For P waves traveling downdip of deep low-frequency earthquakes (LFEs) from intraslab earthquakes in western Shikoku, Japan, we observed distinct later arrivals with slow apparent velocities comparable to velocities of the lower crust. The apparent velocities of predominant S waves were also in the range of lower crust speeds. Such observations suggest that the low-velocity oceanic crust connects with the island arc crust beneath the western Seto Inland Sea, which may be due to direct contact or the presence of low-velocity wedge mantle between the crusts. As high-velocity mantle is not likely near deep LFEs in the region, deep LFEs occur either at the crust-crust boundary or at the boundary between oceanic crust and the overriding low-velocity mantle. Observations of later arrivals differed between earthquakes in eastern and western Shikoku, implying that the location of the high-velocity mantle can vary and affect temperatures along the zone where deep LFEs occur.