The rapid expansion of humanity’s technological capabilities over the last 100 years has gifted the populations of the world’s developed nations with a staggering amount of quickly available information. Not least amongst this is the ability to learn of horrendous natural disasters occurring in widely diverse regions of the planet mere moments after they happen. This has also resulted in a globalization of terms; what was almost always called a tidal wave in the English speaking nations of the world in the 1960s is predominantly referred to as a tsunami now.
Tsunami is a Japanese word that means “harbor wave.” While this may sound mild, technically it is a very good description. While flowing across the ocean, tsunamis are often not all that high; despite the representations of movies such as the Poseidon Adventure. The constriction of seaside landforms such as bays and harbors compress the water into a narrower front, combined with the tidal affects that suck the seawater away from the shore prior to the wave’s arrival, this can lift the wave’s height to the terrifying degree that causes so much damage on the affected coast.
While tsunamis can be created by undersea volcanoes, gas expulsions on the seabed, the very rare large meteorite strikes, and accidents in human marine endeavors involving drilling or pipelines, the vast majority are the result of earthquakes whose epicenters lie within or close to the marine environment.
The largest recorded earthquake, occurring on May 22nd 1960, was centered in Valdivia in Chile and measured 9.5 on the Richter scale. Despite its epicenter being land based, it generated a tsunami that crossed the Pacific ocean. The earthquake killed 1655 people in Chile, the tsunami it generated killed many more in other nations; for example, 61 died in Hawaii, 138 in Japan and at least 32 in the Philippines.
As anyone who has ever been within the effective range of an earthquake knows, they cause tremors or vibrations in the solid surface of our planet. These tremors are basically wave forms flowing through the ground, slowly dissipating as they progress further from the hypocenter of the earthquake. The hypocenter is the actual focal point or source location of the earthquake event within the Earth’s lithosphere, the outer layer of the planet. The epicenter, commonly referred to in news reports, is the point on the Earth’s surface that lies directly above the hypocenter.
In general terms, the deeper the hypocenter, the less damage occurs on the Earth’s surface, although this is also dependent on the magnitude (strength) of the earthquake and the composition of the Earth’s crust above it. The seismic waves generated by an earthquake will diminish quite rapidly when traveling through granite, they may cause far more damage over a greater distance if passing through a less dense rock strata such as limestone.
Liquids readily and easily restructure themselves around waveforms. When the waveform generated by an earthquake transmutes from the solid rock of a crustal tectonic plate to the waters of the sea, it increases in amplitude and effective strength. The atoms or molecules in a liquid are further apart than those in a solid, but energy transfer is similar. The energy generated at the focal point of an earthquake will therefore travel a much larger distance through a liquid than it can through a solid, which is why the Valdivian earthquake resulted in a tsunami killing people on the other side of the largest ocean on the planet, the Pacific.
Besides the ready transference of waveform from solid crust to liquid ocean, earthquakes with an epicenter located below the ocean waves can also generate tsunamis through water displacement. An earthquake is caused by earth movement, this may result in a sharp drop in the level of the seabed or a sharp rise. Either situation can result in a tsunami. Both have a similar effect to a pebble dropped in a puddle, producing a circular ripple wave effect, although of a much higher magnitude.
When the seabed is raised the volume of water that can remain above it is reduced, as the surface level of the seas is a constant. The displaced water flows out from the raised seabed creating an expanding tsunami. When the seabed drops, it causes an inflow of seawater from the surrounding ocean to fill the hole. The inflow exceeds the space caused by the drop causing a rebound effect that again results in the creation of an expanding wavefront resulting in tsunamis striking the coastlines of neighboring land masses. The tsunamis generated by seabed drops caused by earthquakes of comparable magnitudes are usually higher and more devastating than those resulting from seabeds thrust up. Either variety can produce more significant tsunamis than underwater earthquakes that cause neither result, even when they may register as considerably more powerful on the Richter scale.
