Abstract
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)
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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.
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