The back-slip is nominal slip which is opposite to the direction of plate convergence between subducting and overriding plates [Savage 1983]. The back-slip can be interpreted as the slip deficit from the plate convergent. If the plate boundary is completely locked (e.g. no real slip), the rate of back-slip is the same as the convergence rate between two plates. The slip faster than the convergence rate including afterslip and slow slip events is called as the forward-slip.