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We're Living Here on a Broken Eggshell

February 24, 1985|Terence M. Green

The next time you hard-boil an egg--for a salad, say--don't crack the shell against the pan or the sink before you peel it.

12p,10,,1 Alternative Shelter Instead, take the egg in your hand and squeeze it until the shell cracks; maybe turn and squeeze once or twice more until the shell is broken into a number of pieces, large and small, all sticking to the egg.

You've taken the first step toward a layman's understanding of earthquakes.

It's not the actual cracking of the shell; that must have happened millions, perhaps billions of years ago, when there was no life yet on earth. Rather, it's the result you have come up with, a shell broken into a number of irregular pieces. For that is the state of the earth's shell, according to current scientific thinking.

The theory is called "plate tectonics" and is the study of the earth's crust--the stony shell surrounding the semi-molten and molten interior. The equivalents to the pieces of eggshell are called plates; earthquakes and volcanoes result from their rubbing against each other and colliding with each other as they jiggle around.

The analogy is not so far-fetched, either, for the crust of the earth, even though it is many miles thick, is far thinner in relation to the size of the earth than the shell is to the egg. And the crust is made of far less stiff material than the eggshell.

This exact example is not used in the late-1984 book "Terra Non Firma: Understanding and Preparing for Earthquakes," by James M. Gere and Haresh C. Shah, but the subject is made very clear on a more scientific and scholarly level, in an easily understandable and enjoyable way. (The book is published by the Stanford Alumni Assn. and W. H. Freeman & Co.; the price is $11.95 in soft cover and $19.95 hard-bound, and it is available in or through bookstores.)

The plates slide past each other, pull apart from each other, or collide with each other. When they collide, one dips underneath, dissolving in the molten interior but raising the other. When they pull apart, the earth's molten interior bubbles out--or explodes out a la volcano. It is when they slide past each other that Californians are concerned; our state is on two plates, the North American and the Pacific, and the border is the infamous San Andreas Fault.

The Pacific Plate is moving northwest past the North American Plate at a speed of five to eight centimeters (almost two to a little more than three inches) a year. In the central part, they slip fairly freely, causing numerous small to tiny earthquakes, but in the north, the San Francisco Bay area, and the south, here where we are, they stick without moving for years, then let go catastrophically. The last times they did that were, in the north, the San Francisco quake of 1906 and in the south, at Fort Tejon in 1857.

Recent studies of the San Andreas and associated faults covering about the last 1,200 years indicate that the average time between catastrophic quakes is about 140 years. But that is an average; actual incidents range from perhaps 50 to 300 years.

The authors of Terra Non Firma say: "Current estimates place the probabilities of a great earthquake in Southern California at about 40% in the next 30 years; in the San Francisco region, the probability is about half as great. For the entire state, the probability is about 50% in the next 30 years."

(Some others narrow it down more than that. The leading article in the February Scientific American magazine, "Predicting the Next Great Earthquake in California" by Robert L. Wesson and Robert E. Wallace, estimates a probability of 50% for a great earthquake in the southern portion of the San Andreas Fault in the next 30 years.)

All this is only a small part of "Terra Non Firma." Topics other than the causes of earthquakes include other quake hazards, tsunamis (seismic sea waves, often incorrectly called tidal waves), measurement of earthquakes and their effects, earthquake prediction (it has promise but there's a long way to go yet), engineering design, community and individual planning, and preparing for the next earthquake. The book's 203 pages include many photographs, charts, diagrams and tables and suggestions for additional reading.

If you don't know much about earthquakes but feel that people living on top of one should know more about them, this book is a very good place to start.

Now you can slice that egg into your salad.

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