Figure 3: Plot of earthquakes recorded (closed circles) versus not recorded (open circles) by the AS1 seismograph operating at Boston College. The grey curve is an estimate of the threshold for how big an earthquake must be at a given distance in order for the AS1 to record it at this particular site. Dashed line corresponds to unusually strong signals from "PKP" waves that travel through the Earth's core and are focused in that narrow range of distances.