The name tsunami comes from two Japanese words, tsu means harbor and nami means wave, so tsunami means harbor wave. A tsunami is not usually found being created in a harbor though as most often tsunamis are caused by a displacement of water caused by a magnitude seven or eight undersea earthquake. What would be considered a “noticeable” tsunami would be of about a seven on the Richter scale while for an undersea earthquake to cause significant damage to inland areas it would most often have to be an eight or greater.
While the undersea earthquake is the most common reason for a large tsunami they can also be caused by other factors such as underwater landslides, volcanic eruptions and even a large explosion of some kind. Vertical upheaval of the ocean’s floor is also another element most often seen in large tsunamis. This is why tsunamis are not often seen along the San Andreas Fault in California, as the plates there often slide horizontally not vertically. In the case of horizontal shifting this has almost no affect on displacing water and will cause no significant noticeable changes to the area.
The only exception that can be noted from a horizontal shift is if it causes an underwater landslide of significant proportions, then this may cause tsunami. Most often tsunami occurring earthquakes happen at what are called subduction zones, a subduction zone is the juncture where two lithospheric plates come together, one atop the other in most cases. The younger of the two plates will be less dense than the older plate and is often the one that will ride over the edge of the older plate which can sometimes include it being thrust vertically causing a tsunami to form.
The subduction zone plates have also been known to shift as much as 60 feet such as happened with the Sumatra-Andaman earthquake on December 26, 2004, the India plate slipped below the Burma plate what is suspected to be of about 60 feet causing tsunami waves estimated at 80 feet tall to hit off the northern coast of Sumatra killing an estimated 300,000 people. While the United States did not see significant damage the water levels on the east and west coasts did fluctuate significantly.
While the United States has not often seen tragedy in such staggering numbers due to tsunami there have been deaths related to tsunami in both California in 1964 and Hawaii in 1960. Today the United States does have a tsunami warning system in the hopes that these tragedies will never again be seen here.