Here is a tsunami warning sign at the beach one block from our house.
That green round thing in the picture at the end of the lollipop is Mauao, a large distinctive hill rising 232 meters above the surrounding ocean. The lighter area on the map is the spit of land we live on which is only several meters above sea level. The sign says a tsunami may arrive fifty minutes after a major earthquake. It also says watch for signs of the ocean level rising and falling rapidly, or the ocean making loud groaning sounds like a jet aircraft! In case of a tsunami, the emergency plan is for us to rush over to the tall hill, Mauao, and climb up it.
A tsunami occurs when the ocean floor is disrupted by sudden violent activity such as an earthquake, volcanic eruption, a giant undersea landslide or even a meteorite striking the sea. This causes an upheaval in the ocean surface, often far out at sea, and creates a series of fast moving waves (termed a wave train) which radiate outward similar to when a pebble is thrown into a pond.
The undersea event that initiates a tsunami often occurs in the areas where the tectonic plates meet. In a process termed subduction, at their junctures these giant earth plates slip under each other causing a tremendous buildup of pressure and subsequent release often resulting in earthquakes.
The earthquake or the initiating event can occur deep under the crust of the earth at a depth of several hundred miles. In these cases, the surrounding magma and rock absorb much of the energy. More dangerous are seismic events that occur at shallower depths, for example perhaps 18 miles under the surface.
As stated above, giant undersea landslides, pyroclastic flows from volcanoes, and meteorites landing in the ocean can also produce tsunamis by what is termed mass effect. Basically this is like when you drop a large rock into a pond—in this case a large rock into a large pond—and waves spread outward from the disturbance.
It is estimated that a meteorite 200 meters in diameter landing in the ocean could produce a tsunami 50-100 meters high. The meteorite that created the famous Chixulub Crater in the Yucatan Peninsula, Mexico 65.4 million years ago and which extinguished the terrestrial dinosaurs was estimated to be 10 km in diameter. What is the chance of something like this happening again? Not as remote as you think. And statistically more likely than you winning the lottery.
It is important to understand that in a tsunami, not just the top of the water is moving as when we see waves on the ocean surface, but the whole ocean from top to bottom is moving.
In the example of a pebble thrown into a pond, the wavelength, the distance between wave crests, may be only several inches. A tsunami’s wavelength, however, can exceed 60 miles (100 km). To say the same thing in a different way, one wave can occur and the next in the wave train may not occur until an hour later. This can lead people on a shoreline already devastated by an initial wave to be lulled into complacency. The first wave to arrive may also not be the largest.
Far out at sea, a tsunami may not even be seen or felt. Because of the depth of the ocean, a ship might just feel a slight swell or nothing at all.
But as the tsunami approaches a shoreline, the shallow water compresses the wave causes a braking effect. The wavelength decreases while the amplitude (height of the wave) grows enormously. The word, tsunami, is actually made up of the two Japanese words, ‘tsu’ for harbor, and ‘nami’ for wave. In the past, Japanese fishermen would encounter nothing unusual while sea fishing, only to return to find their harbor and village devastated by a huge wave.
Waves can be up to 100 feet (30 meters) high, as high as a ten story building.
Depending on various factors, rather than the classic giant wave, a tsunami may also present at the shoreline simply as a rapidly rising tide or sea level. The term ‘tidal wave’ however is a misnomer, since as you now understand, tsunamis have nothing to do with the tides.
The effect and timing of a tsunami on a given shoreline depends on the strength of the underlying event and its location. A nearby seismic event can trigger a tsunami that arrives in minutes. A larger disturbance is necessary to cause damage at greater distances.
On the shoreline, the signs of a tsunami are ominous. If the first part of the wave to arrive at the shore is a ridge, then sudden flooding or a massive breaking wave will be seen. However if a trough arrives first, a drawback of the water will occur exposing normally submerged areas of the sea leaving the ocean bottom bare for hundreds of meters and allegedly fish flapping on the dry ocean bottom.
One person said it’s like someone pulling the plug, akin to a stopper in a bathtub, on the ocean itself and the water draining out. A giant sucking sound has also been described. If you see this occur, it is a sign to immediately rush to higher ground because usually within 12 minutes, the ridge will arrive and water sweep up onto the land.
The sound of a tsunami has been described as that of jet aircraft or as multiple freight trains.
Low-lying heavily populated coastal areas are obviously particularly vulnerable. Most recently, the 2004 Indian Ocean tsunami was among the deadliest killing 290,000 people in 14 different countries bordering the Indian Ocean. In some places the initial surge was approximately 33 meters (108 feet) in height.
The Pacific Tsunami Warning Center in Honolulu monitors seismometers throughout the Pacific Ocean in an attempt to predict tsunamis based on earthquake acitivity and warn at-risk countries. Unfortunately, at that time there was no organized alert system in place covering the Indian Ocean to warn people.
How about the evidence that animals have a sixth sense about an approaching tsunami and escape to higher ground? It remains unproven.
During World War II, New Zealand actually attempted to use tsunamis as a weapon by using explosions to create small tsunamis but without success.
All countries bordering the Pacific Ocean (80% of tsunamis occur in the Pacific) are particularly at risk including New Zealand. In the area in which Rebecca and I now live, the “Tsunami hazard for the Bay of Plenty and eastern Coromandel Peninsula, NIWA Client Report, June 2004,” presents geological evidence of at least two large tsunamis in the last 4000 years, with wave heights larger than 5 meters. In more recent times since 1840 tsunamis have affected areas along Bay of Plenty at least eleven times. Some have been small (less than .5 meters in wave height) only detected by sea-level gauges. Larger more distant seismic events in 1868, 1877, 1883 and 1960 have caused run-up heights of up to 1.8 meters in the Bay of Plenty region.
The same report also lists the areas of highest vulnerability including “Open coast between Mt. Maunganui/Mauao and Maketu”, the area in which we live.