Guided Wave Effects in the Subducted Nazca Plate: Their Potential and Danger for Deciphering Slab Structure and Mineral Composition

We use guided waves traveling up-dip along the surface of the Nazca slab to image subducted oceanic crust at the Chile-Peru subduction zone. Observed P onsets of intermediate depth events near 21\deg S in Northern Chile reveal wave guide behavior: with growing focal depth, low frequency energy (< 2 Hz) becomes more and more dominant, higher frequencies arrive delayed, sometimes resembling two distinct phases. 2D finite difference (FD) simulations of complete P-SV wave propagation along an up-dip profile of the subduction zone are employed to shed light on several basic issues regarding crustal wave guides: We show that the observed guided wave energy must decouple from the wave guide near 100 km depth to reach the deployed stations and that the observability of up-dip guided waves in subduction zones is related to variations in subduction angle. The guided wave effect has considerable influence on p-arrival times and thus is of major importance for source location and tomographic methods at subduction zones in general. Further, simulations yield constraints on source locations relative to the low velocity structure and wave guide thickness. The results indicate that a structure of less than 4.5 km average width and 7 % low velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km. Further, only seismic sources within or close to the low velocity structure (distance < 4 km) are able to produce the effect. The layer is interpreted as the unaltered lower part of the subducted oceanic crust, suggesting that complete eclogite transformation in the Chile-Peru subduction zone is unlikely to take place until beyond the volcanic front.