In the simulation of the aftershock, we used a simple constant absorbing boundary condition in our model specified by visco dampers. Although there is not much reflection from the boundaries for the San Fernando basin, a nearly closed region, an improved absorbing boundary condition needs to be adapted to deal with other kinds of basins in more general cases with higher accuracy.
In finite-difference models such as those in [33], absorbing boundary conditions developed by Clayton and Engquist [13] were applied to the sides of their computational domain padded with a damping zone of attenuative material [12]. Stacy [44] presented improved Clayton and Engquist-type absorbing boundary conditions which allowed more accurate finite difference formulations at internal boundaries and corners, and he verified that these formulations were stable over a wide range of parameters.
I plan to adopt one of the Stacy's more efficient boundary conditions into finite element formulations; then, I will implement them into my FEM code. To further reduce artificial reflections, a similar damping zone will be combined with the absorbing boundaries conditions as necessary.