Tomographic inversion of P and pP data for aspherical mantle structure below the northwest Pacific region

• Published in: Geophysical Journal International
• Main Authors: van der Hilst, R. D., Engdahl, E. R., Spakman, W.
• Format: Text
• Language: English
• Published: Oxford University Press 1993
• Subjects: Articles; Pacific; Kamchatka
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/115/1/264; https://doi.org/10.1111/j.1365-246X.1993.tb05603.x

To investigate the morphology of subducted slab in the mantle below northwest Pacific island arcs we inverted traveltime residuals for aspherical variations in P -wave propagation velocity relative to the radially symmetric iasp91 reference model. The tomographic method used is based on a step-wise linearization of the inversion problem. First, we relocated ISC (International Seismological Centre) hypocentres with re-identified P and pP phase data using the iasp91 traveltime tables. The variance of P residuals relative to iasp91 traveltimes was 17 per cent less than the variance of P data reported by the ISC relative to the Jeffreys-Bullen (J-B) traveltime tables. Second, we performed a linearized (LSQR) inversion for Earth structure and source relocation with the P and pP residuals obtained from the first step, using iasp91 as the reference model for seismic velocities. The incorporation of the depth phase pP in the tomographic inversions has two major advantages: (1) the pP data provide constraints on focal depth and thus reduce the trade-off between source relocation and structure; and (2) the pP ray paths improve the sampling of Earth structure in the shallow mantle and transition zone. We used more than 2 × 106 and about 1 × 105 P − and pP -wave traveltime residuals, respectively, from about 40 000 earthquakes with epicentres in the study region that were recorded at one or more of the 2300 globally distributed seismological stations considered in this study. We assessed the spatial resolution in the tomographic images with checker board-type sensitivity tests. These tests reveal high resolution of upper mantle and transition-zone structure, particularly below the central part of our study region. Structure with wavelengths of the order of 100 km is resolved below Japan, whereas structure with wavelengths of the order of 300 km is well resolved below the Kuril, Izu Bonin and Ryukyu arcs. Small-scale structure is poorly resolved in depth below the northern part of the Kuril-Kamchatka arc and below the Izu ...