Slab low-velocity layer in the eastern Aleutian subduction zone

Local earthquakes in the vicinity of the Alaskan Peninsula's Shumagin Islands often produce arrivals between the main P and S arrivals not predicted by standard traveltime tables. Based on traveltime and polarization, these anomalous arrivals appear to be from P-to-S conversions at the surface of the subducted Pacific Plate beneath the recording stations. The P-to-S conversion occurs at the top of a low-velocity layer which extends to at least 150 km depth and is 8 +/- 2 per cent slower than the overlying mantle. The slab is similar to 7 per cent faster than the mantle. The low-velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines PS ray paths to lie within it. These observations indicate that layered structures persist to positions well past the surface location of the volcanic front. Reactions forming high-pressure minerals do not yield slab-like velocities until beyond the point that subduction zone magma genesis occurs, If the subducted oceanic crust forms the layer it is subducted essentially intact.