Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff)

Abstract

Abstract. The mapping of variations in P wave speed in the deep mantle is restricted by the uneven sampling of P waves, in particular beneath the Southern Hemisphere. To enhance data coverage, we augmented the • 1.6 million summary rays of P, pP, and pwP that we used in previous studies with differential travel times of diffracted and refracted core phases. For the core-refracted differential travel time residuals (PKP^e-PKPr)v and PKP^•3-PKP•3c) we used 1383 cross-correlated digital waveforms as well as •27,000 routinely processed bulletin data. We used the waveform data to define quality criteria for the selection and reduction of the bulletin PKP data. For PKPDF-Pdiff we only considered 543 records derived from waveform cross correlation. No PcP data were used in this study. We used optical ray theory to calculate the ray paths associated with the P, pP, pwP, and PKP data, which are measured at 1 Hz. However, to account for the large Fresnel zones of the low-frequency (•50 mHz) PKPDF-Pdiff data we estimated the three-dimensional shape of the Fr6chet sensitivity kernels from kernels calculated by normal mode summation. The use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high-frequency data to constrain small-scale structure. The differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers, and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary (CMB). However, images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle. Here, we do not make a priori assumptions about the mantle source of anomalous time differentials. From test inversions we conclude that (both upper and lower) mantle structures that are poorly resolved by P data can be mapped into the core along PKP paths but that the effect of outer core structures, if any, on the mantle model is small. Compared to the inversion of the P, pP, and pwP alone, the inclusion of the PKPA•-PKPr)• and PKPA•-PKP•c and PKPr)v-Pdifr data improves the resolution of structure beneath 2200 km depth. In particular, the joint inversion puts better constraints on the long-wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the CMB that is in agreement with but still smaller than inferences from shear wave studies. Resolution tests indicate that in some regions the enhanced definition of structure is significant, but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle.

Rheologic controls on slab dynamics

“…[3] Sinking of the lithosphere into the mantle is characterized by asymmetric subduction of one plate beneath another with slab dips of 30°to 90°, as constrained by seismicity to depths of 670 km [Isacks and Barazangi, 1977] and seismic tomography [Gudmundsson and Sambridge, 1998;Kárason and van der Hilst, 2001;Fukao et al, 2001]. The relationships between the present-day shape of slabs and slab buoyancy, surface plate motions, seismic coupling and history of subduction provide insight into the physical processes controlling the time evolution of slabs.…”

“…These models show that depending on (1) the magnitude of the viscosity jump or clapeyron slope for the phase changes, (2) the rate of trench migration, and (3) the strength of the slab, the slab can either cross unperturbed into the lower mantle, form temporary piles, or lie flat in the transition zone. All three of these slab morphologies are inferred from seismic tomography [e.g., Fukao et al, 2001;Kárason and van der Hilst, 2001].…”

Constraining P-wave velocity variations in the upper mantle beneath Southeast Asia

“…We use weighted composite rays (Kárason and van der Hilst, 2001) to better balance the sampling and further reduce the size of the sensitivity matrix. Because of noise in the data, we apply norm and gradient damping: norm damping favors a result that is close to the reference model and thus tends to minimize the amplitude of the model, while gradient damping reduces the differences between adjacent blocks and thus produces smooth variations, both laterally and radially.…”

“…The results presented here are a subset of the new Pwave global model; compared to our previous resultssee, for instance, Kárason and van der Hilst (2001) -the use of an irregular grid, the addition of PP-P and ABCE (P) data, and the crustal corrections combine to provide more detail in the upper mantle region of our current interest. The global model will be presented elsewhere (van der Hilst, Li, and Kárason, in preparation) but is freely available upon request.…”

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

“…Resolution of these small-scaled structures are important in understanding the material circulation and the thermal and chemical structure of the mantle. In particular, these differences make it difficult to address unambiguously the issue of whether the subducted slabs penetrate to the lower mantle, the mid-mantle, or the lowermost mantle (1)(2)(3)(4)(5)(6), or the issue of whether plumes rise up from the lowermost mantle to the surface (7)(8)(9).…”