Mechanism of deep focus earthquakes: numerical simulation of physical process and deformation in subducted slabs

我们研究了俯冲板块的变形和物理过程。2004年，重点建立了俯冲融入地幔对流系统的动力学模型。为此，我们需要一个分辨率为1 ~ 2 km的数值代码来计算地幔对流系统。因此，我们利用两层数值网格系统开发了新的地幔对流代码。接下来，我们建立了自发融入地幔对流的二维动态俯冲模型。比较了俯冲岩石圈的应力场和应变场，考察了流变参数。基于这一自洽俯冲模式，研究了2005 ~ 2006年板块与地幔过渡带的相互作用。我们重点研究了(1)自由海沟运动对俯冲板块结构的影响，(2)660 km相界的黏度分层，(3)410 km和660 km以上相转变导致的晶粒尺寸减小。我们还考虑了与温度和压力相关的热膨胀率对俯冲板浮力的影响。在本研究中，我们揭示了地幔过渡带中陆块停滞的形成机制。海沟的运动使俯冲板块弯曲，使深板块的倾角变浅。浅角和海沟向后运动产生的剪切应力与660 km相变相互作用，降低了板块尖端的向下应力。黏度跳变是过渡带板应力场产生的必要条件，与地震观测结果相似。在无粘滞跳变的情况下，板坯的弯曲决定了板坯内的应力。在强应力位置，往往出现一对挤压应力和拉应力。需要660 km相界处的黏度跳变，才能在过渡带尖顶处产生下倾压缩应力场。然而，粘度的跳跃减缓了表面板块的运动。过渡带板的非牛顿变形机制可能是填补这一差异的重要途径。少

We have investigated deformation and physical processes in subducted slabs. We concentrated our efforts to construct dynamic model of the subduction integrated into the mantle convection system in FY2004. For this purpose, we require a numerical code to calculate the mantle convection system with 1 to 2 km resolution. We therefore developed a new code for mantle convection utilizing a 2-layer system of numerical grid. We next developed 2-D dynamic subduction model spontaneously integrated into the mantle convection. The rheological parameters are examined comparing strain and stress field of the subducting lithosphere. Based on this self-consistent subduction model, we investigated interaction of the slab with the mantle transition zones in FY2005 and FY2006. We focused on the effects of (1) effects of free trench motion on the structure of the subducted slab, (2) viscosity layering at the 660 km phase boundary, (3) weakening by the grain-size reduction associated with the 410 and 660 … More km phase transitions. We also consider effects of temperature-and pressure-dependent thermal expansivity on the buoyancy of the subducted slab.In this study, we have revealed the mechanism to generate the slab stagnating in the mantle transition zone. The motion of the trench unbend the subducted slab so that it makes a dip angle of the deep slab shallower. The shallow angle and the shear stress by the backward trench motion reduce downward stress at the tip of the slab interacting with the 660 km phase transition. The viscosity jump is essential to generate stress filed in the transition zone slab resembling that obtained by the seismic observation. In the case with no viscosity jump, the bending of the slab determines the stress in the slab. A couple of compressional and tensile stress always appears in the location with strong stress. The viscosity jump at 660 km phase boundary is needed to generate stress field of down-dip compression in the transition zone stab. The viscosity jump, however, slows the motion of the surface plate. Non-Newtonian deformation mechanism in the transition zone slab may be important to fill this discrepancy. Less