-seismology In The Research Of Crust Structure Of The Philippine Sea Plate

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.