Some coastlines with earthquake zones off-shore can act as "tsunami magnets", to produce devastating flooding as much as 50% deeper than expected.

In this animation, the trough of the tsunami wave propagates towards the shoreline.

And the wave energy is focused and amplified.

The focal point where the tsunami wave is most amplified can be located on-shore or off-shore, depending on the distance of the earthquake origin from the shoreline.

This explains the unexpectedly large runups observed for some tsunamis

such as the great Japan tsunami of 2011

or the Java tsunami in 2006.