Generation and application of automatically determined depth phase picks (pmP, pP, sP, sS)

Depth phases (near-source surface reflections, e.g. pP) are crucial for accurate determination of intermediate-depth earthquake (occurring between 60–300 km depth) locations using global seismic data. However, detection of these phases suffers from poor signal-to- noise ratios in the direct wave coda. To address this limitation, I developed an automated approach to group globally-distributed stations at teleseismic distances into ad-hoc arrays with apertures of 2.5 degrees, before optimising and applying phase-weighted beamforming techniques to each array. Resultant vespagrams allow automated picking algorithms to determine P, pmP, pP, sP, S and sS arrivals, which are subsequently used to determine high resolution event locations (using 1D and 3D approaches) and Moho depths. Improvements in event location and Moho depths, from incorporating ad-hoc array-determined phases, enhance our ability to establish subduction zone geometry, investigate the governing controls on intra-slab deformation, and analyse the rheology of the slab and mantle wedge.

I demonstrate my approach by relocating 1046 intermediate-depth events (based upon 68,075 depth phase detections) and generating Moho depth maps (using 2965 pmP detections) on a continental-scale for the South American Subduction Zone, with the Peruvian flat slab and northern Chile acting as validation regions. The final 3D event locations indicate a deeper subducting slab than previously suggested, highlighting limitations in current global slab models. In Peru, the relocated seismicity nucleates on the bounding bends of the flat slab, indicating the influence of flexural bending in the presence of dehydration embrittlement, and in-line with hydrated slab bathymetric features. In contrast, Chilean earthquakes occur continuously along the normally subducting slab and show signs of a double seismogenic zone, alongside other slab features. Amplitude analysis of pmP and pP arrivals also indicates a cold, dry, isolated mantle wedge overlying the Peruvian flat slab, which transitions into warmer, viscously coupled mantle beyond the distal bend and into Chile.