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Abstract: The structure of the Philippine Sea Plate in the offshore region adjacent to the Boso Peninsula in central Japan is resolved by comparison of recent oceanic seismic survey data with natural repeating earthquake data. This region is characterized by a range of seismic and aseismic phenomena associated with subduction motion of the Philippine Sea Plate, including slow slip events (SSEs), large backslip events, and repeating earthquakes. This offshore region is also the site of the largest aftershock of the 1923 Kanto earthquake (M7.9). The detailed structure of the Philippine Sea Plate is successfully resolved, and the upper boundary of plate is clearly traceable to depths of 20 km. The reflection intensity of the upper boundary varies spatially in accordance with the depth of the reflector. The upper boundary continues smoothly with a line of repeating earthquake hypocenters regarded as an indicator of subduction shear. A continuous model extending into the offshore region can thus be proposed. The variation in reflection intensity on the upper boundary of the Philippine Sea Plate also correlates well with variation in event types. A weak reflection zone overlaps the area of large backslip, while a moderately strong reflection zone encroaches into the large slip area of the Boso SSE. The reflectivity of the upper boundary is thus considered to be dependent on the physical properties and conditions of the reflector, which in turn affect the event types on the plate boundary. A source fault model is proposed for the largest aftershock of the 1923 Kanto earthquake. Based on geodetic data, the source fault is located eastward of the mainshock, with dimensions of 40×50 km and moment magnitude of 7.5. The source fault lies within the region of large backslip and the large slip area of the Boso SSE. [Copyright &y& Elsevier]