Study on Seismic Heterogeneity in the Lower Mantle

在日本岛弧地震台阵J台阵的资料中，利用叠加技术找出了未知震相并进行了分析。由于未知相的慢度与PPP相的慢度相近，但比PPP相的慢度稍小，因此认为未知相是PPP相的前驱相（<1200>PPP或<1200>PPP）。根据地震波走时分析和地震射线追踪，推测该反射波的来源约为1200 km深的下地幔。利用这两次地震的震相，研究了下地幔的不均匀性。通过对南桑威奇群岛地震的P_<1200>P和P_<1200>P分析，认为在南美洲大陆北方下1200 km处存在地震间断，俯冲板片可能已穿入下地幔。地震层析成像研究表明，俯冲板片尖部到达其下1200- 1300 km处，与PPP前兆推断的地震不连续面深度相对应。1300 km深度处的地震间断面应存在于非洲大陆南部或印度南部之下，其中，从P 'P'的前兆中提出了1250 km不连续面存在的可能性，但支持中地幔存在地震不连续面的证据非常少。下地幔的不均匀性有待于进一步的观测和分析。

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.