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What Causes a Tsunami to Get Larger as It Approaches Land?

You are enjoying a nice day at the beach with friends when you notice that the sea gulls have abandoned their pursuit of food and left the area. The undulating sound of the waves ceases or slows and you know that something is wrong. The water's edge is quickly retreating into the depths of the ocean revealing shoreline you wouldn't see at the lowest of tides. The horizon grows and you see it, the giant swell of water rushing into shore. Chances are, you will die in a tsumani today, you just don't know it yet.

Displacement of Water
- Tsunamis generally begin far out in the ocean as a result of an earthquake or a seismic disruption. The tectonic plates under the ocean shift. Hundreds of millions of gallons of seawater are heaved outward, causing a ripple. The displaced water races across the ocean at speeds that match or exceed that of jet planes. In the ocean, this volume of water may only reach a height of 3 feet. The length may be several miles, as there is nothing standing in the way of the wave.
Rippling
- The ripple travels in all directions until it hits land.
  
  The ripple created by this massive movement of water spreads outward in all directions, much in the same way that a pebble thrown into a pond would create. The ripple travels across deep ocean water, indiscriminately moving outward and growing larger in size. Theoretically, the same seismic event could cause tsunamis on opposite sides of each other, as this ripple spreads outward towards shore.
Shoaling
- As the edge of the first wave reaches shallow waters, it slows down, fighting its way through the narrow channels of the seashore. The back of the tsunami, however is still traveling at breakneck speeds, forcing the leading edge of the wave to rise up as a swell. This is called shoaling and results in one of the most devastating natural forces on Earth.
Change in Area, Not Volume
- The tsunami maintains a constant volume of water pushing outwards. In the deeper parts of the ocean there is enough room to spread out the volume of the moving water. Once the water reaches the shallow seashore, the area of the spread is reduced, but the volume of the water moving remains the same. Forcing the same volume of water into a smaller area causes the water to rise. The volume of water that was moving swiftly several miles under the ocean now is moving in depths of feet instead. The wind-driven waves you see normally at the beach are pulled into the mammoth tsunami as it approaches land. If you are on the beach and close enough to see this retreat of the waves, it is seconds away from crashing into the shore. The impact alone will crush trees, walls, cars and unfortunate beach-goers with a brute force.

Nature