-Volcanic Eruptions have the potential to create awesome, enormous, potentially lethal tsunamis.
"Although relatively infrequent, violent volcanic eruptions represent also impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area. According to this mechanism, waves may be generated by the sudden displacement of water caused by a volcanic explosion, by a volcano's slope failure, or more likely by a phreatomagmatic explosion and collapse/engulfment of the volcanic magmatic chambers. One of the largest and most destructive tsunamis ever recorded was generated in August 26, 1883 after the explosion and collapse of the volcano of Krakatoa (Krakatau), in Indonesia. This explosion generated waves that reached 135 feet, destroyed coastal towns and villages along the Sunda Strait in both the islands of Java and Sumatra, killing 36, 417 people. It is also believed that the destruction of the Minoan civilization in Greece was caused in 1490 B.C. by the explosion/collapse of the volcano of Santorin in the Aegean Sea." (kaitoku)
-quick shifts in underwater soil may cause or amplify tsunamis
"Less frequently, tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps. Such events may be caused impulsively from the instability and sudden failure of submarine slopes, which are sometimes triggered by the ground motions of a strong earthquake. For example in the 1980's, earth moving and construction work of an airport runway along the coast of Southern France, triggered an underwater landslide, which generated destructive tsunami waves in the harbor of Thebes. Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that the 1998 tsunami , which killed thousands of people and destroyed coastal villages along the northern coast of Papua-New Guinea, was generated by a large underwater slump of sediments, triggered by an earthquake. In general, the energy of tsunami waves generated from landslides or rock falls is rapidly dissipated as they travel away from the source and across the ocean, or within an enclosed or semi-enclosed body of water - such as a lake or a fjord. However, It should be noted, that the largest tsunami wave ever observed anywhere in the world was caused by a rock fall in Lituya Bay, Alaska on July 9, 1958. Triggered by an earthquake along the Fairweather fault, an approximately 40 million cubic meter rock fall at the head of the bay generated a wave, which reached the incredible height of 520-meter wave ( 1,720 feet) on the opposite side of the inlet. A initial huge solitary wave of about 180 meters (600 feet) raced at about 160 kilometers per hour (100 mph) within the bay debarking trees along its path. However, the tsunami's energy and height diminished rapidly away from the source area and, once in the open ocean, it was hardly recorded by tide gauge stations." (kaitoku)
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Sources:
UNESCO Intergoernmental Oceanographic Commission & International Tsunami Information centre & Laboratoire de geophysique, France & U.S. National Oceaninc & Atmospheric Administration. "Great Waves." June. 2005.
[http://ioc3.unesco.org/itic/files/great_waves_en_2005_small.pdf]
Tammy Kaitoku. "How Do Volcanic Eruptions Generate Tsunamis?" 23 March 2005. [http://ioc3.unesco.org/itic/contents.php?id=159]
Tammy Kaitoku. "How Do Submarine Landslides, Rock Falls and Underwater Slumps Generate Tsunamis?" 23 March 2005. [http://ioc3.unesco.org/itic/contents.php?id=160]