After the earthquake and tsunami disasters which occurred in Southeast Asia in 2004 and Japan in 2011, the relationship between earthquakes and tsunamis has become both a subject of great popular concern and new scientific research. In coastal regions, tsunamis caused by earthquakes can cause far more damage than the earthquake itself, inundating miles of coastline and destroying entire cities. This makes predicting and planning for tsunamis an urgent public priority for coastal cities, especially in zones that are prone to earthquakes, like the Pacific coast.
In general, the relationship between earthquakes and tsunamis is actually quite easily understood. An earthquake is a tremor, ranging from barely perceptible to extremely violent, which occasionally occurs along the boundaries of tectonic plates: the massive subterranean rock plates which form the surface of the Earth. The plates drift along the surface of the planet at an extraordinarily slow speed. At the boundary where one plate meets another, sometimes they catch against one another. Pressure builds until the rock gives way, causing an earthquake. The Pacific coasts of North America and Asia, where several such boundaries occur, are particularly prone to earthquakes.
When this quake occurs underwater, the sudden deformation of the ocean floor can suddenly displace an enormous quantity of water. The sea floor may shift sharply upward, pushing water in all directions, or sink downward, drawing water inward. In either case, according to the Earth sciences program at the University of Washington, waves are created as water moves under the influence of gravity back toward equilibrium. If water was pushed up by the sea floor during an earthquake, this means large waves will travel outward as a result of the water suddenly being displaced at the site of the earthquake. These large waves are known as tsunamis. According to Dan McKenzie and James Jackson of Cambridge University, there is a high risk of large tsunamis whenever an underwater earthquake shifts the ocean floor more than 30 feet along a fault line.
Tsunamis are usually measured by the height above normal level of the waves when they reach the shore. Traveling through the deep ocean, the waves move extremely quickly and are seldom dangerous to ships on the surface. As they move toward the shore, however, the ocean grows shallower and the tsunami begins to climb above the surface of the surrounding water. Dangerous tsunamis can come ashore at a height of a dozen feet or more above normal water levels. The waves which struck Indonesia in 2004 and Japan in 2011 were reported to be as much as several dozen feet high in some of the hardest-hit areas.
Tsunamis can be caused by any sudden displacement of water at sea, and earthquakes under the ocean floor do not necessarily result in large tsunamis. The largest tsunamis, however, are usually caused by so-called megathrust earthquakes occurring in subduction zones - regions were one tectonic plate is sliding beneath another one, rather than side by side. All of the major tsunamis in recent years were caused by megathrust earthquakes. The Smithsonian Institution has prepared an online video description of the relationship between a megathrust earthquake and the Japanese tsunami of 2011, and the University of South Carolina's Tsunami Research Group says that a similar earthquake was responsible for a tsunami in Alaska in 1964.
It is widely believed that the greatest earthquake-related danger to the Pacific northwest, in both the United States and Canada, is posed by a tsunami caused by a large offshore megathrust earthquake.