“…The grid imposed on the half plane as used in the centred difference, one-sided approximations (a) and implicit approximation (b). Engquist and Majda 1977, Reynolds 1978, Liao et al 1984, Cerjan et al 1985, Higdon 1991, Zhang et al 1993, 1999, Berenger 1994, Cao and Greenhalgh 1998, Yang et al 2002, Komatitsch and Tromp 2003, Wu and Liang 2005, Tian et al 2008, Chen and Bording 2010, numerous methods to deal with the free-surface boundary condition have been proposed in the past four decades since the development of finite-difference approximations of the elastodynamic equations in second-order formulation (Alterman andKaral 1968, Alterman andRotenberg 1969). These schemes can be divided into two kinds: (1) adding a fictitious layer above the free surface (figure 1), which includes a centred finitedifference approximation (Alterman and Karal 1968), a onesided approximation (Alterman and Rotenberg 1969) and an implicit boundary update technique (Vidale and Clayton 1986).…”