ALBERT

Description: The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On April 1, 2014, this region was struck by a large earthquake following a two-week-long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami and seismic data, to produce a reliable kinematic slip model of the M w = 8.1 mainshock and a static slip model of the M w = 7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Green's functions, both static and dynamic, while avoiding non-physical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and up-dip of high-frequency sources inferred by back-projection analysis. Both the mainshock and the M w = 7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region.

Description: Meandering fluvial channels and their meander belts are common in modern continental sedimentary basins, yet compose a minor constituent of the reported fluvial rock record. Here we document exhumed amalgamated meander belt deposits from the upper Jurassic Morrison Formation, Utah (United States). The size of the amalgamated meander belt (9000 km 2 ) is significantly larger than any documented previously and comparable in size to those from modern sedimentary basins. We describe a representative outcrop of sandy point bar deposits that shows features considered characteristic of both braided and meandering fluvial systems. Lateral accretion sets compose 〈5% of the outcrop area, yet point bar morphology is clearly visible in plan view. We suggest that difficulties in the identification of sandy, amalgamated meander belt deposits indicate that they have gone largely unrecognized in the rock record. Their recognition has important implications for basin-scale reconstructions of fluvial systems and interpretation of tectonic setting.

Description: More than 1800 detrital zircon uranium-lead (U-Pb) ages collected from Franklinian Basin sedimentary strata of the Canadian Arctic Islands provide important insights into the depositional and tectonic evolution of the northern margin of Laurentia from the late Neoproterozoic to the Late Devonian. The Franklinian Basin succession is composed of strata with three distinctly different U-Pb age provenance signatures, which have implications for the tectonic and paleogeographic evolution of the entire Arctic region. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations of 1750–1950 Ma and 2650–2800 Ma, which are consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland–northeast Canadian Shield. The Lower Silurian to Lower Devonian Danish River Formation contains a dominant population of 900–2150 Ma detrital zircons with scattered Archean ages. The 900–2150 Ma zircons were likely transported axially along the foreland basin of the East Greenland Caledonides (Caledonian orogen) and deposited in a deep-water basin between the Pearya terrane and northern Laurentian margin. Middle Devonian to Upper Devonian strata contain detrital zircon populations of 900–2150 Ma, similar to the Danish River Formation, but these units also contain 370–450 Ma and 500–700 Ma detrital zircons. The 900–2150 Ma zircons were likely derived from the East Greenland Caledonian Mountains, the uplifted foreland of the East Greenland Caledonides, and the Pearya terrane. The population of 370–450 Ma detrital zircons potentially comes from uplifting granites in the Caledonian Mountains and Pearya terrane. The 500–700 Ma detrital zircons were likely derived from the continental landmass responsible for the Ellesmerian orogen. The 500–700 Ma age of the zircons suggests that the northern landmass likely had a connection to rocks of the Timanide orogens, located in the Timan Range of northwestern Russia. A dominant population of 365–450 Ma and 500–700 Ma ages in Upper Devonian strata suggests that the Pearya terrane and the northern continental landmass became the dominant source by the end of Franklinian Basin sedimentation.Because detrital zircons are often recycled from older strata into younger deposits, these data provide the basis for understanding the sedimentary provenance of younger units of the Sverdrup Basin and sedimentary wedges along the present Arctic continental margin.

Description: This study proposes a framework for estimating how much tropical cyclone intensification could result from the amount of energy released inside of a convective burst. A convective burst is a sequence of vigorous convective cells occupying one portion of a tropical cyclone's eyewall for approximately 9 to 24 h. On the basis of Tropical Rainfall Measuring Mission (TRMM) satellite radar observations and previous modeling studies, a typical convective burst may release in 12 h an extra 6 × 1017 J of latent heat. TRMM observations suggest that this extra energy represents an increase of 25% or more in the rate that the eyewall releases latent heat prior to the convective burst. Previous studies suggest that 4.5% to 11% of this extra latent heat may be transformed, after a lag of several hours, into an increase in the kinetic energy of the tropical cyclone's inner-core tangential wind. On the basis of the H*wind analysis of aircraft and dropsonde observations, an increase in kinetic energy of this magnitude may be associated with an intensification of 9–16 m s−1 (17–31 kt) in a tropical cyclone's maximum surface wind. This conservative estimate takes into account the increase in ocean surface friction during the period of intensification and assumes that the associated increase in ocean surface enthalpy flux does not counteract any of the frictional loss. Despite sources of uncertainty, it still appears that significant intensification is possible from the amount of energy released inside of a typical convective burst.

Description: We present results from the first systematic survey of proton and electron pitch angle distributions in the magnetotail, based on Cluster CIS and PEACE data binned by proton plasma β (βp). The proton distributions conform to the canonical picture of magnetotail ions - a boundary layer made up of Earthward streaming and bidirectional field-aligned particles, consistent with recent observations of time-varying beamlets, which gives way to a broadly isotropic central plasma sheet when βp ∼ 3. The electron distributions are significantly different from the canonical picture. A “boundary layer” made up of bidirectional field-aligned electrons is observed to values of βp as high as 17. This boundary quickly gives way to perpendicular-dominated electrons close to the neutral sheet. Hence, our results suggest that, on average, there is no extended, isotropic electron plasma sheet and that the proton plasma sheet is not routinely encountered until higher βp than commonly assumed.

