Abstract: The subduction of the Pacific Plate beneath the North American Plate causes frequent occurrence of interplate megathrust earthquakes in the Alaska forearc. However, the distribution of megathrust earthquakes is not uniform and their causal mechanism is still not clear. To clarify this issue, we collected teleseismic waveform data recorded at 268 seismic stations deployed in Alaska during 2018-2019, and measure the teleseismic fundamental mode Rayleigh-wave amplitude and phase data at periods of 25～100 s. We applied a seismic tomographic method to determine a new three-dimensional shear-wave velocity model of the upper mantle beneath Alaska. Our tomographic results revealed lateral structural variations along the trench beneath the Alaska forearc. Beneath the source zones of megathrust earthquakes, the subducting slab exhibits an obvious high-velocity anomaly and is supported by a subslab with obvious low-velocity anomalies. These features suggest possible convergence of the asthenosphere beneath the source zones of the megathrust earthquakes, which may strengthen the coupling between the subducting slab and the overlying plate by increasing the slab buoyancy, and thus affect the generation of the megathrust earthquakes.