Two discontinuities have been discovered which are closely related to the Philippine Sea (PHS) plate that subducts beneath the Kanto district in Central Honshu, Japan. A seismic transmission survey method is used to image discontinuities in the Earth from mode converted transmission (MCT) waves, and this method is applied to aftershocks of an event which occurred beneath the southern Tokyo Bay at a depth of 100 km. As a result, two predominant interfaces at depths of 24 km and 35 km appear in the section of P→SV MCT waves. This example demonstrates that the seismic transmission survey method proposed by Katsumata (1992) should be very useful in studying fine structure in the Earth. Moreover, it indicates that even microearthquakes can be used as sources for seismic surveys. ¿ American Geophysical Union 1993