Tsunamis • Vol. 17: #47

The word ‘tsunami’ comes from the Japanese phrase meaning ‘harbor wave.’ On average, two tsunamis occur per year throughout the world capable of causing numerous deaths and catastrophic damage to cities, villages and coastal landscapes. On average a destructive ocean-wide tsunami occurs about once every 15 years. This week Tidbits explores the facts about these devastating natural phenomena and what causes them to occur.

• Tsunamis are giant waves generated by any large displacement of the sea level, usually by earthquakes, landslides, and volcanic eruptions.

• Undersea volcanoes heave up massive amounts of lava, forming unstable islands prone to collapse. Oceanographic surveys show that the 1,360-mile-long stretch of islands from Hawaii to Midway Island in the South Pacific has experienced over 70 mammoth underwater landslides, some of which are over 200 miles in length. One such slide occured thousands of years ago north of Oahu and measures 150 miles long. It covers 14,300 square miles and is over a mile thick, making it one of the largest underwater avalanches ever discovered. The tsunami generated by this single event would likely have reached heights of 65 feet along the entire western coast of the U.S.

• Undersea landslides are particularly common since the end of the last Ice Age, when sea levels rose by about 425 feet inundating shorelines that hadn’t previously been underwater. These now-saturated areas are particularly prone to landslides and collapse.

• Submerged volcanoes release super-heated lava which causes the cold sea water to instantly erupt into steam and rise to the surface in an enormous heave. When the huge volcano on the Indonesian island of Krakatoa erupted in 1883, hundreds of millions of gallons of sea water suddenly rushed into the massive underwater magma chamber.

The resulting explosion was so violent that the sound impact was heard some 2,000 miles away in Australia. The tsunamis that immediately followed killed more than 36,000 people living on Krakatoa and surrounding islands. Significant other effects were also felt around the world in the days and weeks following, including high tides, rough seas, and seismic activities.

TSUNAMI ANATOMY

• Normal waves caused by surface winds rise, break, and retreat, followed by a succession of waves that are an average of 300 feet from crest to crest (called the wavelength). However, tsunami waves may be so large that it takes a full hour for the wave to finish washing by, with the next wave following up to 500 miles behind. It’s common for the first wave to be the smallest, with succeeding waves becoming progressively larger.

• The longer the wavelength distance, the faster the waves travel across the ocean. Normal waves move at about 55 mph, but the biggest tsunami waves can travel at speeds of up to 600 mph, which is faster than most airliners.

• Shorelines that lie on the edge of a long and shallow stretch of a continental shelf suffer far less damage than shorelines that lie on the edge of a deep, steeply declining stretch of seafloor. If the sea bottom rises slowly to shallow water, much of the energy of the wave is dissipated through gravity and friction. The island of Tahiti escapes the worst of tsunamis because of this fact. If the ocean floor rises steeply to shore, the inflowing rush of the seas are forced upwards into a towering wave.

• The deadly force of tsunamis is exacerbated with the absence of natural barriers such as coral reefs, mangrove swamps, coastal forests, and sand dunes, all of which serve to break up the force of the inrushing water.

• Tsunami inflows are greatly amplified when they enter a wide shallow bay that narrows towards the shore. The land pattern of this shape creates a funnel effect that squeezes the energy of the incoming wave into a significantly taller crest. When the 1964 earthquake in Alaska sent out tsunami waves, most of the California coast was hit with waves that measured about only two feet, but the bay bordering California's Crescent City funneled the same waves into 21-foot crests that wiped out its downtown area and killed 11 people.

• Several minutes before a tsunami reaches land, the water often recedes out to sea at unprece-dented levels, leaving anchored marina boats resting on a muddy bottom while unwary people rush to the shore to witness the curious spectacle. Those unaware of the impending danger in time to retreat to higher land before the massive wave rushes in are sadly likely to be counted among the casualties.

• Tsunamis travel in groups with crests often an hour or more apart. Unfortunately, people who hurry back to survey the damage or search for survivors after the first wave hits may be caught in the second surge.

RECORD TSUNAMIS

• On April 1, 1946, a 50-foot tsunami struck Hilo, Hawaii, killing 150 people and wiping out one-third of the town. The wave had been generated in Alaska, and had taken more than four hours to reach Hawaii, travelling at the rate of 500 mph.

• Lisbon, Portugal sat near the epicenter of an earthquake on November 1, 1755. Buildings fell and fires started. People fled to the waterfront. An hour after the earthquake, the water drained back out of the harbor, followed ten minutes later by a wall of water 50 feet high, which swept over the harbor and rushed ten miles upriver. When it washed back out to sea, it dragged debris and people into the ocean, only to be followed by two more giant incoming waves. Between the original earthquake, the aftershocks, the fires, and the tsunami, about 60,000 people died, equal to a quarter of Lisbon's population.

THE GREAT CHRISTMAS QUAKE

• The magnitude 9.1 earthquake on the day after Christmas, December 26, 2004 near Sumatra in the Indian Ocean singularly released more energy than all the earthquakes on the planet in the past 25 years combined. The force of two opposing undersea tectonic plates caused a 750-mile long segment of the seafloor the size of California to heave upward and sideways by 50 feet, generating the most destructive tsunami ever known and causing one of the deadliest disasters in modern history.

• The massive quake displaced seven cubic miles of seawater, generating a wall of water up to 100 feet high traveling up to 500 mph, which in total took the lives of some 283,000 people.

• Some areas of northern Indonesia such as Banda Aceh were hit by the waves only eight minutes after the quake, while areas to the south such as Sri Lanka weren’t struck until 90 minutes later. The tsunami took 16 hours to reach South Africa with a five-foot surge, and even far-distant Antarctica recorded three-foot waves. In Banda Aceh, the rush of waves reached 2.5 miles inland. In many of these cases the rapid rush of water retreating back to the sea was more destructive than the original incoming waves.

• About 30 minutes elapsed between each surge, and in most areas the third wave was the largest and most devastating. Some areas experienced the rapid draining of bays prior to the arrival of the waves; others did not. Some had towering waves crashing over shore while others saw only a rapid rising of water levels.

• Because there was no tsunami warning system in place at the time there was no way to warn residents of the impending danger. Those who tried to phone or use the internet for help found all systems down, and all government offices were closed because it was Sunday morning. Even had they been open, there was no system available to get emergency word out to warn the public. The devastating events of this monumental disaster resulted in the Indonesian government finally putting a high-tech warning system in place. □