While writing this article, my father sent me a chilling text message saying a 9.0-magnitude earthquake (according to the US Geological Survey) had just hit the Pacific Ocean near Northeastern Japan on March 11. Tsunami alerts were promptly sounded in 50 countries from Alaska to South America. However, nothing could adequately brace the people of Japan along the east coast for what was to come.
It would have been the most natural thing for the Japanese to write this off as ‘one of the usual’ tsunami alerts. After all, they are accustomed to these things living in one of the most active seismic areas in the world where over a thousand quakes of at least 4.0 magnitudes have hit in and around the country since 2009.
However, this occasion was different. It was a quake of unprecedented proportions. The waves that were launched as a result of the March 11 earthquake (the fifth largest recorded quake since 1900) reached towering heights of 10 metres, eviscerating the northeast coast of Japan leaving over 15,000 dead and over half a million homeless in its wake .
Within the next three days, hundreds of aftershocks were experienced in the Honshu Island alone (the largest island of Japan). The brute force of the tsunami pummelled cars like tin cans, uprooted trees and laid waste hundreds of homes—tearing them from their foundations. Cranes had snapped in two, sign posting what was left of thriving manufacturing centres. Sirens wailed and thick plumes of smoke poured from various factories.
In some areas, the advancing wall of water looked like a black shroud of sludge and debris, churning everything in its path into mush. And as Japan began to grasp the scale of devastation from the earthquake and tsunami, it also had to contend with explosions at and radiation leaks from its nuclear plants. The resultant loss in energy has also left the country with blackouts and power outages.
Elsewhere, the tsunami’s far reaching effects were observed as it raced across the ocean, flooding low-lying areas in Hawaii and striking northern California, sweeping people out to sea, damaging boats and wrecking the harbour of Crescent City. Peru experienced a surge in wave height and over 300 homes were damaged. Most other countries experienced larger-than-normal waves but suffered little or no damage.
While tsunamis like the ones as a result of the Japan and Sumatra quakes overshadow other tsunamis in recent history, it’s clear to see that they have followed the same basic sequence of events and left behind a trail of destruction and death.
By the same token, however, numerous people have survived to tell their tale by understanding the way tsunamis are formed and learning to look out for the give-away signs of its impending arrival. You must be wondering just exactly what gives rise to tsunamis and what some of the give-away signs are. Well, you’ll be pleased to know that I’ve detailed them in the next bit…
Formation of Tsunamis
The word, ‘tsunami’ was coined from the Japanese words tsu (harbour) and nami (waves). Sometimes they are incorrectly referred to as ‘tidal waves’ but tsunamis are not caused by the tides (which are affected by the gravitational force of the moon on the sea).
Rather, they are caused by underwater earthquakes. To understand underwater earthquakes, let’s briefly explore the theory of plate tectonics. The top layer of the earth which is made up of a series of huge plates moves most dramatically along fault lines. These plates make up the seafloor and the continents.
When two plates come into contact at a region known as a plate boundary, a heavier plate can slip under a lighter one through a process called subduction. In some instances, this subduction can result in part of the seafloor connected to the lighter plate to ‘snap up’ due to pressure from the sinking plate.
This results in an earthquake. The energy from the earthquake vertically jolts the seabed, displacing a large body of water. The energy of that force is transferred to the water. This pushes the water above the normal sea level and gravity disperses the energy horizontally, creating waves that do not increase much in height in ocean depths but which stack up to towering heights as they travel inland and the depth of the ocean decreases.
Their speed is phenomenal (up to 965km/hr in some cases), travelling as fast as jet planes over deep waters. And their ability to maintain such speeds is directly influenced by the depth of the water, moving faster in deeper water and slower in shallower water.
Typically, a tsunami is no more than one metre high in ocean depths. However, once it starts closing in on the shore, it takes on a deadly form. The topography of the seafloor and shape of the shore influence the tsunami’s appearance and behaviour.
The shallow water and coastal land compresses the energy travelling through the tsunami, forcing the water upward as it slows down considerably from say 965 km/hr to around 50 km/hr, and rises to dizzying heights of up to 30 metres above sea level!
The shore will experience a stark rise and fall of water when a tsunami is about to hit. Sometimes the coastal water will disappear as it gets drawn into an onrushing tsunami. When it makes landfall and the first tsunami wave crashes ashore, the wave pushes a large amount of water onto the shore above sea level causing tremendous damage inland. Other waves can follow from anywhere between five to 90 minutes after the initial strike.
Thanks to scientists, there are now warning centres established to continuously monitor seismic events and changes in tide level. Also out in the ocean, there are open-ocean buoys and coastal tide gauges that relate information to stations within a particular region. Tide stations record sea level changes while seismograph stations record earthquake activity. There are also pressure recorders that sit on the ocean floor to detect changes in the overlying water pressure.
While tsunamis cannot be stopped once they are in motion, these measures help us make a fairly accurate tsunami risk assessment. Thereafter we can make informed decisions on the need to notify civil defence agencies, set off sirens and implement the necessary evacuation procedures via radio and television announcements.
Some countries have even opted for structural measures such as sea walls, shelters and ‘quake-resistant’ buildings to protect lives, properties and significant infrastructures from tsunami. However, these measures offer scant protection in the face of larger tsunamis (40% of Japan’s coastline which is lined with concrete seawalls, is a case-in-point).
In any case, once an alert has been sounded or that all the visual cues (like the retreating of coastal water, experiencing one abnormally point to an impending tsunami, coastal residents should quickly head inland and to higher ground.
Despite being versed in everything tsunami-related, there will still be some individuals that will probably chase after tsunamis—ones that will probably jump in a boat and head out to sea in an attempt to catch the action up close in person. But for the vast majority of us who know better than to throw caution to the wind, I daresay that while your knowledge and respect for tsunamis may not guarantee your safety, you’re still better off knowing these things as they can boost your chances of survival in the likelihood of an impending tsunami.