Preliminary Study of the causes of absence intermediate and deep focus earthquakes in the Manila subduction zone

The Manila subduction zone is a seismic-prone area in South China Sea whose formation and tectonic evolution have close relationship with the earthquake causes.In this paper,the seismic data and tomography results of Manila subduction zone were conducted in-depth analysis.The results showed that: the earthquakes in Manila subduction zone are mainly intensive shallow earthquakes without intermediate and deep focus earthquakes.Further analysis revealed that: 1) Dehydration and eclogite formation of South China Sea ocean crust largely cease before contacting with asthenosphere.In the south of Manila subduction zone,South China Sea oceanic crust changed to plastic deformation in shallow depth and stopped diving while the South China Sea plate continued to dive due to the oblique obduction of the Philippine plate in the north.2) Subducting plate of Manila subduction zone was tore in the North Luzon Trench with a depth of nearly 100km and extended southward to the ancient spreading ridge of South China Sea.These two mechanisms combined responsible for the absence of intermediate and deep focus earthquakes of the Manila subduction zone.