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On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2019. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. With the advent of AGU's data policy, many reviewers have helped immensely to evaluate the accessibility and availability of data associated with the papers they have reviewed, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics.

On behalf of the journal, AGU, and the scientific community, the Editors would like to sincerely thank those who reviewed manuscripts for Geophysical Research Letters in 2018. The hours reading and commenting on manuscripts not only improves the manuscripts but also increases the scientific rigor of future research in the field. We particularly appreciate the timely reviews, in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics. In total, 4,484 referees contributed to 7,557 individual reviews in journal. Thank you again. We look forward to the coming year of exciting advances in the field and communicating those advances to our community and to the broader public.

Seismic tomography is the most abundant source of information about the internal structure of the Earth at scales ranging from a few meters to thousands of kilometers. It constrains the properties of active volcanoes, earthquake fault zones, deep reservoirs and storage sites, glaciers and ice sheets, or the entire globe. It contributes to outstanding societal problems related to natural hazards, resource exploration, underground storage, and many more. The recent advances in seismic tomography are being translated to nondestructive testing, medical ultrasound, and helioseismology. Nearly 50 yr after its first successful applications, this article offers a snapshot of modern seismic tomography. Focused on major challenges and particularly promising research directions, it is intended to guide both Earth science professionals and early-career scientists. The individual contributions by the coauthors provide diverse perspectives on topics that may at first seem disconnected but are closely tied together by a few coherent threads: multiparameter inversion for properties related to dynamic processes, data quality, and geographic coverage, uncertainty quantification that is useful for geologic interpretation, new formulations of tomographic inverse problems that address concrete geologic questions more directly, and the presentation and quantitative comparison of tomographic models. It remains to be seen which of these problems will be considered solved, solved to some extent, or practically unsolvable over the next decade. [ABSTRACT FROM AUTHOR]

We employ a new thermodynamic method for self-consistent computation of compositional and thermal effects on phase transition depths, density, and seismic velocities. Using these profiles, we compare theoretical and observed differential traveltimes between P410s and P([T.sub.410]) and between P600s and P410s ([T.sub.660-410]) that are affected only by seismic structure in the upper mantle. The anticorrelation between [T.sub.410] and [T.sub.660-410] suggests that variations in [T.sub.410] and [T.sub.660-410] of ~8 s are due to lateral temperature variations in the upper mantle transition zone of ~400 K. If the mantle is a mechanical mixture of basaltic and harzburgitic components, our traveltime data suggest that the mantle has an average temperature of 1600 [+ or -] 50 K, in agreement with temperature estimates from magma compositions of mid-ocean ridge basalts. We infer a 100 K hotter mantle if we assume the mantle to have a homogeneous pyrolitic composition. The transition-zone temperature beneath hotspots and within subduction zones is relatively high and low, respectively. However, the largest variability in [T.sub.410] and [T.sub.660-410] is recorded by global stations far from subduction zones and hotspots. This indicates that the 400 K variation in upper mantle temperature is complicated by tilted upwellings, slab flattening and accumulation, ancient subduction, and processes unrelated to present-day subduction and plume ascent.

A 200-400-kilometer-wide thermal anamoly has been discovered beneath the eastern branch of the East African rift system. The mantle in this location has the geochemical aspects of a plume equal to or less than 150 kilometers below northern Tanzania, which may explain this phenomenon.

A number of seismological studies have indicated that the ratio R of S -wave and P -wave velocity perturbations increases to 3–4 in the lower mantle with the highest values in the large low-velocity provinces (LLVPs) beneath Africa and the central Pacific. Traveltime constraints on R are based primarily on ray-theoretical modelling of delay times of P waves (Δ T P) and S waves (Δ T S), even for measurements derived from long-period waveforms and core-diffracted waves for which ray theory (RT) is deemed inaccurate. Along with a published set of traveltime delays, we compare predicted values of Δ T P, Δ T S, and the Δ T S/Δ T P ratio for RT and finite-frequency (FF) theory to determine the resolvability of R in the lower mantle. We determine the FF predictions of Δ T P and Δ T S using cross-correlation methods applied to spectral-element method waveforms, analogous to the analysis of recorded waveforms, and by integration using FF sensitivity kernels. Our calculations indicate that RT and FF predict a similar variation of the Δ T S/Δ T P ratio when R increases linearly with depth in the mantle. However, variations of R in relatively thin layers (< 400 km) are poorly resolved using long-period data (T > 20 s). This is because FF predicts that Δ T P and Δ T S vary smoothly with epicentral distance even when vertical P -wave and S -wave gradients change abruptly. Our waveform simulations also show that the estimate of R for the Pacific LLVP is strongly affected by velocity structure shallower in the mantle. If R increases with depth in the mantle, which appears to be a robust inference, the acceleration of P waves in the lithosphere beneath eastern North America and the high-velocity Farallon anomaly negates the P -wave deceleration in the LLVP. This results in a Δ T P of about 0, whereas Δ T S is positive. Consequently, the recorded high Δ T S/Δ T P for events in the southwest Pacific and stations in North America may be misinterpreted as an anomalously high R for the Pacific LLVP. [ABSTRACT FROM AUTHOR]