Examples of Tsunamis generated by Earthquakes

April 1, 1946 - A magnitude  7.3  earthquake occurred near Unimak Island in the Aleutian Islands west of Alaska, near the Alaska Trench.  Sediment accumulating in the trench slumped into the trench and generated a tsunami.  A lighthouse at Scotch Gap built of steel reinforced concrete was located on shore at an elevation of 14 m above mean low water.  The first wave of the tsunami hit the Scotch Gap area 20 minutes after the earthquake, had a run-up 30 m and completely destroyed the lighthouse.  4.5 hours later the same tsunami reached the Hawaiian Islands after traveling at an average speed of 659 km/hr.  As it approached the city of Hilo on the Big Island, it slowed to about 47 km/hr (note that even the fastest human cannot run faster than about 35 km/hr) and had a run-up of 18 m above normal high tide. It killed 159 people (90 in Hilo) and caused $25 million in property damage.

May 22, 1960 - A magnitude 8.6 earthquake occurred  along the subduction zone off South America.  Because the population of Chile (movie clip) is familiar with earthquakes and potential tsunamis, most people along the coast moved to higher ground.  15 minutes after the earthquake, a tsunami with a run-up of 4.5 m hit the coast.  The first wave then retreated, dragging broken houses and boats back into the ocean.  Many people saw this smooth retreat of the sea as a sign they could ride their boats out to sea and recover some of the property swept away by the first wave.  But, about 1 hour later, the second wave traveling at a velocity of 166 km/hr crashed in with a run-up of 8 m.  This wave crushed boats along the coast and destroyed coastal buildings. This was followed by a third wave traveling at only 83 km/hr that crashed in later with a run-up of 11 m, destroying all that was left of coastal villages.  The resulting causalities listed 909 dead with 834 missing.  In Hawaii, a tsunami warning system was in place and the tsunami was expected to arrive at 9:57 AM.  It hit at 9:58 AM and 61 people died, mostly sightseers that wanted to watch the wave roll in at close range (obviously they were too close).  The tsunami continued across the Pacific Ocean, eventually reaching Japan where it killed an additional 185 people.  March 27, 1964 - The Good Friday Earthquake in Alaska had a magnitude of 8.5 on the Richter Scale.  This earthquake also occurred along the subduction zone, and as we saw in our study of earthquakes, caused deformation of the crust where huge blocks where dropped down as much as 2.3  m.  Because the coastline of Alaska is sparsely populated, only 122 people died from the tsunami in Alaska.  With a tsunami warning system in place in Crescent City, California, all the townspeople moved to higher ground.  After watching four successive waves destroy their town, many people returned to the low lying areas to assess the damage to their property.  The fifth wave had the largest run-up of 6.3 m and killed 12 people. September 2, 1992 - A magnitude 7 earthquake off the coast of Nicaragua in Central America occurred along the subduction zone below the Middle America Trench. The earthquake was barely felt by the residents of Nicaragua and was somewhat unusual.  A 100 km-long segment of the oceanic lithosphere moved 1 m further below the over riding plate over a period of two minutes.  Much energy was released but the ground did not shake very much.  Seawater apparently absorbed some of the energy and sent a tsunami onto the coast.  Residents had little warning, 150 people died and 13,000 people were left homeless.

How do tsunamis differ from other water waves?

Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.
As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.

What is a Tsunami?

Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.
A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water.  Tsunamis are sometimes called "seismic sea waves", although, as we will see, they can be generated by other mechanisms than earthquakes.  Tsunamis have also been called "tidal waves", but this term should not be used because they are not in any way related to the tides of the Earth.  Because tsunamis occur suddenly, often without warning, they are extremely dangerous to coastal communities.

Physical Characteristics of Tsunamis                             top...
All types of waves, including tsunamis, have a wavelength, a wave height, an amplitude, a frequency or period, and a velocity.
Wavelength is defined as the distance between two identical points on a wave (i.e. between wave crests or wave troughs). Normal ocean waves have wavelengths of about 100 meters.  Tsunamis have much longer wavelengths, usually measured in kilometers and up to 200 kilometers