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.

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