On the afternoon of March 11th 2011 a massive tsunami was generated by an estimated 8.9 intensity earthquake that occurred out to sea about 380 km to the northeast of Tokyo. Chains of giant waves up to 10 metres high swept across the adjacent shoreline of Japan around Sendai during the afternoon, producing widespread damage and an unknown loss of life.
This is shaping up to one of the worst natural disasters of modern times.
The tsunamis struck Hawaii overnight, producing only minor damage, but some 12 hours later reached the US west coastal area of Crescent City, near Oregon, where extensive damage resulted in the local harbour.
This event was probably as severe as the infamous Boxing Day tsunami that raced across the Indian Ocean on December 26 2004.
This Indian Ocean event was one of the worst ever natural disasters in recorded history, killing more than a quarter of a million people over eleven countries and this event in Japan is also a major tsunami disaster.
In September 2009 another deadly tsunami created devastation on a smaller scale around the Pacific Island nation of Samoa.
Once again the terrible phenomenon of a tsunami has captured the worlds headlines.
What is a tsunami?
Tsunami is a Japanese word meaning “harbour wave”. A tsunami is a wave or series of waves generated in the ocean by such phenomena as earthquakes, undersea land-slides, volcanic eruptions and meteor impacts. It should not be confused with ocean swell waves, which are generated by the action of wind on the surface of the sea, or tides, which are produced by gravitational effects of the Sun and Moon.
Undersea earthquakes, as we have just seen off the coast of Japan, are common around the so called Pacific Ring of Fire, a geologically unstable region that extends from New Zealand, across the Tongan region, Papua New Guinea, Indonesia and then northwards to Japan. It then travels east across the northern Pacific and moves down the west coast of both North and south America.
The Pacific Ring of Fire
(Image from Wikipedia Commons - click to enlarge)
This zone is largely generated by the movement of the Pacific tectonic plates against other adjacent plates. It has been unusually active this year, also producing the devastating earthquake across Christchurch.
Tsunamis are sometimes referred to as “tidal waves” or “seismic waves” but both of these terms are inaccurate descriptions.
Above - The Pacific tectonic plate runs close to Japan.
(Image from Wikipedia Commons - click to enlarge)
Tsunamis have produced tremendous devastation throughout history, causing massive damage along shorelines, wrecking many coastal townships and killing untold thousands of people through drowning.
The tsunami as a shallow water wave
When an earthquake occurs near or under the ocean, a tsunami can be generated, and the characteristics of this type of wave are markedly different to the “normal” waves we are used to seeing down at the beach. These waves may break on the shore say every 10 or 12 seconds (called the “wave period”), and have a distance of around 100 to 120 metres between wave crests (called the “wavelength”).
But tsunamis may have a period of up to an hour and a wavelength of around 100km, with the second and third waves still maintaining massive power. This wavelength is very much greater than the depth of the ocean through which the wave is travelling, which for the Pacific Ocean can be around 3000 to 4000 metres.
Waves with this characteristic are called shallow water waves, which is somewhat confusing because here we are talking about the deep ocean. But “shallow” in this case is only a relative term, meaning the ocean depth (~3000m) is shallow compared with the wavelength (~100km or 100, 000m)
It can be demonstrated through the physics of wave theory that shallow water waves move at a speed which is directly proportional to the depth of the water through which they are moving, and in a water depth of 3000 m, this translates to a wave velocity of ~ 170 m/s or over 600 kph. If a wave of this type is encountered by a ship at sea, it may be barely noticed, as the wave is spread out through the entire depth of the ocean and may only form a slight disturbance on the surface, although moving at great speed.
Another characteristic of shallow water waves is that they lose energy at a rate that is inversely proportional to the wavelength – meaning the longer the wavelength the further the wave can travel. The extremely long wavelengths of tsunamis means that they can travel extended distances, in the order of thousands of kilometers, or, in other words, across entire oceans.
Tsunamis approaching the shoreline
Whilst tsunamis are markedly different from ocean waves in all the ways described, the normal physics of wave motion still applies. All waves contain a certain amount of energy that is dependant on the mass of water being displaced (which is closely related to the height of the wave), and the velocity of the wave. This energy is largely conserved, apart from a slight dissipation over time.
