Three‐dimensional seismic velocity structure of the Earth's mantle using body wave travel times from intra‐plate and deep‐focus earthquakes

Abstract

This paper combines the recent measurements of P and S wave travel times from intra-plate earthquakes with travel times from deep-focus earthquakes reported earlier to infer the velocity structure of the mantle. It is reasonable that the source bias for intra-plate earthquakes is less pronounced than that for other shallow-focus earthquakes such as those occurring in island arcs. The inclusion of intra-plate earthquakes compensates for the lack of uniform areal distribution exhibited by the deep-focus earthquake data and augments the number of high quality travel time observations. The features observed in the variation of teleseismic travel times from intra-plate earthquakes are consistent with the travel time variation determined from deep-focus earthquakes (especially the P wave travel time variation). The combined intra-plate and deep-focus earthquake travel time data were converted to determine an average (spherically symmetric) compressional and shear velocity model and, subsequently, a three-dimensional compressional velocity model for the earth's mantle. Lateral variation in the mantle accounted for most of the scatter in the observed P wave travel times. The average compressional velocity model suggests two rapid velocity increases at 500 and 670 km depths and a small velocity gradient in the depth interval of 100-450 km. The 3-D model determined from the augmented travel time data set corroborated the earlier findings (1) that the lateral heterogeneities are more pronounced near the upper-mantle and near the core-mantle boundary and (2) that the surface tectonic features are correlated with compressional velocity anomahes near the upper mantle, but not with velocity anomalies below the upper mantle. A. WESTERN AUSTRALIA i i i i ß ... ß ß ß ß ß ß ß ß ß ß. ß i ß b) GABON ß ß ß •ß ß ß ß ß ß ß ß ß ß ß ß ß : ß ß I t ß ß ßß ß ß ß I ß d) HAWAIIAN ISLANDS ß i , I i , i i i i i i I ß ß ß t t ß ß ß ß ß ß ß ß . ß ß ß ßß •,ß ß ß ß ß ß ß ß d•ß ß ß & ß ß ß ß ß eee e•e I ß ß ß ß e " ß ß ß ß ß e e ß ee . ß ß . ß • ß ß ß ß ee ß ß • ß ß ß ß ' ee ß ß e•e ee • ß ß .. . • ß ß ß e ß ß ee ß ß ß ß •e e ß e) SOUTHWEST OF SUMATRA s/Sz, Bruce Julian, and Bob Sheppard for technical assistance during this study, Belle Lasher for drafting assistance, and Charlotte Scott and Dot Lyle for typing the manuscript. The authors also would like to thank the anonymous reviewers for their comments which have im-. proved the manuscript.

Evidence for a shear velocity discontinuity in the lower mantle beneath India and the Indian Ocean

“…ters (Alexander and Phinney, 1966; Cleary and Most contemporary velocity models for the lower -Haddon, 1972;Doornbos and Vlaar, 1973; Hadmost mantle have either slightly positive or slightly don and Cleary, 1974;King et al, 1974;Wright, negative velocity gradients in D", with smooth 1975;Husebye et al, 1976;Bolt and Niazi, 1984).…”

Lateral heterogeneity and azimuthal anisotropy in the North Atlantic determined from SS‐S differential travel times

“…If such an anomaly exists, we would expect it to be detected in global studies of P wave velocity structure. In the three-dimensional compressional velocity model of Sengupta et al [1981] in which velocity is represented by 10 ø by 10 ø, 500 km thick discrete blocks, in the depth range between 500 and 1000 km all the blocks in this area have more than five sampling rays and consistently show slightly slow rather than fast velocities with respect to the normal mantle. The models of Dziewonski [1984] also show no significant P wave anomaly in this area at this depth.…”

Tomographic inversions for mantle P wave velocity structure based on the minimization of l2 and l1 norms of International Seismological Centre Travel Time Residuals

“…All of the mantle tomographic studies carded out so far [e.g., Aki et aJ., 1977;Clayton and Comer, 1983;Dziewonski et al, 1977;Dziewonski, 1984;Grand, 1987;Inoue et al, 1990;Sengupta and Toksoz, 1976;Sengupta et al, 1981;Tralli and Johnson, 1986] employ least squares algorithms for the inversion of travel time residuals. Least squares approaches can be considered maximum likelihood estimators of mantle velocities when the errors on the data are normally distributed [Constable, 1988] but the errors on the ISC travel times are clearly not Gaussian [Jeffreys, 1932[Jeffreys, , 1939Bolt, 1960;Buland, 1984Buland, , 1986.…”

“…Those severely restrict the number of parameters available to describe the model due to limitations of existing computers. Increased detail became possible when iterative, approximate techniques were employed to solve the system of constraining equations (e.g., Sengupta et al [1981] and Clayton and Comer [1983] and presented by Hager and Clayton [1989]). The cost of this increased detail was that resolution and covariance could not be calculated formally and presented alongside the mantle model.…”