What warnings do you get before a tsunami hits land?

Because the shock waves-- "seismic signals," that is-- from an earthquake travel faster through the rocks beneath the sea than the tsunami travels, seismographs around the world will usually detect an earthquake below the ocean before a tsunami can reach shore. If the earthquake happens to be near shore this may not work. Also, not all earthquakes below the ocean cause tsunamis.

Warning buoys at sea, that continuously monitor the elevation of the sea's surface, can detect a tsunami passing while it's still far out at sea. Oddly enough, a tsunami is not a threat at sea. It is only when the wave gets into shallow waters, slows down from the drag of the sea floor, and therefore starts piling up on top of itself that it becomes dangerous. Nevertheless, it is high enough even in the deep sea that a properly equipped buoy can detect it.

As a tsunami approaches land, the first sign is that the water level DROPS. The water can pull away from the shore, leaving a wide expanse of what is normally seabed exposed to the air. Often, people run out to explore and to gather in the fish that are trapped on land when the water recedes. Of course, they'll all be killed when the tsunami hits. My advice is that if you're at the beach and you see the water start pulling away far from the normal shoreline, run for the hills!