Existence of an S wave reflector near the upper plane of the double seismic zone beneath the Southern Kanto District, Japan

The amplitude of coda from local shallow earthquakes in the southern Kanto district of Japan does not decay smoothly with time. A clear phase appears in the S coda at lapse time from 15 to 45 s measured from the S wave arrival, and it is predominant for frequencies around 8 Hz. This phase seems to be an S wave reflected from a discontinuity inclined westward at upper mantle depths since it is predominantly on the horizontal components and propagated westward with a high apparent velocity. Applying an inversion method to the travel times of the phases read from 8‐Hz band seismograms, we locate the S wave reflector near the upper plane of the double seismic zone associated with the subducting Pacific plate. The S wave reflector is resolved east of the volcanic front at a depth of 80–100 km. If we put Qs−l = 10−3, the reflection coefficient is estimated to be around 0.5; however, the scatter is large. The S wave impedance contrast at the interface seems to be very large. It may suggest the existence of a zone containing many liquid bodies.