The seismogram shown here of a magnitude 6.8 aftershock of the magnitude 8.3 Sea of Okhotsk earthquake (see previous post) has two spikes that tell us we recorded a deep earthquake.
The quake is very deep (>600 km), and when a deep earthquake occurs there are seismic “P waves” that go directly from the quake to a seismograph, and also P waves that go from the quake to the surface, bounce off the surface, and then go to the seismograph (pP). The difference in timing of those two waves (pP-P) gives an estimate of the depth. For this case, based on this one seismogram, the difference between the times of the two spikes is about 2 minutes, which we calculated from this BC-ESP seismogram to correspond to about 620 km. The official USGS depth, based on many seismograms all around the world is 623 km.
So, this one observation, on this one seismogram, gives a pretty good estimate of the depth of the quake.