Why should an exploration gephysicist take an interest in earthquakes, deep or shallow? One reason is that sound professionalism demands that we should be aware of what is going on in related fields; another is that some ideas described in this volume could well be applicable to our own branch of geophysics. For example, in chapter 6 Frohlich describes how earthquake seismologists determine source radiation patterns from seismic recordings. There is a lucid and ingeniously illustrated treatment of the seismic moment tensor, which serves to describe the many possible earthquake source radiation patterns. Some of these techniques might well be relevant to the exploration industry's current interest in the use of microearthquakes for fault and fracture delineation. Chapter 8 deals with seismic tomography and how earthquake seismologists treat lateral velocity variations in a structure -- again, these ideas could apply to our own use of the tomographic method. Even while the book's emphasis is on deep earthquakes, the author neatly describes the basic ideas and tools in use today by earthquake seismologists. A feature which I enjoyed particularly is his use of numerous text boxes to provide detail for points of special importance.

Frohlich defines deep earthquakes as those having focal depths between 60 and 700 km, and states that this is the first book exclusively devoted to the study of such deep quakes.

While most quakes causing major damage are of shallower origin, there have been a number of devastating deep quakes as well. One such deep event in southern Chile in 1939 claimed some 28000 victims. On the other hand, another deep quake in Bolivia in 1994 resulted in very few victims, yet was felt as far away as Minneapolis by a woman who was living on the twelfth floor of a condominium. Even today we don't really know why some deep events can be so devastating, while others cause almost no damage. An open question is why there are no quakes with focal depths greater than 700 km. It has been generally assumed that rocks become too ductile at such depths so that quake-causing brittle fracture is unlikely to occur. However, this may not be the whole story - some seismologists (the author not among them) believe that quakes occur at such depths not because of brittle fracture, but because of phase transitions as minerals convert from a less dense low pressure to a more dense high pressure form.

This book is considerably more than yet another text in earthquake seismology: It also emphasizes the human element as science is being done. For example, Frohlich takes pains to narrate how the thinking of some of the leading players evolved over time. Thus the illustrious Sir Harold Jeffreys believed for a decade that since the theory of isostasy required a plastic mantle, deep earthquakes could not occur, even though they did in fact occur. Knott (he of the Knott-Zoeppritz equations fame) first thought that P- and S-waves would become so jumbled together during propagation that they could never be studied separately. Even the great minds of science often stray up the wrong creek, a consoling thought for those of us struggling with today's tough problems.

The volume might have benefited from a much more complete index, while a glossary of technical terms would have been of great help to the nonspecialized reader. On occasion terms are introduced in the text before they are defined. But these are minor flaws, easily corrected in a future edition. My only serious gripe is the book's price: At US$150 it is beyond the reach of most students. A pity that commercial publishers choose to rely on library sales rather than on students who are far more likely to purchase such volumes in larger quantities, were they only affordable.

Sven Treitel

Tulsa, USA