TRIGGERING CAUSES OF EARTHQUAKES ALONG THE IZU-BONIN-MARIANA SUBDUCTION ZONE

The correlation coefficient between shallow earthquakes and plate subduction rates along the IBM is calculated in this paper. It suggests that the plate subduction rate is an important parameter to determine the spatial distribution of earthquakes. Statistic of earthquakes reveals that intermediate to deep (60~300 km) seated earthquakes along the Mariana Arc show an obvious feature of segmentation corresponding to the relief of sea floor. It is inferred to be caused by over supply of fluids during the seamount subduction, which leads to the break and local deformation of the subducting slab and change in coupling of thermal pattern. A counter surface of 8.0 km/s from the global P-wave model beneath the Mariana Arc is extracted and presented by the Paraview software. Two gaps are observed under the north and south ends of the Mariana Arc, owing to the tearing up of the subducting slab, which is stronger in the north and weaker in the south, and the difference in subduction rate in the northern Ogasawara Plateau and the southern Caroline Ridge respectively. The gravity and earthquake data also reveals that in the southern part of the Mariana Arc there may be strong interplate coupling, and 3D P-wave speed map shows that the 410~670 km remanent Pacific Plate beneath the Philippine Sea Plate extend to the southwest and there is an indirect contact with the subducting Pacific plate from the southern Mariana Arc. A preliminary deduction is that the deep structure and the buoyant Caroline Ridge may determine the existence of the rotation pole located at 8°N, 137.3°E in the southern Mariana.