Figure 8a: Effect of the subducted slab beneath Alaska on regional seismic waveforms that travel along the subducted crust (Abers and Sarker, 1996). Red circles denote earthquakes at 100-150 km depth within the slab. Seismo-grams recorded at COL are simple for P waves that do not travel along the slab (green arrow), but pulse dispersion characterizes waves that travel along the slab (red arrows). Inset shows structure needed to produce this effect, a layer of subducted crust, ~5% slower than surrounding mantle. Velocities are consistent with lawsonite blueschist (Helffrich, 1996) but not gabbro or eclogite.