During a large offshore earthquake, horizontal sea-floor deformation combined with bathymetry contribute to vertical water displacement in addition to the vertical sea-floor deformation (Tanioka and Satake, GRL, 1996). This contribution can be negligible or on the order of the contribution of vertical sea-floor deformation. The importance of this contribution only relies on geometrical parameters: the slab dip angle and the bathymetry gradient. Thus, it can be evaluated for synthetic earthquakes along the main subduction zones of the world. If you want to know if this effect is negligible in your area of study you can refer to the figure below and the associated article (Bletery et al., PAGEOPH, 2015).