The most common of tsunamis are underneath quakes. To understand underwater-earth quakes, you may first understand plate tectonics. The theory of plate tectonics suggests the lithosphere or top layer of the Earth, is made up of a series of huge plates. There plates make up the continents and seafloor. They rest on underlying viscous layer called the asthenosphere.
Think of a pie cut into eight slices. The pie crust would be the lithosphere and the hot sticky pie filling underneath would be the asthenosphere. On the Earth, these plates are constantly in motion, moving along each other at a speed of 1 to 2 inches (2,5 centimeters) per year. The movement occurs most dramatically along fault lines (where the pie is cut). These motions are capable of producing earthquakes and volcanism, which when they occur at the bottom of the ocean, are two possible sources of tsunamis.
When two plates come into region known as a plate boundary, a heavier plate can slip under a lighter one. This is called subduction. Underwater subduction often leaves enormous “handprint” in the form of deep ocean trenches along the seafloor. In some cases of subduction, part of the seafloor connected to the lighter plate may “snap up” suddenly due to pressure from sinking plate. This result is an earthquake. The focus of earthquake is the point within the Earth when the rupture first occurs, rock break, and the first seismic is the point on the seafloor directly above the focus.
When this piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water. The energy pushes to the water upward above normal sea level. This is the birth of a tsunami. The earthquake that generated the December 26, 2004 tsunami in the Indian ocean was 9.0 on the richer scale – one of the biggest in recorded history.
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