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Author |
Houser C, Masters G, Shearer P, Laske G, |
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Title |
Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms
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Type |
Journal Article |
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Year |
2008 |
Publication |
Geophysical Journal International |
Abbreviated Journal |
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Volume |
174 |
Issue |
1 |
Pages |
195-212 |
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Keywords  |
Mantle processes, Body waves, Seismic tomography, |
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Abstract |
We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique. This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23° to 100°) and SS and PP (from 50° to 170°). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the coremantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.
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133 |
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Blackwell Publishing Ltd |
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ISSN |
1365-246X |
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Approved |
yes |
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Call Number |
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Serial |
2812 |
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Author |
Mohiuddin Anwar, Long Maureen D, Lynner Colton, |
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Title |
Mid-mantle seismic anisotropy beneath southwestern Pacific subduction systems and implications for mid-mantle deformation
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Type |
Journal Article |
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Year |
2015 |
Publication |
Physics of the Earth and Planetary Interiors |
Abbreviated Journal |
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Volume |
245 |
Issue |
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Pages |
1-14 |
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Keywords |
Mantle deformation, Mid-mantle, Seismic anisotropy, Shear wave splitting, Subduction zones, |
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Abstract |
Observations of seismic anisotropy can offer relatively direct constraints on patterns of mantle deformation, but most studies have focused on the upper mantle. While much of the lower mantle is thought to be isotropic, several recent studies have found evidence for anisotropy in the transition zone and uppermost lower mantle (the mid-mantle), particularly in the vicinity of subducting slabs. Here we investigate anisotropy at mid-mantle depths in the Tonga-Kermadec, Sumatra, New Britain, New Hebrides, and Philippines subduction zones using the source-side shear wave splitting technique. We measure splitting of direct teleseismic S phases originating from deep events (>300km) that have been corrected for the effect of upper mantle anisotropy beneath the seismic stations. We find evidence for considerable anisotropy at mid-mantle depths in all subduction systems studied, with delay times averaging ∼1.0–1.5s. Several measurements originating from depths greater than 600km exhibit delay times greater than 1s, suggesting a significant contribution from anisotropy in the uppermost lower mantle. We combine our results with those documented in previous studies into a quasi-global set of source-side shear wave splitting measurements that reflect mid-mantle anisotropy. We document significant variability in the dominant fast directions both within and among individual subduction systems, suggesting different deformation geometries among different subduction systems. As further constraints on the elasticity and deformation of mid-mantle minerals become available, our dataset can be used to constrain patterns of mid-mantle flow associated with subduction. |
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133 |
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ISSN |
0031-9201 |
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Approved |
yes |
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Call Number |
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Serial |
6152 |
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Author |
Thébault, E.; Schott, J.J.; Mandea, M. |
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Title |
Revised spherical cap harmonic analysis (R-SCHA): Validation and properties |
Type |
Journal Article |
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Year |
2006 |
Publication |
Journal of geophysical research-atmospheres |
Abbreviated Journal |
J. Geophys. Res. |
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Volume |
111 |
Issue |
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Pages |
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Keywords |
magnetism; regional modeling; Spherical Cap Harmonic Analysis; 1214 Geodesy and Gravity: Geopotential theory and determination; 1517 Geomagnetism and Paleomagnetism: Magnetic anomalies: modeling and interpretation; 1532 Geomagnetism and Paleomagnetism: Reference fields: regional, global; 1541 Geomagnetism and Paleomagnetism: Satellite magnetics: main field, crustal field, external field; 1545 Geomagnetism and Paleomagnetism: Spatial variations: all harmonics and anomalies |
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Abstract |
We recently proposed a technique able to represent the spatial variations of the magnetic field at regional scales. However, we pointed out that these preliminary developments were not suited for the complete representation of the geomagnetic field. In this paper, we propose a complete revision, the revised spherical cap harmonic analysis (R-SCHA), which introduces slight changes in order to rectify the previous shortcomings. In addition, some discussions shed a new light on the former spherical cap harmonic analysis (SCHA) and help us to demonstrate its deficiencies and approximations. We finally show that R-SCHA now fully satisfies the natural properties of potential fields. R-SCHA also yields analytical relationships with the spherical harmonics. Taking advantage of the mathematical equivalence of both representations, we explore the relevance of fundamental concepts like spectrum, minimum wavelength, or internal/external field separation. We conclude that these concepts are misleading and must be handled with care in regional modeling. A prime goal being the ability of R-SCHA to represent real data sets, we also investigate and illustrate the effect of finite series expansions. A norm for the regularization of the inverse problem is proposed as well. The conclusions drawn in this paper allow us to validate the method and to assert that the present proposal is suited for modeling and studying the lithospheric magnetic field from ground to satellite altitudes at regional scales. |
