You'll notice on the frequency charts that some of the islands picked up Canada's quakes quite well...while others barely registered it at all. We learned about 200 pages ago that this is because of large pools of magma beneath some of the islands. The magma pools stop the flow of the seismic waves and therefore don't show up on some island's seismographs or frequency charts. It's great way to tell which islands have the greatest amounts of magma beneath them.