Are deep earthquakes useful? That is, do they provide us with information that helps solve problems of practical importance to humankind? Or, are deep earthquakes just another abstruse phenomenon, justifying attention from scientists only “because they are there”?

This chapter's main thesis is that a disproportionate fraction of what we know about the structure and dynamics of the Earth's interior comes specifically from observations of intermediate- and deep-focus earthquakes. Thus, this chapter briefly reviews several topics where deep earthquakes provide the essential information that constrains our present knowledge.

Structure of the Earth

Gross earth models and the structure of the core

What do we know about the structure of the Earth, and from whom did we learn it? Between 1930 and 1945 three individuals, Harold Jeffreys, Beno Gutenberg, and Kiyoo Wadati were responsible for many of the seismological observations used to construct travel time tables and develop models of gross earth structure (see Frohlich, 1987). One thing is very clear if you reread their papers published in journals such as Gerlands Beiträge zur Geophysik, Geophysical Magazine, and Monthly Notices of the Royal Astronomical Society, Geophysical Supplement – records of deep earthquakes were fundamentally important in the construction of these models.