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Programme |
139;905 |
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Publisher |
American Geophysical Union |
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Abbreviated Series Title |
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Edition |
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ISSN |
0148-0227 |
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Approved |
yes |
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Call Number |
IPEV @ Thierry.Lemaire @ |
Serial |
5626 |
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Author |
International Association of Geomagnetism and Aeronomy Working Group V-MOD Participating members, Finlay C C, Maus S, Beggan C D, Bondar T N, Chambodut A, Chernova T A, Chulliat A, Golovkov V P, Hamilton B, Hamoudi M, Holme R, Hulot G, Kuang W, Langlais B, Lesur V, Lowes F J, Lühr H, Macmillan S, Mandea M, McLean S, Manoj C, Menvielle M, Michaelis I, Olsen N, Rauberg J, Rother M, Sabaka T J, Tangborn A, Tøffner-Clausen L, Thébault E, Thomson A W P, Wardinski I, Wei Z, Zvereva T I, |
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Title |
International Geomagnetic Reference Field: the eleventh generation
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Type |
Journal Article |
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Year |
2010 |
Publication |
Geophysical Journal International |
Abbreviated Journal |
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Volume |
183 |
Issue |
3 |
Pages |
1216 -1230 |
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Keywords |
Magnetic field, Satellite magnetics, |
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Abstract |
SUMMARY The eleventh generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2009 by the International Association of Geomagnetism and Aeronomy Working Group V-MOD. It updates the previous IGRF generation with a definitive main field model for epoch 2005.0, a main field model for epoch 2010.0, and a linear predictive secular variation model for 2010.02015.0. In this note the equations defining the IGRF model are provided along with the spherical harmonic coefficients for the eleventh generation. Maps of the magnetic declination, inclination and total intensity for epoch 2010.0 and their predicted rates of change for 2010.02015.0 are presented. The recent evolution of the South Atlantic Anomaly and magnetic pole positions are also examined.
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Programme |
139;905 |
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Corporate Author |
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Thesis |
Bachelor's thesis |
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Publisher |
Blackwell Publishing Ltd |
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Abbreviated Series Title |
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Series Issue |
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Edition |
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ISSN |
1365-246X |
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Notes |
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Approved |
yes |
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Call Number |
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Serial |
2683 |
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| Permanent link to this record |
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Author |
Thébault, E.; Schott, J. J.; Mandea, M.; Hoffbeck, J. P. |
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Title |
A new proposal for spherical cap harmonic modelling. |
Type |
Journal Article |
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Year |
2004 |
Publication |
Geophysical journal international |
Abbreviated Journal |
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Volume |
159 |
Issue |
2 |
Pages |
83-103 |
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Keywords |
magnetic field, regional modelling, spherical cap harmonic analysis |
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Abstract |
The geomagnetic field above the surface of the Earth in the current-free region may be expressed as the gradient of a scalar potential solving Laplace's equation. For regions with a fairly dense coverage of data at different altitudes, a regional model ought to offer a better spatial resolution of the regional field over the volume under study than a global field expanded in spherical harmonics (SH). The spherical cap harmonics analysis (SCHA) is an attractive regional modelling tool having close relationship with global SH. With the SCHA adopted so far, difficulties arise in upward continuation and in establishing a relationship between global and local Gauss coefficients. Such a relationship would be useful, for instance, for introducing prior constraint on an inverse problem dealing with the estimation of local Gauss coefficients based upon a local data set. In this paper, we show that these difficulties are overcome if the SCHA modelling is formulated as a boundary value (BV) problem in a cone bounded radially by the surface of the Earth and an upper surface suitable for satellite data, and bounded laterally in order to encompass a specific region of study. Although the example worked out here applies only to a limited class of fields, which verifies some special flux condition, the ideas behind this formalism are quite general and should offer a new way of processing data in a bounded region of space. |
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Programme |
139;905 |
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Abbreviated Series Title |
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Series Volume |
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Series Issue |
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Edition |
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ISSN |
0956-540X |
ISBN |
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Notes |
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Approved |
yes |
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Call Number |
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Serial |
4706 |
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Author |
