Study on Seismic Heterogeneity in the Lower Mantle

在日本岛弧上安装的J阵地震资料中，利用叠加技术找出了未知地震相，并进行了分析。由于未知相的慢度与PPP的慢度接近且略小于PPP的慢度，因此可以将未知相识别为PPP的前兆（P_<1200>PP或PP_<1200>P）。根据对传播时间的分析和对地震射线的追踪，假设反射源位于下地幔深处约1200公里处。利用这两个事件的相位，研究了下地幔的非均质性。其中一次发生在南桑威奇群岛，另一次发生在南美洲。根据南桑威奇群岛事件的P_<1200>PP和P_<1200>P的分析，在1200km深度的地震不连续应该存在于南美洲大陆北部下方，俯冲板块被认为渗透到下地幔中。地震层析成像研究表明，俯冲板的尖端到达其下方1200 ~ 1300km深度，与PPP前兆推断的地震不连续深度相对应。从发生在南美洲的事件的前兆到PPP的分析来看，1300公里深度的地震不连续应该存在于非洲大陆南部或印度南部，其中从前兆到P‘P’可能存在1250公里的不连续。支持中地幔地震不连续存在的证据非常少。下地幔的非均质性有待进一步的观察和分析。

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.