When a deep earthquake happens, what happens? At its focus or “source,” how long does it take? And over how big a region in space does it happen? Does it happen smoothly, or in a series of jerks? Does it radiate seismic waves of different strength in different directions? If so, what is the pattern? These are some obvious questions about the nature of the deep earthquake source. This chapter focuses on the observed properties of the deep earthquake source. Then, the next chapter will review the underlying physical processes that may be responsible for deep earthquakes.

Only a few features of an earthquake's source are directly observable from individual seismograms. These include the source duration, the scalar moment, the radiated energy (Fig. 6.1), and sometimes the relative arrival times of various subevents within the rupture process. The uncertainties in these observations can be large; for individual earthquakes different investigators often report values that differ by a factor of two or more. All other source parameters – static stress drop, moment tensor elements, etc. – are derived. Determining these requires the introduction of models that often introduce systematic errors that produce even greater uncertainty. As we evaluate the literature on the deep earthquake source it is important for the reader to keep in mind which source parameters are observed and which are derived.