V. Haberle, A. Marchaudon, A. Chambodut, P.-L. Blelly |
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Title |
Direct Determination of Geomagnetic Baselines During Quiet Periods for Low- and Mid-Latitude Observatories |
Type |
Journal |
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Year |
2022 |
Publication |
Journal of Geophysical Research: Space Physics |
Abbreviated Journal |
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Volume |
127 |
Issue |
8 |
Pages |
e2022JA030407 |
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Keywords |
magnetic baseline magnetic observatory data quiet geomagnetic field Sq current |
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Abstract |
The geomagnetic field is composed of a variety of sources that act on a wide range of timescales and amplitudes. The separation of magnetic storm effects from quiet variations is needed to accurately quantify impacts of space weather events. The extraction of such quiet contributions within geomagnetic measurements is achieved by the determination of baselines, which, ideally, is done by a simple algorithm which captures quiet sources suitably well, while being applicable to an extensive network of magnetic observatories independent of the period of time. In this work, we apply signal filtering techniques on the horizontal components of geomagnetic field measurements from low- and mid-latitude observatories to determine baselines. The variations within the baseline are investigated for magnetically quiet periods between 1991 and 2019, focusing on long-term trends, seasonal and local time dependencies, and day-to-day variability. The analysis confirms that the contributing quiet sources include the secular variation and the solar quiet (Sq) current system. The non-negligible day-to-day variability, that is typical for Sq in low- and mid-latitudes, is embedded within the baseline. Thus, the filter approach extracts quiet magnetic field variations well. Comparisons with other baseline methods show good agreements. We conclude that the filter approach can be used to determine baselines automatically during magnetically quiet periods without the need of further apriori information and is applicable on a wide network of magnetic observatories. It marks the first step for deriving magnetic indices for (near) real-time space weather applications. |
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Programme |
139 |
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Series Title |
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Abbreviated Series Title |
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Series Volume |
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Series Issue |
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Edition |
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ISSN |
2169-9402 |
ISBN |
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Notes |
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Approved |
yes |
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Call Number |
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Serial |
8568 |
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Author |
R. Sulzbach, H. Dobslaw, M. Thomas |
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Title |
High-Resolution Numerical Modeling of Barotropic Global Ocean Tides for Satellite Gravimetry |
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Journal |
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Year |
2021 |
Publication |
Journal of Geophysical Research: Oceans |
Abbreviated Journal |
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Volume |
126 |
Issue |
5 |
Pages |
e2020JC017097 |
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Keywords |
M2-tide minor tides pole-rotation self-attraction and loading tide-generating potential topographic wavedrag |
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Abstract |
The recently upgraded barotropic tidal model TiME is employed to study the influence of fundamental tidal processes, the chosen model resolution, and the bathymetric map on the achievable model accuracy, exemplary for the M2 tide. Additionally, the newly introduced pole-rotation scheme allows to estimate the model’s inherent precision (open ocean rms: 0.90 cm) and enables studies of the Arctic domain without numerical deviations originating from pole cap handling. We find that the smallest open ocean rms with respect to the FES14-atlas (3.39 cm) is obtained when tidal dissipation is carried out to similar parts by quadratic bottom friction, wave drag, and parametrized eddy-viscosity. This setting proves versatile to obtaining high accuracy values for a diverse ensemble of additional partial tides. Using the preferred model settings, we show that for certain minor tides it is possible to obtain solutions that are more accurate than results derived with admittance assumptions from data-constrained tidal atlases. As linear admittance derived minor tides are routinely used for de-aliasing of satellite gravimetric data, this opens the potential for improving gravity field products by employing the solutions from TiME. |
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Programme |
688 |
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Abbreviated Series Title |
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Series Issue |
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Edition |
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ISSN |
2169-9291 |
ISBN |
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Approved |
yes |
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Call Number |
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Serial |
8605 |
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Author |
Gómez-Díaz Elena, Doherty Jr Paul F, Duneau David, McCoy Karen D, |
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Title |
Cryptic vector divergence masks vector-specific patterns of infection: an example from the marine cycle of Lyme borreliosis
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Type |
Journal Article |
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Year |
2010 |
Publication |
Evolutionary Applications |
Abbreviated Journal |
Evol Appl |
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Volume |
3 |
Issue |
4 |
Pages |
391 -401 |
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Keywords |
lyme disease bacteria, pathogen detection, seabirds, site occupancy models, transmission, |
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Abstract |
