ALBERT

Description: We invert Global Positioning System (GPS) velocity data to estimate fault slip rates in California using a fault-based crustal deformation model with geologic constraints. The model assumes buried elastic dislocations across the region using Uniform California Earthquake Rupture Forecast Version 3 (UCERF3) fault geometries. New GPS velocity and geologic slip-rate data were compiled by the UCERF3 deformation working group. The result of least-squares inversion shows that the San Andreas fault slips at 19–22 mm/yr along Santa Cruz to the North Coast, 25–28 mm/yr along the central California creeping segment to the Carrizo Plain, 20–22 mm/yr along the Mojave, and 20–24 mm/yr along the Coachella to the Imperial Valley. Modeled slip rates are 7–16 mm/yr lower than the preferred geologic rates from the central California creeping section to the San Bernardino North section. For the Bartlett Springs section, fault slip rates of 7–9 mm/yr fall within the geologic bounds but are twice the preferred geologic rates. For the central and eastern Garlock, inverted slip rates of 7.5 and 4.9 mm/yr, respectively, match closely with the geologic rates. For the western Garlock, however, our result suggests a low slip rate of 1.7 mm/yr. Along the eastern California shear zone and southern Walker Lane, our model shows a cumulative slip rate of 6.2–6.9 mm/yr across its east–west transects, which is ~1 mm/yr increase of the geologic estimates. For the off-coast faults of central California, from Hosgri to San Gregorio, fault slips are modeled at 1–5 mm/yr, similar to the lower geologic bounds. For the off-fault deformation, the total moment rate amounts to 0.88 x 10 19 N·m/yr, with fast straining regions found around the Mendocino triple junction, Transverse Ranges and Garlock fault zones, Landers and Brawley seismic zones, and farther south. The overall California moment rate is 2.76 x 10 19 N·m/yr, which is a 16% increase compared with the UCERF2 model. Online Material: Table of geological slip rates.

Description: Abstract Suprathermal electron bursts (STEBs), characterized by a board energy spectrum and a field‐aligned pitch angle distribution, have been well recognized to be associated with electron acceleration by inertial Alfvén waves and are thus conventionally termed as “Alfvénic aurora.” In this study, we report joint Enhanced‐Polar‐Outflow‐Probe (e‐POP) and ground‐based optical observations of Alfvénic auroras. In particular, we highlight the prominence of 630‐nm red line emissions under low‐energy Alfvénic auroral precipitation. During the event interval, e‐POP traverses two arcs. One bright arc dominated by green line emissions is clearly seen by all optical instruments; it is embedded in upward field‐aligned currents (FACs) yet leaves little imprint on the e‐POP suprathermal electron imager (SEI), likely due to that the precipitation is well above the upper energy limit of SEI. On the other hand, there is a red line arc that is pronounced only in 630‐nm images. Such a red‐line‐only arc is located in a transition from large‐scale upward FACs to downward FACs and is associated with a prominent STEB structure detected by e‐POP SEI. The STEB features an inverse energy time dispersion, namely, that lower‐energy electrons are seen earlier while higher‐energy electrons appear later. The red‐line‐only arc and its separation from the green line arc evolve in a repeatable fashion, each stemming from a poleward auroral intensification (PAI) propagated from higher latitudes. Following each poleward auroral intensification the green line arc progressively moved southward, while the red‐line‐only arc is quasi‐stationary and stayed relatively stable in latitude. We propose tentative interpretations of the above features based upon stationary inertial Alfvén waves.

Description: Abstract Our understanding of the tectonic development of the African continent and the interplay between its geological provinces is hindered by unevenly distributed seismic instrumentation. In order to better understand the continent, we used long‐period ambient noise full‐waveform tomography on data collected from 186 broadband seismic stations throughout Africa and surrounding regions to better image the upper mantle structure. We extracted empirical Green's functions from ambient seismic noise using a frequency‐time normalization method and retrieved coherent signal at periods of 7–340 s. We simulated wave propagation through a heterogeneous Earth using a spherical finite‐difference approach to obtain synthetic waveforms, measured the misfit as phase delay between the data and synthetics, calculated numerical sensitivity kernels using the scattering integral approach, and iteratively inverted for structure. The resulting images of isotropic, shear wave speed for the continent reveal segmented, low‐velocity upper mantle beneath the highly magmatic northern and eastern sections of the East African Rift System (EARS). In the southern and western sections, high‐velocity upper mantle dominates, and distinct, low‐velocity anomalies are restricted to regions of current volcanism. At deeper depths, the southern and western EARS transition to low velocities. In addition to the EARS, several low‐velocity anomalies are scattered through the shallow upper mantle beneath Angola and North Africa, and some of these low‐velocity anomalies may be connected to a deeper feature. Distinct upper mantle high‐velocity anomalies are imaged throughout the continent and suggest multiple cratonic roots within the Congo region and possible cratonic roots within the Sahara Metacraton.

Description: We invert Global Positioning System (GPS) velocity data to estimate fault slip rates in California using a fault-based crustal deformation model with geologic constraints. The model assumes buried elastic dislocations across the region using Uniform California Earthquake Rupture Forecast Version 3 (UCERF3) fault geometries. New GPS velocity and geologic slip-rate data were compiled by the UCERF3 deformation working group. The result of least-squares inversion shows that the San Andreas fault slips at 19–22 mm/yr along Santa Cruz to the North Coast, 25–28 mm/yr along the central California creeping segment to the Carrizo Plain, 20–22 mm/yr along the Mojave, and 20–24 mm/yr along the Coachella to the Imperial Valley. Modeled slip rates are 7–16 mm/yr lower than the preferred geologic rates from the central California creeping section to the San Bernardino North section. For the Bartlett Springs section, fault slip rates of 7–9 mm/yr fall within the geologic bounds but are twice the preferred geologic rates. For the central and eastern Garlock, inverted slip rates of 7.5 and 4.9 mm/yr, respectively, match closely with the geologic rates. For the western Garlock, however, our result suggests a low slip rate of 1.7 mm/yr. Along the eastern California shear zone and southern Walker Lane, our model shows a cumulative slip rate of 6.2–6.9 mm/yr across its east–west transects, which is ~1 mm/yr increase of the geologic estimates. For the off-coast faults of central California, from Hosgri to San Gregorio, fault slips are modeled at 1–5 mm/yr, similar to the lower geologic bounds. For the off-fault deformation, the total moment rate amounts to 0.88 x 10 19 N·m/yr, with fast straining regions found around the Mendocino triple junction, Transverse Ranges and Garlock fault zones, Landers and Brawley seismic zones, and farther south. The overall California moment rate is 2.76 x 10 19 N·m/yr, which is a 16% increase compared with the UCERF2 model. Online Material: Table of geological slip rates.

Description: By determining the location and size of the Region 1 (R1) and Region 2 (R2) large-scale field-aligned currents (FACs) from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) data, we are able to study the small-scale magnetic fluctuations observed by the Swarm satellites embedded within the large-scale FACs. A statistical comparison of R1 and R2 high-frequency fluctuations is presented in terms of different solar wind conditions and geomagnetic activities. We find that: (1) the amplitude of high-frequency fluctuations in both R1 and R2 increases as the large-scale R1 and R2 FACs intensify; (2) high-frequency fluctuations in R1 peak near dayside dawn and dusk, while those in R2 peak around noon; (3) the location of the largest high-frequency fluctuations in R1 shifts in local time in response to IMF By, indicating a connection between the R1 fluctuation and the driving solar wind most likely explained by magnetic reconnection; (4) high-frequency fluctuations in R2 are enhanced in a small region near local noon and respond clearly to nightside drivers, as characterized by the auroral electrojet index. Our analysis shows that the intensity of R1 and R2 high-frequency magnetic fluctuations is directly connected to the intensity of FACs, which implies that the magnetic fluctuations are closely related to the magnetospheric processes that drive them.

Description: Particle filters (PFs) constitute a sequential data assimilation method based on the Monte Carlo approximation of Bayesian estimation theory. Standard PFs use scalar weights derived from the likelihood of the approximate posterior probability density functions (PDFs) of observations and use resampling schemes to generate posterior particles. However, the scalar weights approach interferes with the localization algorithm and often results in filter degeneracy. Recently, a localized particle filter (LPF) was developed by extending the scalar weights of PF to vector weights, which produces various (local) posterior PDFs for different model grids and variables. With a sampling and merging approach in the resampling, an LPF can effectively solve the filter degeneracy problem and offer a practical, efficient algorithm for localization. However, this algorithm assumes the variations in the weights of a state variable of neighbouring grids to be continuous and uses a spatially linear interpolation of PF weights to determine the local weights. In this paper, we first analyse the possible concerns associated with the linear continuity of PF weights. This assumption is found to challenge the theoretical properties of nonlinear and non-Gaussian variations in weights and alleviate the intrinsic spatial variations of PF weights. On this basis, we propose a new algorithm to produce vector weights for PFs for neighbouring grids. Numerical experiments using the Lorenz ’96 model show that our new localized particle filter performs better than the existing LPF algorithm, indicating the advantages and potential applications of this new algorithm of vector weights in the field of data assimilation.

Description: ABSTRACT Maintenance of hematopoietic stem cells (HSC) takes place in a highly specialized microenvironment within the bone marrow. Technological improvements, especially in the field of in vivo imaging, have helped unravel the complexity of the niche microenvironment and have completely changed the classical concept from what was previously believed to be a static supportive platform, to a dynamic microenvironment tightly regulating HSC homeostasis through the complex interplay between diverse cell types, secreted factors, extracellular matrix molecules and the expression of different transmembrane receptors. To add to the complexity, non-protein based metabolites have also been recognized as a component of the bone marrow niche. The objective of this review is to discuss the current understanding on how the different extracellular matrix components of the niche regulate HSC fate, both during embryonic development and in adulthood. Special attention will be provided to the description of non-protein metabolites, such as lipids and metal ions, which contribute to the regulation of HSC behavior. This article is protected by copyright. All rights reserved