Parametric investigations of tsunami wave modeling are performed to discuss an important issue related to the sensitivity of tsunami simulation results to varied top‐edge depths of a shallowly dipping fault plane (seabed surface rupture versus buried fault rupture). Input boundary conditions (i.e., vertical seabed deformation) for tsunami simulation are calculated using so‐called Okada equations. The analysis results for the 2011 Tohoku, Japan, tsunami case study highlight the significant effects of varied top‐edge depth parameters on vertical seabed deformation and tsunami wave heights. In particular, the uplifted water outside of the fault plane for the buried rupture case, in comparison with the seabed surface‐rupture case, causes large tsunami waves. The results are applicable to other tsunamigenic earthquakes that occur on a gently dipping fault plane at a shallow depth (e.g., anticipated Nankai–Tonankai earthquake) and have important implications on how tsunami inversion should be carried out and how developed source models should be interpreted.