Vector organisms are implicated in the transmission of close to a third of all infectious diseases. In many cases, multiple vectors (species or populations) can participate in transmission but may contribute differently to disease ecology and evolution. The presence of cryptic vector populations can be particularly problematic as differences in infection can be difficult to evaluate and may lead to erroneous evolutionary and epidemiological inferences. Here, we combine site-occupancy modeling and molecular assays to evaluate patterns of infection in the marine cycle of Lyme borreliosis, involving colonial seabirds, the tick Ixodes uriae, and bacteria of the Borrelia burgdorferi s.l. complex. In this cycle, the tick vector consists of multiple, cryptic (phenotypically undistinguishable but genetically distinct) host races that are frequently found in sympatry. Our results show that bacterial detection varies strongly among tick races leading to vector-specific biases if raw counts are used to calculate Borrelia prevalence. These differences are largely explained by differences in infection intensity among tick races. After accounting for detection probabilities, we found that overall prevalence in this system is higher than previously suspected and that certain vector-host combinations likely contribute more than others to the local dynamics and large-scale dispersal of Borrelia spirochetes. These results highlight the importance of evaluating vector population structure and accounting for detection probability when trying to understand the evolutionary ecology of vector-borne diseases.
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Programme |
333 |
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Series Title |
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Abbreviated Series Title |
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Edition |
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ISSN |
1752-4571 |
ISBN |
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Approved |
yes |
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Call Number |
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Serial |
2324 |
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| Permanent link to this record |
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Author |
Lynner Colton, Long Maureen D, |
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Title |
Lowermost mantle anisotropy and deformation along the boundary of the African LLSVP
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Type |
Journal Article |
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Year |
2014 |
Publication |
Geophysical Research Letters |
Abbreviated Journal |
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Volume |
41 |
Issue |
10 |
Pages |
3447-3454 |
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Keywords |
lowermost mantle anisotropy, African LLSVP, discrepant SKS-SKKS splitting, 7208 Mantle, 7203 Body waves, |
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Abstract |
Shear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we present SKS and SKKS splitting measurements for 233 event-station pairs at 34 seismic stations that sample D″ beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D″ region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed). |
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Programme |
133 |
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Place of Publication |
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Abbreviated Series Title |
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Series Issue |
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Edition |
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ISSN |
1944-8007 |
ISBN |
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Approved |
yes |
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Call Number |
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Serial |
5026 |
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Author |
Frandsen MS, Fort J, Rigét FF, Galatius A, Mosbech A, |
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Title |
Composition of chick meals from one of the main little auk (Alle alle) breeding colonies in Northwest Greenland
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Type |
Journal Article |
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Year |
2014 |
Publication |
Polar Biology |
Abbreviated Journal |
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Volume |
37 |
Issue |
7 |
Pages |
1055-1060- |
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Keywords |
Little auk, Chick diet, High arctic, Seabirds, Calanus species, |
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Abstract |
During the last decade, increasing information on little auk (Alle alle) biology, ecology and behaviour has been reported. However, only a few of these studies have focused on the breeding population in the Avanersuaq (Thule) district of Northwest Greenland, where 80 % of the global little auk population is estimated to breed. This study reports on the chick diet composition from one of the largest colonies, the Paakitsoq colony, located on the south-eastern margin of the North Water (NOW) Polynya. Results revealed the highest proportion of Calanus hyperboreus, a large lipid-rich copepod, in chick diet reported for any little auk colony. Results confirmed that the cold,
highly productive waters of the NOW Polynya are favourable foraging grounds for the little auks during the breeding season. Species diversity within and between the chick meals was low, which probably reflects a high availability of a few preferred prey species. Individual chick meals were generally low in number of prey items and total energy content compared with other published results. This may be explained by a higher feeding frequency or by the samples being collected late in the breeding period (during late chick rearing), when chicks have a reduced growth rate and may require less energy than at earlier developmental stages. |
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388 |
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Publisher |
Springer Berlin Heidelberg |
Place of Publication |
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Edition |
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ISSN |
0722-4060 |
ISBN |
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Approved |
yes |
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Call Number |
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Serial |
5313 |
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| Permanent link to this record |
