July 9, 1958 in Lituya Bay, Alaska, an earthquake caused what would be the biggest tsunami in history, with a wave reaching 1,720 feet tall. On May 22, 1960, an earthquake off the coast of Chile triggered a tsunami that killed 200 people. More than 122 people lost their lives to an earthquake-induced tsunami in Prince William Sound of Alaska on March 28, 1964. More recently, on March 11, 2011, approximately 24,000 people were confirmed dead or missing in Japan due to a tsunami. What causes a tsunami? Why are they so deadly?
What causes a tsunami?
Tsunamis are caused by motion that occurs on the ocean floor, such as an earthquake. These earthquakes happen as oceanic plates push up against each other until they become stuck. Instead of releasing pressure gradually by being free to move, when the plates get stuck the pressure continues to build tremendously.
These plates can be stuck for decades or centuries, unable to release the energy, until it finally gives. When this happens, it causes a great motion, much like placing a board over a knee and applying pressure until it snaps. This motion sets off waves in both directions from the epicenter. As the waves move, they gain speed and size until they come crashing onto land, leaving a path of destruction.
There are many factors that determine the severity of the earthquake and resulting tsunami. NOAA’s National Weather Service says, “The tsunami generating process is more complicated than a sudden push against the column of ocean water. The earthquake’s magnitude and depth, water depth in the region of tsunami generation, the amount of vertical motion of the sea floor, the velocity of such motion, whether there is coincident slumping of sediments and the efficiency with which energy is transferred from the earth’s crust to ocean water are all part of the generation mechanism.”
Tsunamis can travel at speeds of 500 mph or more. As the energy and subsequent speeds escalate through the waters, it also slows as it approaches the shore, to approximately 45 mph. However, when it hits a coastline, that energy is concentrated into a rather small area when compared to the vast amount of room it had while traveling in the ocean waters.
As the water shallows when approaching land, the waves have nowhere else to go but straight up, causing the extremely high waves we associate with tsunamis. It’s that power and height combined that causes so much damage, and often, death. Sometimes, the waves can crash down up to a mile inland, leveling towns in it’s path.
Because of the many fault lines, the Pacific Ocean is especially prone to earthquakes and tsunamis. While most people are familiar with earthquakes and have experienced several in their lifetime, tsunamis are less familiar. They don’t happen as often as earthquakes do, therefore, they don’t get as much exposure until a serious one happens.