Distributions of parameters describing individuals within a population generally contain information about the processes that affect the individuals; e.g., the age distribution of an animal species will depend on whether individuals most often succumb to episodes of bad weather, to random predation of adults, or to “old age.” In this chapter, the individuals are earthquakes, the populations reside in earthquake catalogs, and the parameters of interest are focal depth and “size,” i.e., magnitude or moment. What can the distributions of focal depth and size tell us about the physical processes that control deep seismicity?

What is focal depth?

Although Wadati originally established that deep earthquakes existed by evaluating (S-P) isochrons and felt intensity data, nowadays most catalogs report depths determined using one of three types of data. These are: (1) P arrival times; (2) (pP–P) intervals; and (3) body and surface waveforms.

Depths from P arrival times

The most common method to locate earthquakes – the method used to construct the catalog of the International Seismological Centre (ISC) – is to compare observed P arrival times with arrivals predicted by a reference earth model. The location is then the latitude, longitude, depth, and origin time that produces the best fit between model and observations; in principle this is unique if there are P times recorded at four or more stations. The advantage of this method is that it is widely applicable since P arrivals from numerous stations are often available for earthquakes with magnitudes as small as 4 or less and as large as 8 or more.