Earthquakes & Tsunamis
The earth’s crust is made up of huge slabs of rock called tectonic plates that fit together like jigsaw puzzle pieces and float atop the molten rock of the earth’s mantle. The continents and oceans sit on these plates, which vary in thickness from around 10 to 125 miles deep. Since they are always shifting their position, moving from 2-8 inches per year, the plate boundaries are subjected to different stresses.
When two of the earth’s plates collide, the crust at the edge of the plates crumples and folds, creating mountains. If the plates pull apart from each other, fissures and deep trenches are formed, such as those under the ocean. Plates may slide past each other in opposite directions, creating a crack along the fault line. Any of these events can shake the earth’s crust for miles around. Earthquakes may also occur in connection with a volcanic eruption.
If you mark on a world map the places where earthquakes and volcanoes have been, you will notice that most are concentrated on either side of the Pacific Ocean. Another earthquake belt runs along the Mediterranean Sea and across Asia. Several of the worst earthquakes in history occurred in China, the most devastating of which took place on January 23, 1556 in the Shensi province, resulting in over 830,000 deaths.
In general, a medium quake close to a large city will cause more damage and injury than a larger quake in an unpopulated area. Sometimes earthquakes are so severe that landforms are changed and whole cities are destroyed. The Indian Ocean earthquake on December 26, 2004, actually moved some small islands. Scientists say the massive release of energy and shift in mass even altered the Earth's rotation slightly.
Fortunately, not all earthquakes are that powerful. Most are so mild that they are barely noticeable. Approximately 150,000 earthquakes occur each year. Of these, 40,000-50,000 are strong enough to be felt. About a hundred of these earthquakes are somewhat destructive, but only twenty or so do a large amount of serious damage. Small earthquakes often last for less than a second and even major quakes may only last for a minute or two. Weak tremors called aftershocks can follow a larger earthquake for several days or weeks.
Earthquake vibrations are measured and recorded using sensitive scientific instruments (seismometers and seismographs). A seismogram is a chart that provides a record of the earth’s movement. The Modified Mercalli Scale is a nonscientific ranking of twelve levels of intensity from imperceptible shaking to catastrophic destruction. The Richter Scale is a mathematical measure of the earthquake’s magnitude, or amount of energy released. The strongest earthquake recorded to date was the 1960 Chilean earthquake, with an estimated magnitude of 9.5. The December 26, 2004, Indian Ocean earthquake had a magnitude of 9.0.
Structural engineers have learned to design special kinds of buildings for earthquake-prone areas. Earthquake resistant structures are often made of steel-reinforced concrete, with diagonal cross-bracing in the walls for added support. Some buildings have a broad base to keep them from toppling over, such as the pyramid-shaped Transamerica tower in San Francisco. Others are built on shock-resistant foundations made of spongy or springy materials like rubber. These buildings are able to shift slightly without collapsing.
Did You Know…?
The famous American architect Frank Lloyd Wright was the first to integrate architecture and engineering into a comprehensive earthquake-proof design strategy when he built the Imperial Hotel in Tokyo.
Earthquakes that occur under the ocean and cause a sudden shift in the seafloor will create seismic sea wave or tsunamis. (They are sometimes called tidal waves, but that term is technically incorrect.) These waves move at speeds of 200 to 450 miles per hour and can travel thousands of miles across the ocean.
When the tsunami (pronounced soo-nahm-ee) is over deep ocean water, it may be only 1-3 feet above the normal surface level. But when it hits a sloping shore, it rises up and the shallow sea water underneath is sucked into the wave. This makes it appear as if the tide is suddenly going out, exposing the ocean floor. Experts believe that a receding ocean may give people as much as five minutes' warning to evacuate the area.
The tsunami crests at an average of 30-120 feet – that’s 3- to 12-stories high! This huge wall of water rushes in over the coast and comes crashing down, washing everything in its path out to sea. The December 26, 2004, Indian Ocean tsunami reached a height of about 50 feet.
Around 1500 BC, a volcanic eruption in the Mediterranean Sea triggered a tsunami which struck the island of Crete, home of the Minoan Empire. This wealthy, sophisticated civilization was completely wiped out in a matter of seconds by colossal 200-foot-high waves that swept over the land. The story of the lost city of Atlantis, as told by the Greek philosopher Plato in 355 BC, is probably based on an ancient town that was destroyed in a similar manner.
Did You Know…? The world’s largest recurring surfable wave in Maui, Hawaii, created by a uniquely shaped underwater ridge, reaches a height of 70 feet about a dozen times a year. Storm waves on the open sea are often 50 feet high and can be as high as 100 feet. The largest wave on record occurred on July 9, 1958, at Lituya Bay in southeastern Alaska. An earthquake with a magnitude of 8.3 triggered a landslide of rock and ice which fell into the bay, displacing an enormous amount of water. This made a massive wave that splashed to a maximum height of 1,720 feet, washing away about 4 square miles on either side of the bay.
Amazing Earth: Earthquakes, by Sandra Markle, 2002. (This book contains lots on interesting facts, photos, and easy hands-on activities to help children understand the natural processes related to earthquakes.)
Earthquakes, by Seymour Simon, 1991. (An award-winning science writer examines the history and mystery of earthquakes.)
Volcanoes and Earthquakes, by Michael Carroll, 1997. (A child’s guide to God’s power beneath our feet – a science journalist/astronomical artist explains earth science from a biblical perspective.)
www.cnn.com/SPECIALS/2004/tsunami.disaster/ (Waves of Destruction: a special report with news, eyewitness accounts, and educational features including earthquake magnitudes, an animation showing how a tsunami forms, and a chronology of previous tsunamis going back to 1755.)
www.fema.gov/kids/tsunami.htm (A colorful introduction to tsunami science and safety for elementary-age kids.)
http://earthquake.USGS.gov/eqinthenews/2004/USSlav/ (An excellent report with explanation of exactly how the Asian earthquake/tsunami happened on December 26, 2004, maps that you can enlarge, and the extent of the tsunami.)
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These pages are a continuous work in progress.