We apply surface wave ray-tracing techniques to tsunami propagation in an ocean of variable depth. Because tsunami velocities are proportional to the square root of the depth of the water column, the juxtaposition of oceanic plateaux and basins generates strong lateral velocity gradients, and bathymetric features can act as lenses focusing or defocusing the tsunammi waves. We present a reconstruction of the tsunami wave field from the great Chilean earthquake of 1960: an order-of-magnitude computation based on the concept of ray-density provides qualitative agreement with observations at several sites in the pacific Ocean. We also demonstrate the possibility of using ''reciprocal'' maps to identify tsunamigenic zones that present particular danger for a given receiving shore. ¿American Geophysical Union 1987