Study on Seismic Heterogeneity in the Lower Mantle

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In the data of J Array which is sismic array installed in Japanese islands arc, unknown seismic phases were found out and analyzed using stacking techniques. Since the slowness of the unknown phases was nearly equal to that of PPP and a little smaller than that, the unknown phases were recognized as precursor to PPP (P_<1200>PP or PP_<1200>P). The origin of the reflection was assumed as approximately 1200 km in depth in lower mantle, based on the analyzes of travel times and tracing the seismic ray. Using these phases from two events, the heterogeneity in the lower mantle was investigated. One of these events occurred in the South Sandwich Islands and the other did in the Southern America.Based on analysis of the P_<1200>PP and P_<1200>P of the event occurring in the South Sandwich Islands, the seismic discontinuity at the depth of 1200km should exist beneath the northern part of the Southern American Continent, where the subducting slab thought to be penetrated into the lower mantle. The research on the seismic tomography showed that the tip of the subducting slab reached at the depth of 1200-1300km beneath there, which corresponded to the depth of the seismic discontinuity inferred from the precursor to PPP.From analysis of the precursor to PPP of the event occurring in the Southern America, the seismic discontinuity at the depth of 1300km should exist beneath south off of the African continent or south of India, where the possibility of existence of 1250km discontinuity had been proposed from the precursor to P'P'.The evidence which supports the existence of the seismic discontinuity in the mid-mantle is very rare. The further observations and analyzes are requested to investigate the heterogeneity in the lower mantle.