Description: Abstract Damaged and hydrothermally altered rocks are ubiquitous in fault zones, with the degree of damage and type and intensity of alteration varying in space and time. The impact of damage and alteration on hydromechanical properties of fault zones is difficult to assess without characterizing the associated changes to rock and fracture mechanical parameters. To evaluate the mechanical properties of fault rocks from different alteration regimes, we conducted 1) double‐torsion load‐relaxation tests to measure mode‐I fracture toughness (KIC) and subcritical fracture growth index (SCI), 2) uniaxial testing to measure unconfined compressive strength (UCS) and static elastic parameters, and 3) mineralogic and textural characterization of rock from four sites in the footwall of the Dixie Valley‐Stillwater fault zone. Alteration at these sites includes: acid sulfate alteration and silicification associated with active fumaroles, intense silicification after calcite and chlorite alteration in an epithermal setting, quartz‐kaolinite‐carbonate alteration from an intermediate depth system, and a calcite‐chlorite‐hematite assemblage containing abundant unhealed damage. Silicification is associated with high KIC, SCI, UCS, and increased brittleness, and in precipitation‐dominated settings produces fault cores that are as strong or stronger than adjacent damage zone material. Calcite‐chlorite‐hematite assemblages containing abundant unsealed microfractures are approximately 4‐5 times weaker than the granodiorite protolith. Mechanical properties are not predicted by mineralogical composition alone; a key control is the accumulation of damage and degree of healing. Measures of strength increase when mineral precipitation reduces microfracture porosity to 〈10‐15% of total microfracture area. These results show that fault‐proximal weakening or strengthening is influenced by hydrothermal setting.

Description: [1]  The solar wind electron distribution is observed near and within 1 AU to consist of three components: a thermal core, and a suprathermal halo and strahl. The former two components are isotropic, while the strahl is field-aligned and flows outward along the interplanetary magnetic field. The evolution of solar wind electrons with heliocentric distance is poorly understood; although the halo is thought to be formed through pitch angle scattering of the strahl, the responsible physical process hasn’t been conclusively identified. Measurements of solar wind electrons throughout the heliosphere are required to solve this problem. We present the first observations of the suprathermal components of the solar wind electron distribution made outside5 AU . We find indications of a strahl component narrower than predicted by extrapolating observations and models of electrons in the inner heliosphere, suggesting the rate of electron pitch angle scattering in the solar wind can decrease with increasing heliocentric distance.

Description: Field-aligned beams of suprathermal electrons, known as 'strahl', are a frequently observed constituent of solar wind plasma. However, the formation and interplanetary evolution of the strahl electron populations has yet to be fully understood. As strahl electrons travel away from the Sun, they move into regions of decreasing magnetic field strength and thus are subject to adiabatic focusing. However, the widths of strahl pitch angle distributions observed at 1 AU are significantly broader than expected. Previous investigations have found that the average observed strahl pitch angle width actually increases with heliocentric radial distance. This implies that strahl electrons must be subjected to some form of pitch angle scattering process or processes, details of which as of yet remain elusive. In this paper, we use Cassini electron measurements to examine strahl beams across a distance range of approximately 8 AU, from its Earth Flyby in 1999 until its insertion into orbit around Saturn in 2004. We find that, in general, there is a relatively constant rate of broadening of strahl pitch angle distributions with distance between ∼1 - 5.5 AU. Our results from beyond this distance indicate that the strahl population is likely to be completely scattered, presumably to form part of the halo. We find multiple energy dependences at different radial distances implying that there are multiple strahl scattering mechanisms in operation.

Description: During a magnetic storm on 23 June 2015, several very intense substorms took place, with signatures observed by multiple spacecraft including DMSP and MMS. At the time of interest, DMSP F18 crossed inbound through a poleward-expanding auroral bulge boundary at 23.5h MLT, while MMS was located duskward of 22h MLT during an inward crossing of the expanding plasma sheet boundary. The two spacecraft observed a consistent set of signatures as they simultaneously crossed the reconnection separatrix layer during this very intense reconnection event. These include: 1) energy dispersion of the energetic ions and electrons travelling Earthwards, accompanied with high electron energies in the vicinity of the separatrix; 2) energy dispersion of polar rain electrons, with a high-energy cutoff; and 3) intense inward convection of the magnetic field lines at the MMS location. The high temporal resolution measurements by MMS provide unprecedented observations of the outermost electron boundary layer. We discuss the relevance of the energy dispersion of the electrons, and their pitch angle distribution, to the spatial and temporal evolution of the boundary layer. The results indicate that the underlying magnetotail magnetic reconnection process was an intrinsically impulsive and the active X-line was located relatively close to the Earth, approximately at 16-18 R E .