Since the 2011 M9.0 Tohoku-Oki megathrust earthquake, many normal-faulting earthquakes have occurred off northeastern Japan in the upper plate, indicating a trench-normal tensional stress state opposite to that in the inland back-arc area. Determining the spatiotemporal evolution of the stress field is essential for understanding the earthquake occurrence mechanisms in the upper plate of the megathrust. However, the focal depths of normal-faulting earthquakes are not well-constrained due to the lack of observational data directly above the earthquakes. In this study, we first developed a high-frequency waveform modeling method to constrain earthquake depths by reproducing sP depth phases. We then applied the method to M{greater than or equal to}3 earthquakes off southern Tohoku, where intense normal-faulting earthquakes occurred. By comparing the observed waveform envelopes with their synthetic counterparts, we estimated focal depths of 300 events. The estimated focal depths are consistent with those estimated from seafloor seismic observations, supporting the validity of our estimates based solely on inland data. We then conducted relative hypocenter relocation using the events whose focal depths were estimated by sP depth phases as master events to obtain hypocenters of 8,599 events. The results showed that these normal-faulting earthquakes occurred near the surface to a depth of approximately 30 km. This suggests that a tensional stress state prevails from the shallow to deep regions above the megathrust. One possible cause of the tensional stress is gravity force. The effect of plate convergence may not be sufficiently large to make the fore-arc region a compression regime.