俯冲带地震循环的数值模拟——以日本Tohoku M_W9.0地震为例

Numerical Simulations about Subduction Earthquake Cycles:The Case of Japan Tohoku M_W9.0 Earthquake

基于滑移弱化摩擦准则,以日本2011年Tohoku M_W9.0地震为例,建立一个以物理规律控制地震循环过程的二维有限元数值模型。结果显示,参考模型在1 000a间的6次大地震表现出特征地震的规律,地震重复周期约161a,单位破裂长度地震矩为1.13×1020 Nm/km,在两次大地震中间会发生一次5.62×1018Nm/km的小地震。参考模型的数值结果与地表同震GPS位移、震间GPS速度分布具有较好的一致性。模型弹性参数的不确定性对同震以及震间形变的影响有限,模型粘性参数主要影响震间形变场。数值计算也显示,假设震间形变仅由断层运动规律所决定,那么在一个地震周期内,模型空间重力异常基本上随时间均匀变化,在大陆一侧距海沟100km处可达-370μGal;速度场的变化主要发生在震后约5a的时间内,此后基本保持稳定增加。

Earthquake cycles based on the site of the 2011 M_W9.0earthquake in Tohoku,Japan was simulated with a 2Dfinite element model in which motions of the fault is controlled by a slip weakening friction law.The numerical results show that earthquakes occur in a typical pattern of characteristic earthquakes：there are 6large earthquakes during the 1 000 asimulation,the intervals between two adjacent earthquakes are about 161±4a,and the seismic moments of per unit length for each earthquake is about 1.13×1020 Nm/km.There is a small earthquake with a seismic moment of 5.62×1018 Nm/km occurred between each two-adjoin large earthquakes.The coseismic and interseismic surface deformations of the numerical model agree well with the GPS observations.The uncertainty of the elastic parameters has limited effects on the coseismic and interseismic deformations,while variations of the viscosity influence the interseismic deformations.The numerical results also show that if the interseismic deformations were only controlled by the motion of the fault,the gravity anomaly of this model would decrease linearly during the interseismic period and reach about-370μGal at about100 km from the trench on the continent.Variations on velocity mainly happened within the first 5a and velocities change little about 5aafter each earthquake.