LA Yoga. Special Section. What is a Tsunami?

:: March/April 2005 Volume 4/Number 2

SPECIAL SECTION:
What is a Tsunami?

By Laura Faye

In trying to make some sense out of the recent devastating event, it is helpful to understand a number of interrelated social and scientific concepts.

One is simply our attraction to the ocean. People are fascinated with the ocean’s great power and mystery and want to be near it. The ocean borders most of Earth’s largest cities and nearly half of the planet’s 6 billion human inhabitants live near a coastline. Our love for the ocean ensures that when a tsunami strikes, the effected areas will most likely be populated.

The Facts.
A tsunami is an extremely long wavelength (typically 200 km or 125 miles) wave caused by the rapid displacement of ocean water such as a sudden vertical movement of Earth along a fault. (The other causes are volcanic eruption, avalanche or iceberg crashing into the sea.) The destructive megathrust earthquake of December 26, 2004 occurred on the interface of the India and Burma plates. When two oceanic plates converge, one of the colliding plates will be older and denser and therefore slip steeply below the other. This event consisted of a downward sliding of the India plate relative to the overlying Burma plate.
The world's largest recorded earthquakes have all been megathrust events, caused by the release of stresses that develop when one tectonic plate slips beneath another. The vast majority the world’s megathrust earthquakes occur in the Pacific Ocean which is part of the reason why there is currently no Indian Ocean warning network. In this instance, the India plate began its descent into the mantle at the Sunda trench, which lies to the west of the earthquake's epicenter. A huge area, 1200 km, of the plate boundary slipped as a result of the earthquake.
A rupture along the fault then causes a depression in the sea surface. The oscillating ocean surface generates progressive waves that radiate from the epicenter in all directions. It is equally likely for either the crest or trough to strike the shore first. The depth of the water determines the wave speed of a tsunami: the deeper the water, the faster the wave. In 4000 meters, the average depth of the ocean, a tsunami will travel almost 500 miles per hour. As a tsunami approaches shore, the seafloor becomes shallow which causes the wave to slow down and increase greatly in height. The shape of the coast also affects the size of the wave. A gently sloping beach will produce a larger wave.

Destruction and Creation.
These same deadly processes that caused the earthquake and subsequent tsunami are what also created the beautiful Indonesian archipelago with all its glorious abundance and lush tropical diversity. As these two tectonic plates converge over geologic time, seawater and melting rock from the subducting plate turn to a fluid mix of magma, which powers vigorous volcanoes that emerge from the seafloor and form these curving island arcs made of mineral rich volcanic soil. The mutually creative, destructive and transformational concepts represented by Brahma and Shiva in Hindu philosophy can also be understood from a scientific perspective as the forces of our planetary cycles in a geological time frame.