Earthquakes
| Earthquakes (i.e. Indian Ocean, 12.26.04): Seismic disturbances can cause sudden rises of the earth's crust under the ocean, raising the water level and pushing away water. Usually moving in all directions away from the underwater crust movement, the water level will rise or fall as water is displaced by the creation of an underwater valley or mountain. Occasionally, water will be pushed more in one direction than another, depending on movement of earth's crustal plates. Tsunamis are also called 'seismic sea waves' because earthquakes located beneath the ocean floor may generate tsunamis. Strike-slip faults, occurring when plates rub against each other, rarely cause tsunamis. The 1998 Papua, New Guinea strike-slip fault earthquake killed 3000 people because the epicenter was so close to an enclosed lagoon. The 1906 earthquake in California did not cause a tsunami because it was a strike-slip quake far enough away from an open coastline to cause no substantive waves. Subduction faults ('dip-slip faults'), most often located near the boundaries of continents (i.e., Pacific Rim), cause most tsunamis because of the amount of pressure applied to the water as a plate slips upward in vertical or reverse displacement. Denser seismic plates are subducted under the lighter plates, which are often continental plates. The earth is pushed up or down in vertical columns, in turn, pushing the water up in vertical columns or sucking it down in columns. Pushing away from the subduction point, bands of tsunami waves then ripple, sometimes reaching every coast in the world. Watch a computer animation of the Hokkaido earthquake tsunami [2.4MB QuickTime movie]. Use the animation to understand the formation of the waves and how their collapse generates energy. (Developed by Takeyuki Takahashi of the Disaster Control Research Center, Tohoku University, Japan.) |