A tsunami surging into shallow water
(Image from Wikipedia Commons - click to enlarge)
When the wave approaches a shoreline and the water becomes shallower, the wave begins to slow as a result of friction with the ocean bed. But the overall energy of the wave stays much the same, so to compensate for the slower speed, the height of the wave ramps up.
A tsunami approaching the shore will "arc up" sometimes to great heights
Image: Wikipedia Commons
(Click on image to enlarge)
This means that a tsunami, whilst barely noticeable at sea, quickly grows in height, and cases of waves reaching heights of 30 metres are known. An entire tsunami event may consist of several waves, called a wave train, each of which can carry a major destructive punch.
The final height and shape of the wave are largely determined by the topography of the ocean bed near the shoreline. As well as the celebrated wall of water towering over the beach as is often portrayed in the movies, the wave can also present as a rapidly rising tide moving a long way inland with unstoppable force, destroying everything in its path.
A tsunami train approaching shallow water will "bunch up" but increase in height. (Image from Wikipedia Commons - click to enlarge)
A sign that a tsunami is approaching is a lengthy retreat, or “drawdown” of the ocean along the shoreline, and the water level can retreat more than 300 metres seawards of its normal position when this happens.
Immense drawback of the ocean just before the onslaught of a tsunami wave at Kata Noi Beach, Thailand on December 26th 2004. Image: Wikipedia Commons. (Click on image to enlarge)
Destruction generated by tsunamis
Tsunamis can reach the coast with tremendous amounts of energy. They can create significant shoreline erosion, stripping beaches of sand that may have taken years to accumulate and uprooting trees and other coastal vegetation. Capable of inundating, or flooding, kilometers inland past the typical high-water level, the fast-moving water associated with the breaking tsunami can easily crush homes and disrupt coastal infrastructure such as powerlines and roadways.
As we have seen with the Japanese disaster, after crossing a built up area, tsunamis carry a massive volume of debris with them, and this acts as a colossal "grinder", scouring existing structures away, which then becomes part of the moving debris mountain.
A devastated town on the coast of Sumatra, seen just after the Boxing Day tsunami 2004, graphically illustrates the enormous power of a tsunami. Image: Wikipedia Commons. (Click on image to enlarge)
Tsunami prone areas and warning systems
Throughout history, tsunamis have been recorded in most of the oceans of the world. However because of inherently unstable geological conditions, the Pacific Ocean is particularly notorious, with frequent earthquakes occurring around the Pacific Rim.
No formal warning system was in place until 1960, when the devastation caused by a massive tsunami in Chile during May of that year led to the formation of the Pacific Tsunami Warning System (PTWS) located in Hawaii. This group organises and monitors a network of earthquake detectors and tide gauges which determine where earthquakes occur and whether or not a tsunami may have been generated.
After a major earthquake in Alaska in 1964, the PTWS concept was extended and the International Tsunami Warning System (ITWS) was created. Following the disastrous Boxing Day tsunami of 2004, this system was extended to cover the Indian Ocean.
As soon as an earthquake is detected, it is evaluated as to its location and intensity, and if it is thought that a tsunami may have been generated, calculations are made which estimate the speed of propagation of the wave, based on the depth of the ocean. Warnings are then issued to all countries in the affected area, and depending on the lead-time available, emergency preparations are begun.
The Japanese tsunami was particularly well photographed, including some unique footage of the wave trains approaching the shoreline taken from helicopter. This vision will be closely analysed by experts in an attempt to learn more about these massive and lethal monsters of the deep.
Extraordinary footage showing the tsunami slamming through the port of Kamaishi, in northeastern Japan can be seen here:
An untested hypothesis
An interesting hypothesis has been advanced to explain the "disaster clusters" we sometimes see - this year we have seen severe flooding in Australia, the massive Christchurch earthquake and the the Japanese tsunami - could these be connected in some way?
During times of strong El Ninos and La Ninas sea levels rise for prolonged periods over either the western or eastern sides of the tropical Pacific Ocean. Could this colossal extra weight of water be enough to disturb the Pacific plate and trigger periods of high seismic activity? The strong La Nina we've experienced this year has produced record flooding across much of eastern Australia but could it have also triggered the Christchurch and Japanese disasters?
This hypothesis is totally untested but it could help explain the confluence of natural disasters we sometimes see.
“Disasters, Events and Moments that Changed the World”, Richard Whitaker, New Holland Publishing, 2007