SEISMOLOGICAL ANALYSIS AND NUMERICAL SIMULATION FOR THE MANTLE STRUCTURE WITH SUBDUCTED SLAB

利用宽带地震波形分析方法研究了西北太平洋死板(SS)的构造。M3.11模式具有高速异常(HVA)的区域和660公里的不连续深度凹陷被M2.0模式用HVA但不存在不连续凹陷所划定的区域所包围。这种结构变化与SS的地球化学性质有关，其中富含水的石榴石层与大量橄榄岩的对比涉及。用SS采样后，在一些区域台站观测到了深源事件的异常展宽的P波。异常波的射线在用M3.11或M2.0模拟的区域附近传播。加宽的P波可能是“660公里”不连续面附近异常窄带内的SV转换波。这种非均质性的产生可能与水的相变和SS的冷温异常的复杂结构有关。还进行了数值模拟，以测试与SS相关的各种物理条件。为了解释悬浮物的形成过程，将绝热压缩、粘性耗散和沟槽后退的影响引入到二维热流数值模型中，以阐明各种因素对悬浮物形成过程的影响。我们还建立了考虑密度、粘度等物理性质时空变化的二维数值模型，以及任意形状俯冲板块温度分布的计算方法。使用该模型，我们通过计算横跨日本岛屿的一条剖面的温度分布，测试了导致660公里不连续面凹陷的条件，其中地震速度结构已经通过接收函数和地震层析成像得到了很好的确定。

Broadband seismic waveform analyses were carried out to study the structure associated with stagnant slab (SS) in the northwestern Pacific. The region of model M3.11 with high velocity anomaly (HVA) and the 660 km discontinuity depth depression is bounded by zones that are delineated by model M2.0 with HVA but without depression of the discontinuity. The structural change is associated with the geochemical properties of SS where the contrast of a hydrous garnet rich layer versus a bulk of peridotite involves. Anomalously broadened P waveforms of deep focus events were observed at some regional stations after sampling the region with SS. The rays of the anomalous waves propagated in the vicinity of the regions which are modeled with M3.11 or M2.0. The broadened P waveforms may be SV converted waves within an anomalous narrow zone near the "660 km" discontinuity. The creation of such heterogeneities may involve in the complex structure of phase transformation with water and cold temperature anomaly with SS. Numerical simulations were also performed to test various physical conditions associated with SS. To explain formation process of SS, effects of adiabatic compression, viscous dissipation, and trench retreat were incorporated into a 2-D thermal-fluid flow numerical model to clarify the roles of various factors on formation process of SS. We also developed a 2-D numerical model incorporating spatio-temporal variations of physical properties such as density and viscosity, and a method of calculation for temperature distribution of subducting slab with arbitrary shape. Using the model, we tested conditions to cause depression of the 660 km discontinuity, by calculating temperature distribution along a profile across the Japanese islands where seismic velocity structure has been well determined using receiver functions and seismic tomography.