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Return flow in a deep subduction channel (DSC) has been proposed to explain rapid exhumation of high pressure–low temperature metamorphicrocks, entirely based on the fossil rock record. Supported by thermo-mechanical models, the DSC is envisioned as a thin layer on top of thesubducted plate reaching down to minimum depths of about 150 km. We perform numerical simulations of high-frequency seismic wave propagation(1–5 Hz) to explore potential seismological evidence for the in situ existence of a DSC.  Motivated by field observations, for modelingpurposes we assume a simple block-in-matrix (BIM) structure with eclogitic blocks floating in a serpentinite matrix. Homogenization calculationsfor BIM structures demonstrate that effective seismic velocities in such composites are lower than in the surrounding oceaniccrust and mantle, with nearly constant values along the entire length of the DSC. Synthetic seismograms for receivers  at the surfacecomputed for intermediate depth earthquakes in the subducted oceanic crust for models with and without DSC turn out to be markedlyinfluenced by its presence or absence.  While for both models <i>P</i> and <i>S</i> waveforms are dominated by delayed high-amplitude guided waves,models with DSC exhibit a very different pattern of seismic arrivals compared to models without DSC.The main reason for the difference is the greater length and widthof the low-velocity channel when a DSC is present. Seismic velocity heterogeneity within the DSC or oceanic crustis of minor importance.The characteristic patterns allow for definition oftypical signatures by which models with and without DSC may be discriminated.  The signatures stably recur in slightly modified form for earthquakes at differentdepths inside subducted oceanic crust. Available seismological data from intermediate depth earthquakes recorded in theforearc of the Hellenic subduction zone exhibit similar multi-arrival waveforms as observed in the synthetic seismograms for models with DSC.According to our results, observation of intermediate depth earthquakes along a profile across the forearc may allow to test the hypothesisof a DSC and to identify situations where such processes could be active today

solid earth

Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes