How deep was that earthquake?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

These seismograms show a magnitude 8.3 earthquake in the Sea of Okhotsk and a magnitude 6.8 aftershock. The seismogram of the aftershock has two spikes that tell us we recorded a deep earthquake. The main shock also has two spikes indicating a deep earthquake, but they are off-scale.

Magnitude 8.3 earthquake in the Sea of Okhotsk (near top of seismogram) and a magnitude 6.8 aftershock (in red circle, and above, left).

The earthquake 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.