Tsunami (Chapter 5)
Tsunamis (also known as seismic sea waves) are very large ocean waves triggered by displacement of water by by earthquakes, volcanoes, landslides or, very rarely, meteor/asteroid impacts.
Characteristics of tsunami
Tsunami generally occur as multiple waves
Tsunami wavelengths average 370 km
In deep water, tsunami amplitudes are about 180 km (about ½ the wavelength), so they affect the ocean floor at all depths.
In deep water, most of the wave is below the sea surface. Tsunami extend only a few meters. Given the large wavelengths, the slope of the tsunami in deep water is very shallow.
Tsunami travel at very high speeds, as high as 870 km/hr. This is a fast as many jet airplanes.
In deep oceans, tsunami are barely noticeable. Because tsunami extend to deep parts of the ocean, they are pushed to large heights when they encounter shallow water near coastlines. When water shallows, tsunami waves are pushed up, and slow down. As a consequence, wavelengths become smaller as individual waves “catch up” to one another. Large wave heights (wave crests) are matched by deep wave troughs. As a tsunami crest approach the shore, they are preceded by a trough where the bottom of the trough is below the normal tide level of the coastline. Consequently, water drains into the trough and the sea appears to recede prior to the tsunami.
Wave refraction: Interaction of a wave with the sea floor always slows the wave down. If a wave approaches a shore line at an angle, the part of the wave closest to the shore slows down and falls behind the part of the wave further from the shore. In this way, the wave bends and refracts. In this way, large waves like tsunami can turn and strike a seemingly sheltered side of a coast. This is how the west coast of Sri Lanka was affected by the east-west traveling 2004 tsunami. Web Animation (see hoe the tsunami bends around Sir Lanka and India
Causes of tsunami.
Earthquakes: Mostly earthquakes associated with subduction zones. In these quakes, downward movement of the subducting slab temporarily stops as the plates become locked. As stress builds, a coastal bulge develops as land directly behind the trench uplifts (figure 5-37) as the over riding plate bends. When the quake occurs, the overriding plate snaps back to it original shape, triggering a tsunami (STUDY FIGURE 5-37) Web Animation
Modern examples:
Sumatra, 2004
Anchorage Alaska, 1964
Chile, 1960
Ancient examples:
Cascade region, Geologic evidence including, wave generated sand layers and sheared trees, indicate a massive tsunami ca. 1700. Massive subduction zone quakes, similar to the 2004 Sumatra quake, periodically occur along the Washington and Oregon coasts. A coastal bulge developing off the coasts of Oregon and N. California suggests that stress is building up in the subduction zone, but the exact timing of the next earthquake is unknown.
Landslides: massive landslides can trigger tsunami if large masses of land slide rapidly into the sea. Evidence includes massive slided from the islands of the Hawaiian chain.
Volcanoes: Large explosive eruptions of volcanic islands. Example: Krakatau, 1883. This explosive eruption generated a tsunami that caused 35,000+ fatalities, mainly in Java and Sumatra. this was a very large caldera forming (collapse) eruption.
Meteor and asteroid impacts: Impact of an asteroid or large meteor in the ocean can cause large displacements of water and, subsequently, extremely large tsunami. Evidence, massive, wave generated sand layers along the Cretaceous-Tertiary boundary in the southeastern U.S.
LINKS
A WHOLE BUNCH OF INFO ON THE 2004 SUMATRA TSUNAMI
NATIONAL TSUNAMI HAZARD MITIGATION PROGRAM
