A Seismic Study of the Mantle Transition Zone and Subducted Lithosphere

[804718]本文研究了菲律宾海北部过渡带和汤加俯冲带弧后区异常横波速度(v)的横向和径向分布。在这两个地区，快速p波速度（Vp）的明显异常被认为是俯冲太平洋岩石圈残余物在地幔过渡带保持停滞的证据，至少是暂时的。所提出的研究结果将提供一个难得的机会来比较v和Vp的异常分布和大小，为理解异常体的性质形成约束。这项工作的动机有两点。首先，在倾斜的汤加Wadati-Benioff带外，分散的深源地震活动缺乏相应的Vp异常，说明低温对Vp升高的影响被成分或矿物学效应抵消了。其次，在过渡带的分散地震活动性震源区附近，v可能具有高度的各向异性。因此，将寻求深层地震活动性与泊松比异常或v的各向异性异常之间的联系。

9804718ChenThis research involves the study of the lateral and radial distributions of anomalous S-wave speeds (Vs) in the transition zone beneath the northern Philippine Sea and the back-arc region of the Tonga subduction zone. In both regions, pronounced anomalies of fast P-wave speeds (Vp) have been taken as evidence for remnants of subducted pacific lithosphere that remain stagnant, at least temporarily, in the transition zone of the mantle. Results from the proposed research will offer a rare opportunity to compare the distribution and magnitude of anomalies in both Vs and Vp, forming constraints for understanding the nature of the anomalous bodies. This work is motivated by two observations. First, scattered, deep-focus seismicity off the inclined Tonga Wadati-Benioff zone lacks a corresponding anomaly in Vp, suggesting that the effect of cold temperature, which should raise Vp, is counteracted by compositional or mineralogical effects. Second, Vs may be highly anisotropic near the source regions of this scattered seismicity in the transition zone. Thus, links will be sought between the deep seismicity and anomalies in Poisson's ratio or in anisotropy of Vs.