Mapping the 660 km Discontinuity Under Japan Islands Using Mantle Reflected Waves

The broadband waveform records from deep earthquakes occurred around Japan are analyzed to estimate the depth variations of the 660 km discontinuity (d660) beneath the Japan Islands. A postcursor to sScS, which is identified as a top-side reflection from the d660, is detected in many records. The differential travel time between the postcursor and sScS contains information on the d660 depths beneath the hypocenter and the receiver. We have successfully detected the postcursor and separated residuals of the differential travel times to those from the d660 reflections beneath the hypocenter and receiver, which was not done in previous studies. The residuals are interpreted as the depth change in d660 beneath each hypocenter and receiver. A deeper d660 is found beneath western Japan, which may be caused by cold materials of the stagnant Pacific slab. Beneath central Japan, d660 also exhibits depressions, which is not attributable to the Pacific slab itself because this region is located east of the stagnant slab. The d660 becomes shallower beneath northern and central Japan, which may be caused by a local hot anomaly related to either the cusps of the Pacific slab or a hot upwelling from the lower mantle.