What are deep-focus and intermediate-focus earthquakes?

When we call an earthquake “deep,” what do we mean? As we shall see, there is surprisingly little agreement about this among seismologists. However, in this book I use these definitions:

1. • A shallow or shallow-focus earthquake is any earthquake with a believable reported focal depth less than 60 km;

2. • An intermediate-focus, intermediate-depth, or intermediate earthquake is any earthquake with a believable reported focal depth equal to or greater than 60 km but less than 300 km;

3. • A deep-focus earthquake is any earthquake with a believable reported focal depth of 300 km or more;

4. • A deep earthquake is any deep-focus or intermediate-focus earthquake, i.e., a quake with a believable reported focal depth of 60 km or more.

Although it is confusing for “deep-focus” and “deep” to specify distinctly different depth categories, it is often desirable (as with the title of this book) to specify intermediate- and deep-focus earthquakes together. Since about 1970 a few scientists have called them “mantle” earthquakes instead of “deep” earthquakes (e.g., Isacks and Molnar, 1971). This is misleading since not all shallow-focus earthquakes occur within the crust; e.g., nearly all oceanic intraplate earthquakes are shallow but occur in the mantle. Finally, there are now already hundreds of published papers which used “deep earthquake” to specify both deep-focus and intermediate-focus events.