DEEP EARTHQUAKES

Why write a book on deep earthquakes? On 21 January 1906 an earthquake with A/w of 7.4 occurred at a depth of 340 km beneath Japan. Fusakichi Omori, one of the world’s finest seismologists of that time, located the event using (S-P) intervals, and concluded that at several Japanese seismograph stations the P waves were transverse and the S waves were longitudinal. This puzzled Omori, so he published a paper about this a year later (see Fig. 0.1 and Chapter 3). So began the story of deep earthquakes - Omori’s (1907) paper is among the earliest publications in my collection of references on deep earthquakes. Of course, for many reasons, Omori wouldn’t have told the story of the beginning quite this way. In 1907 the magnitude scale hadn’t yet been invented, and Omori’s paper never uses the terms “P”, “S”, or “(S-P) interval”; rather, it mentions the “1st displacement,” the “2nd displacement,” and the “duration of 1st preliminary tremor.” Omori also didn’t know that the earthquake was deep; indeed, he didn’t even know that “normal” earthquakes occurred at depths of 40 km or less. Our perspective on earthquakes has changed considerably since 1907; my collection of references on deep earthquakes now has more than 2000 entries. We have learned a great deal about earthquakes over the past 100 years. For example, one thing we have learned is that deep earthquakes are quite common - nearly a third of all earthquakes located by the International Seismological Centre have focal depths exceeding 60 km; of these, about one fifth exceed 300 km. The purpose of this book is to summarize what a century of research has taught us about deep earthquakes, i.e., earthquakes with focal depths of 60 km and greater. As far as I can tell, no one has written a summary book about deep earthquakes - ever. Thus, I submit that the passage of 100 years, the appearance of 2000 publications, and the absence of any other book marks a good time to review what we now know about deep earthquakes.