saeedahmed.com

Saeed Ahmed - Staff
Monday, March 12, 2001

From El Salvador to India to Seattle, earthquakes have claimed thousands of lives and caused billions of dollars in damage this year. Each time, they struck without warning.

What could cause the normally stable Earth to shake as if a herd of elephants was stampeding through your bedroom?

The ancient Siberians believed the Earth rested on a sled pulled by dogs with fleas. When the dogs stopped to scratch, they shook the Earth, according to this legend.

In Mozambique, the Earth was considered a living creature that would shake when it got sick and had a fever and chills.

And in parts of China, legend holds that there is a race of people living inside the Earth who shake the ground from time to time to see if anyone is still living on the surface.

Until a few decades ago, there were many theories about the cause of tremors, but none could explain why some places, such as California and Japan, have earthquakes so often while others, such as Sweden and Australia, almost never have any.

A scientific explanation emerged in the late 1960s, when the theory of "plate tectonics" was developed.

Cracks in the Crust

The Earth is divided into three layers --- a very hot center "core," a soft thick middle called the "mantle" and a thin, hard outer layer called the "crust."

Over the years, the crust has cracked, creating seven large stone slabs and more than a dozen smaller slabs called "plates." The plates are about 30 miles thick under land and 1 to 5 miles thick beneath the ocean.

The plates float on "magma," which is molten lava found in the Earth's interior. Like the ocean, magma has currents, which are caused by heat rising from the hot center of the Earth.

These currents move in a circular motion beneath the plates, causing the plates to move.

The point at which two plates meet is called a "fault," or "fault line." Perhaps the most famous fault in the United States is the San Andreas Fault in California. It is where the Pacific and North American plates meet. It is hundreds of miles long and runs almost the entire length of the state.

When plates meet at a fault, they can act in three ways:

1.They can smash into each other and crumple up, forming mountains.
2. One plate can slip under another, then melt and rise up to form a volcano.
3. They can slide past each other.

Mountains, valleys, plains and plateaus have been shaped by the movement of these plates over millions of years. .

Stretching like a rubber band

Earthquakes occur when two plates grind and scrape against each other at a fault line. Sometimes the friction produced is so great that whole sections of the plate get stuck to one another.

Although they seem brittle, the slabs of rocks that make up plates are like a rubber band.

If you pull a rubber band, it stretches and stretches, building up energy in the band. Then suddenly the stretched band whips back to its original shape, releasing the pent-up energy in a loud snap.

In the same way, as the two plates try to slip past each other, stress begins building in the sections that are stuck together. When the friction between the plates is suddenly overcome, a section of one plate breaks loose and the stored energy is released in a sudden jerk.

The ground begins to shake and sway, and the result is an earthquake.Like ripples in a pond, the released energy moves outward from the center. That's why the place where an earthquake starts is called its "epicenter."

Grading a Quake

Seismologists --- scientists who study earthquakes --- estimate there are 500,000 tremors in the world each year. Most of them are too small to be felt, and even fewer cause damage.

One way to tell the size of an earthquake is by its "magnitude," which often is measured by the Richter scale. The scale was devised in 1935 by seismologists Beno Gutenberg and Charles Francis Richter.

The Richter scale is logarithmic. This means that each one-point increase on the scale represents a 10-times change in "magnitude."

So a magnitude 6 earthquake is 10 times stronger than one with a magnitude 5. A magnitude 7 quake is 100 hundred times stronger than a magnitude 5 quake.

The strongest earthquake in the United States was a magnitude 9.2. It struck Alaska in 1964. The strongest in the world was a magnitude 9.5, which struck Chile in 1960.

What's the Georgia Record?

In the United States, Alaska is the most earthquake-prone state, experiencing a magnitude 7 quake almost every year. Florida and North Dakota are rocked the least often.

As for Georgia, the largest quake was a magnitude 5.5 in the Covington/Mansfield area (east of Atlanta) in 1914. Tim Long, a seismologist at the Georgia Tech School of Earth and Atmospheric Sciences, says Georgia is not on a fault line and that quakes here do not result from two plates colliding with each other but instead are caused by activities within a plate. "The explanation of how this happens is not fully agreed upon by seismologists," Long said.

Do's and Don't's

Although scientists can measure quakes, they can't predict them.

Maybe other creatures can. Dogs, cats, snakes and horses have been known to behave strangely just before a quake. And a fish in a high school biology lab in California would flip on its side before some quakes occurred, according to published reports.

If you're ever caught in an earthquake, "drop to the floor and under something for cover," says a spokesman for the Federal Emergency Management Agency. Also remember: