Figure 14.4(a) P and S waves are refracted upward in the mantle and also can be reflected from Earth’s surface. A seismic wave that has been reflected once from Earth’s surface is labeled with a double letter (PP or SS). (b) Seismograms recorded at various distances from an earthquake focus in the Aleutian Islands, Alaska. The colored lines identify the arrival times of the P and S waves, the surface waves, and the PP and SS waves reflected from Earth’s surface.