断层阶区对产生超剪切地震破裂的促进作用

地震时若断层发生超剪切破裂,地震灾害会显著加剧.因此研究超剪切破裂的形成机理有着非常重要的科学意义.本文利用动力有限单元方法,模拟断层破裂从初始断层跳跃传播到另一条平行的次级断层(断层阶区)时破裂速度的变化情况,并分析断层阶区几何特征等物理参数对产生超剪切地震破裂的促进作用.计算结果表明,断层阶区的诸多物理因素(如:重叠长度、相隔距离以及摩擦系数等)都会对破裂的传播速度产生影响.在一定条件下,破裂传播速度会由在初始断层上的亚剪切波速度,转换为在次级断层上的超剪切波速度.在破裂速度转换过程中,断层间隔起着重要作用,当断层阶区中两断层垂直间隔距离小到一定程度时,破裂跳跃阶区后,破裂速度不会发生变化.所以对于分段断层(可视为一种特殊的断层阶区),由于其断层垂直间隔为0,也就不会出现破裂速度变化的现象,模拟结果对此也进行了证实.然而,若断层间隔太大,当其距离超过一定的限度后,破裂通常无法跨越断层阶区继续传播,而是终止在初始断层上.模拟结果还表明,初始断层与次级断层之间的重叠距离也十分重要,只有当断层阶区中两平行断层之间的重叠部分达到一定长度后,断层的破裂速度才有可能发生转换.此外,计算结果显示,破裂过程中断层面上的应力变化可能是破裂传播速度发生转换的直接原因.最后,模拟还发现,当破裂跨越断层阶区发生速度转换时,破裂需要停顿一定的时间,以便积聚足够的能量来实现破裂速度的增快.

Numerical simulation of seismic supershear rupture process facilitated by a fault stepover

Once the fault produces supershear rupture, the earthquake hazard will be significantly increased, so it has very important scientific significance to research the formation mechanism of supershear rupture. This paper used the dynamic finite element method to simulate the changes of rupture velocity during the rupture jump a stepover, and analyze influences of geometric features of stepover on promoting supershear earthquake rupture. The computed results demonstrate that the physical parameters of stepover, such as overlap length, step width, and coefficients of friction, etc., will have effect on the rupture velocity. Under certain conditions, when rupture jumps the stepover, the rupture velocity will change, by the subshear wave velocity on the initial fault into a supershear wave velocity on the secondary fault. However, the simulation results further show that step width plays an important role in the velocity change. When the step width is small to a certain extent, the rupture velocity will not change after the rupture jumped the stepover. For the segmentation faults, due to the vertical interval between the faults is 0, the transformation phenomenon of rupture velocity will not appear either. It also can be confirmed by the simulation results. The rupture usually cannot jump the stepover and arrest on the initial fault, when the step width is too large. The computed results also demonstrate that only when the overlap between faults reaches a certain length, the rupture velocity can transform subshear wave velocity to supershear wave velocity. In addition, the computed results show that change of stress on the interface during the rupture process is the direct cause of the rupture velocity change. Finally, our computed results also reveal that when rupture jump the stepover and the velocity change, the rupture need a pause for a period of time to accumulate sufficient energy, and then to achieve the increase of rupture velocity.