skip to main content

Title: Array‐Based Iterative Measurements of Travel Times and Their Constraints on Outermost Core Structure

Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate awave velocity,, at the top of the outer core (thelayer), which is lower than that in the Preliminary Reference Earth Model. However, these lowmodels do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuringarrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measuredifferential travel times. This approach extracts individualsignal from mixed waveforms of theseries, allowing us to reliably measure differential travel times. We successfully use this method to measuretime delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones.time delays are measured by waveform cross correlation betweenand, and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to syntheticseismograms to investigate the 3‐D mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias theestimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of,, and, supporting a lowat the top of Earth's outer core.

more » « less
Award ID(s):
Author(s) / Creator(s):
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH+ OCHNH/ CHNH + HO, CHNH+ HCH+ NH, CHNHCHNH, and CHNH + OCHNH + HOwere calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNHand its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NHgroup in CHNHcompared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NHin methylamine oxidation.

    more » « less
  2. Abstract

    We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF Bin NH (Bin SH) in most local seasons under both signs of IMF B. For Bin NH (Bin SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During Band Bin NH (Band Bin SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.180.09 in winter and 1.170.09 in autumn. During Band Bin NH (Band Bin SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.160.07 for FAC. We also find an explicit Beffect on auroral currents in a given hemisphere: on average Bin NH and Bin SH causes larger currents than vice versa. The explicit Beffect on divergence‐free current during IMF Bis in very good agreement with the Beffect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer.

    more » « less
  3. Abstract

    The potential for molecular hydrogen () generated via serpentinization to fuel subsurface microbial ecosystems independent from photosynthesis has prompted biogeochemical investigations of serpentinization‐influenced fluids. However, investigations typically sample via surface seeps or open‐borehole pumping, which can mix chemically distinct waters from different depths. Depth‐indiscriminate sampling methods could thus hinder understanding of the spatial controls on nutrient availability for microbial life. To resolve distinct groundwaters in a low‐temperature serpentinizing environment, we deployed packers (tools that seal against borehole walls during pumping) in two‐deep, peridotite‐hosted wells in the Samail Ophiolite, Oman. Isolation and pumping of discrete intervals as deep astobelow ground level revealed multiple aquifers that ranged in pH from 8 to 11. Chemical analyses and 16S rRNA gene sequencing of deep, highly reactedgroundwaters bearing up to,methane () andsulfate () revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported byoxidation coupled toreduction. In shallower, oxidizedgroundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. Highandof(up toand, respectively) indicated microbialoxidation, particularly inwaters with evidence of mixing withwaters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.

    more » « less
  4. Purpose

    We propose a method to acquiredistribution plots by encoding ininstead of image space. Using this method,data is acquired in a different way from traditional spatialmapping, and allows for quick measurement of high dynamic rangedata.


    To encode in, we acquire multiple projections of a slice, each along the same direction, but using a different phase sensitivity to. Using a convex optimization formulation, we reconstruct histograms of thedistribution estimates of the slice.


    We verify in vivodistribution measurements by comparing measured distributions to distributions calculated from reference spatialmaps using the Earth Mover's Distance. Phantom measurements using a surface coil show that for increased spatialvariations, measureddistributions using the proposed method more accurately estimate the distribution than a low‐resolution spatialmap, resulting in a 37% Earth Mover's Distance decrease while using fewer measurements.


    We propose and validate the performance of a method to acquiredistribution information directly without acquiring a spatialmap. The method may provide faster estimates of afield for applications that do not require spatiallocalization, such as the transmit gain calibration of the scanner, particularly for high dynamicranges. Magn Reson Med 77:229–236, 2017. © 2016 Wiley Periodicals, Inc.

    more » « less
  5. Abstract

    Letbe a graph,be an integer, and writefor the maximum number of edges in an‐vertex graph that is‐partite and has no subgraph isomorphic to. The functionhas been studied by many researchers. Findingis a special case of the Zarankiewicz problem. We prove an analog of the Kövári‐Sós‐Turán theorem for 3‐partite graphs by showingurn:x-wiley:10638539:media:jcd21654:jcd21654-math-0009for. Using Sidon sets constructed by Bose and Chowla, we prove that this upper bound is asymptotically best possible in the case thatandis odd, that is,for. In the cases ofand, we use a result of Allen, Keevash, Sudakov, and Verstraëte, to show that a similar upper bound holds for alland gives a better constant when. Finally, we point out an interesting connection between difference families from design theory and.

    more » « less