Deep Earthquakes , Cliff Frohlich , Cambridge U. Press, New York, 2006. $150.00 (573 pp.). ISBN 978-0-521-82869-7
A labor of love by a dedicated researcher with a passion for the topic, Deep Earthquakes by Cliff Frohlich is unusual in its combination of breadth, depth, and tone. The intended audience is broad—from specialists on deep earthquakes to geophysics researchers and graduate students to scientifically literate nonscientists. The book succeeds in offering results and explanations that are up-to-date and, though occasionally technical, still accessible. And I know of no other text devoted to deep earthquakes, or even one that treats them in depth.
Earthquakes in general occur in subducting slabs down to about 670 km below Earth's surface, but those that occur below 50 km present a seismological puzzle. Eighty years after their discovery, deep earthquakes remain mysterious—that is, researchers still do not understand the physical processes behind the seismic signals that have delivered most of the current knowledge of Earth's interior structure. Brittle failure and frictional sliding, the processes that generate shallow earthquakes, are strongly inhibited below depths of about 50 km by the intense stresses produced by the great weight of overlying rock, while ductile flow and deformation are prompted by the high temperatures of depth.
Over the decades scientists have suggested numerous mechanisms for deep earthquakes, including dehydration embrittlement, transformational faulting, thermal shear instability, and frictional melting. But whatever mechanisms operate in those inaccessible depths, the seismic signals emitted indicate sudden, sharp failure and differ only subtly from those of shallow earthquakes, which we geophysicists think are better understood. The research terrain is necessarily interdisciplinary, since the physics of deep earthquakes is related to the disciplines of seismology, geodynamics, and mineral physics through several physical processes, including phase transformations in mantle minerals, mantle circulation, plate tectonics, formation and hydration of the oceanic crust, and subduction of sediments and water.
Frohlich is associate director of the Institute for Geophysics at the University of Texas at Austin. A well-known seismologist with particular expertise in deep earthquakes and global seismicity, he has written a score of papers on deep earthquakes, focusing particularly on their statistics and the geometrical orientation of the seismic sources. He explains that as a graduate student at Cornell University, he “imprinted on deep earthquakes” in the way a duckling imprints on its mother, following her relentlessly. Frohlich admits he has remained obsessed with the topic.
Deep Earthquakes is a lengthy volume, enriched with numerous black-and-white figures, many from current literature and some created expressly for the book. The casual, sometimes quirky tone is unusual for a scientific narrative but refreshing and appropriate for the intended broad audience. All aspects of the topic are covered, some more thoroughly than others, and there is an emphasis on statistical significance throughout. In addition to long chapters on the discovery, properties, physical mechanisms, and relevance of deep earthquakes, an extensive chapter 10 geographically summarizes by subduction zone hundreds of significant deep earthquakes. The chapter also includes deep lunar seismicity, which seems even more mysterious than deep seismicity here on Earth.
For the general audience, the book provides thoughtful descriptions and explanations of the phenomena, including earthquake numbers and sizes, rupture processes, subduction zone structure, and effects of ambient conditions—mainly pressure and temperature—on possible physical processes. Although the book contains no problem sets and is not a textbook, it could be quite useful for a graduate-level topical seminar because of its overview of numerous processes and its synthesis of relevant concepts. For the specialist, the comprehensive literature review and geographic summary are particularly valuable.
Overall, Deep Earthquakes is quite an accomplishment. Bottom line: If, like many geophysicists, you are curious about this mysterious phenomenon, check out Frohlich's book. It will most likely answer some questions and raise many others.
