ALBERT

Notes: 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 times 106 and about 1 times 105P- 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 Bonin and Mariana arcs. This limits the interpretation of slab structure and mantle flow from tomographic images alone.With this limitation in mind, we conclude from the tomographic images that subducted slab deflects in the mantle transition zone below the geographical area encompassed by the Kuril basin, the Japan Sea, and the northern part of the Philippine Sea. This is in good agreement with the results of other recently published tomographic studies, the occurrence of earthquakes several hundred kilometres off the inclined Wadati-Benioff seismic zones, and inferences about ‘660 km’ discontinuity topography. In contrast, slab-like structures of high P-wave velocity are imaged in the lower mantle below the deepest earthquakes of the northern Kuril-Kamchatka and Mariana seismic zones. This is indicative of local slab penetration of the lower mantle. From tomographic images we cannot discern between compositionally or thermally induced variations in seismic velocity. However, with regard to the nature of the boundary between upper and lower mantle, our observations argue against either compositional mantle layering with large contrasts in intrinsic density or phase changes with steep Clapeyron slopes.

Notes: A major problem in P delay-time tomography is the inhomogeneous sampling of mantle structure by the P-wave ray paths resulting in low resolution in images of large regions of the upper mantle. Incorporation of PP and pP phases can improve the quality and reliability of tomographic images because they: (1) sample Earth structure not ordinarily sampled by direct P phases; (2) add rays that are oblique to rays of direct phases, which is especially important where the latter sample mantle structure in selected directions; and (3) pP data better constrain the earthquake focal depths. PP traveltimes have often been used in combination with P data in differential traveltime studies. We show that the assumptions and approximations necessary for this approach are problematic, and that they can be avoided when the P, PP and P data are used in tomographic inversion. We investigated the applicability of PP and pP delay times to the tomographic study of the aspherical mantle structure below the Caribbean region.The success of the application of data of the later arriving reflected waves depends critically on the quality of these data. We examined possible sources of error in the ISC PP and pP data and assessed the contribution to the delay times used in this study. For the Caribbean region, analyses of the ISC PP and pP delay times do not reveal biases due to effects of PP-waveform distortions, the asymmetry of the reflections, or due to misidentifications of phases that reflect at a surface other than that assumed. The noise level of PP and pP data is high with respect to data of the direct P-wave. This is accommodated by weighting with the inverse of the variance of the data of each of the three phases.The independent information that is revealed from the PP and pP data results in modifications of tomographic images based solely on P data. These modifications are important if the tomographic images are being used to understand the geodynamical history of convergent margins in the Caribbean region. We investigated the effect of adding data of later arriving phases to the ISC P data with sensitivity tests: we inverted synthetic delay times to which we added Gaussian noise with a standard error typical for the data of the three seismic phases. These tests demonstrate that the image resolution of shallow mantle structure is enhanced significantly by the incorporation of later arriving phases. Due to the absence of seismicity below 200 km the resolution improved less at deeper levels below the Caribbean region. In some poorly constrained parts of the solution the test results even indicate an apparent decrease of resolution. This is explained by changes in the rate of convergence of the inversion algorithm: in mantle regions where the effective sampling of structure improved by the addition of PP- and pP-wave ray paths, the convergence was speeded up at the expense of the convergence rate in regions where fewer or no PP- or pP-wave ray paths were added.A shortcoming of the resolution tests used in our study is that some specific problems of reported delay times are not reflected in synthetic data. We observe that ISC delay times of later arriving phases are not necessarily consistent with the reported hypocentral parameters, as most ISC-reported earthquake locations are computed from direct P-wave data. For the pP data, the inconsistency with event location not only results in a decrease of focal depths during relocation, but in a bias of the imaged velocity perturbations as well. This property of reported data is not modelled in resolution tests.

Notes: Over the last three years, a major international effort has been made by the Sub-Commission on Earthquake Algorithms of the International Association of Seismology and the Physics of the Earth's Interior (IASPEI) to generate new global traveltime tables for seismic phases to update the tables of Jeffreys & Bullen (1940). The new tables are specifically designed for convenient computational use, with high-accuracy interpolation in both depth and range. The new iasp91 traveltime tables are derived from a radially stratified velocity model which has been constructed so that the times for the major seismic phases are consistent with the reported times for events in the catalogue of the International Seismological Centre (ISC) for the period 1964–1987. The baseline for the P-wave traveltimes in the iasp91 model has been adjusted to provide only a small bias in origin time for well-constrained events at the main nuclear testing sites around the world.For P-waves at teleseismic distances, the new tables are about 0.7s slower than the 1968 P-tables (Herrin 1968) and on average about 1.8–1.9 s faster than the Jeffreys & Bullen (1940) tables. For S-waves the teleseismic times lie between those of the JB tables and the results of Randall (1971).Because the times for all phases are derived from the same velocity model, there is complete consistency between the traveltimes for different phases at different focal depths. The calculation scheme adopted for the new iasp91 tables is that proposed by Buland & Chapman (1983). Tables of delay time as a function of slowness are stored for each traveltime branch, and interpolated using a specially designed tau spline which takes care of square-root singularities in the derivative of the traveltime curve at certain critical slownesses. With this representation, once the source depth is specified, it is straightforward to find the traveltime explicitly for a given epicentral distance. The computational cost is no higher than a conventional look-up table, but there is increased accuracy in constructing the traveltimes for a source at arbitrary depth. A further advantage over standard tables is that exactly the same procedure can be used for each phase. For a given source depth, it is therefore possible to generate very rapidly a comprehensive list of traveltimes and associated derivatives for the main seismic phases which could be observed at a given epicentral distance.