When the Earth Shakes, Rattles and Rolls : NONFICTION : WHY THE EARTH QUAKES: The Story of Earthquakes and Volcanoes, <i> By Matthys Levy and Mario Salvadori (W.W. Norton & Company: $25; 215 pp.)</i>

Since “Earthquake Country,” an out-of-print guide written by Robert Iacopi for Sunset Books, there has been a paucity of books understandable to the layman about the mechanics of quakes and other seismically related events, such as volcanic eruptions and tsunamis.

It is with filling that gap in mind that Matthys Levy and Mario Salvadori, two New York architectural engineers who are particularly expert in making buildings more resistant to quake damage, have written what amounts to an earthquake and volcano primer.

Of course, there is no single elegant way of explaining how the earth shakes. A chemical reaction can be observed in a fishbowl. But as of now, no one can burrow deep within to observe in precise detail what takes place when the earth begins to shake, and there are no scientific instruments that convey a full picture.

The result is that scientists hold widely divergent views and the consensus, where it does exist, is changing all the time. Many old theories simply embraced the facile notion that quakes result from slippage caused by an accumulation of stress on faults.

Today, some scientists see an infusion of pore liquids from deep in the earth into relatively weak fault zones as an important cause. Others debate the perhaps minor “impediments” in faults that may determine whether a small quake becomes a big one. And there has been a preoccupation with the movement of great tectonic plates floating on the Earth’s mantle and “diving” under one another, thus ultimately causing quakes.

What is evident from any cursory look at historic quake and volcano maps is that a huge preponderance of such events takes place within relatively narrow geographic bands.

The most active locales are a ring around most of the Pacific Ocean, and it may surprise most Californians that this state is by no means as frequent a center of earthquakes as some other places, such as off Japan, north of New Zealand, off Central America and around Greece and western Turkey.

Of course, there is a sufficient number of strong quakes in California to cause great concern, and a seismic risk map of the United States included in these pages shows this state, along with Nevada and Alaska, to be the most seismically active places in our country.

The best thing about “Why the Earth Quakes” is that it conveys facts about tectonic movements, mid-ocean ridges, subduction boundaries, volcanic “hot spots” and the like in language that is fairly uncomplicated. It also offers a useful “Risk and Preparedness” chapter as well as striking descriptions of some of the most important seismic and volcanic events of history--such as the Lisbon earthquake of 1755, the Tokyo-Yokohama quake of 1923 and the Mt. St. Helens eruption in Washington state in 1980. There is even a section devoted to our own 1994 earthquake in Northridge.

The book is weakened, however, by a few factual errors. There are some incorrect dates and times, and misspelled names of such places as Louisiana and Klamath Falls, Ore. (One of the authors recently gave assurances these will be corrected if the volume sees a second printing.) And when the authors write about their realm of expertise--e.g., protecting structures from ground shaking by elaborate shock-absorbers--their descriptions grow more elaborate, and in some cases less understandable.

On the whole, however, this is a most readable book--a good start for those interested in finding out more about just what is shaking us up.