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

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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.