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财年，我们集中精力建立了整合到地幔对流系统中的俯冲动力学模型。为此，我们需要一个数值程序来计算1到2公里分辨率的地幔对流系统。因此，我们利用两层数值网格系统开发了一种新的地幔对流计算程序。接下来，我们发展了自发集成到地幔对流中的二维动态俯冲模型。通过对比俯冲岩石圈的应变场和应力场，测定了岩石圈的流变参数。基于这一自洽俯冲模型，我们在2005财年和2006财年研究了板块与地幔过渡带的相互作用。我们着重研究了(1)自由沟槽运动对俯冲板块结构的影响，(2)在660 KM相界的粘性分层，(3)与410和660…相关的颗粒尺寸减小而减弱的影响更多公里的相变。我们还考虑了依赖于温度和压力的热膨胀率对俯冲板块浮力的影响。在这项研究中，我们揭示了板块在地幔过渡区停滞的产生机制。海沟的运动使俯冲板块伸展，从而使深部板块的倾角变浅。浅角和反向沟槽运动产生的剪应力降低了与660公里相变相互作用的板顶端的向下应力。粘性跃变是在过渡带板内产生类似于地震观测得到的应力场所必需的。在没有粘性跳跃的情况下，板的弯曲决定了板中的应力。在应力较大的部位往往会出现一对压应力和拉应力。需要在660公里相边界处发生粘性跃变，才能在转折区产生向下挤压应力场。然而，粘度的跃升减缓了表面板的运动。过渡区板材的非牛顿变形机制可能是填补这一差异的重要原因。较少

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