Tsunamis, sometimes referred to as tidal waves, are crushing waves of water caused by underwater earthquakes, volcanic explosions, landslides, or even asteroid impacts. Tsunamis are almost undetectable until they reach shallow water, at which point the mass of water builds up into a great wave. Depending on the size of the tsunami and the slope of the shore, the wave can travel anywhere from hundreds of yards to more than a mile inland, leaving destruction in its wake. The water then drains back, dragging all manner of debris and creatures far out to sea. The exact damage caused by a tsunami is subject to the GM’s discretion, but a typical tsunami obliterates or displaces all temporary and poorly built structures in its path, destroys about 25% of well-built buildings (and causes significant damage to those that survive), and leaves serious fortifications only lightly damaged. As much as a quarter of the population living in the area (including animals and monsters) perishes in the disaster, either swept out to sea, drowned on shore, or buried under rubble.
A creature can avoid being pulled out to sea with a DC 25 Swim check; otherwise it is pulled 6d6 x 10 feet away from shore. Waters after a tsunami are always treated as rough or stormy, barring magical conf lict. A creature caught in a collapsing building takes 6d6 points of damage (DC 15 Reflex save for half), or half that amount if the building is particularly small. There is a 50% chance that the creature is buried (as for a cave-in), or the tsunami may tear the building apart, freeing the creature from the rubble.