In a subduction zone, a tsunami source is usually situated above the continental bottom slope. Due to the wave refraction, the opposite slope of a deep water trench works like an optical lens for tsunami waves. The tsunami propagation process is studied using the wave-ray approximation. The tsunami amplitude reduction rate strongly depends on a relative (regarding a trench) location of the initial surface displacement. If the source boundary expands closer to a deep trench axis, then the initially circled front line of the leading tsunami wave will be formed into almost a flat shape after passing a trench. The same kind of tsunami behavior can be observed when a wave is generated by a submarine mudslide on a trench slope. A number of hypothetic tsunamis were simulated in the Alaska-Aleutian and in the Kuril-Kamchatka subduction zones. Some locations of possible tsunami sources are estimated as most dangerous for the Hawaiian population and infrastructure.