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  • 1
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    Seismic Effects on the Secular Drift of the Earth's Rotational Pole (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Earthquake‐induced mass redistribution in the Earth excites the polar motion; its cumulative coseismic effect has been found to cause a secular polar drift (SPD) toward ~140°E longitude with strong statistical tendency. Here we find numerically the cumulatively coseismic effect in SPD since 1952 to be at the rate of ~0.75 mas/year (or ~2.3 cm/year), amounting to nearly 20% of the observed SPD that points to the opposite geographical direction and hence is significant in the pursuit of understanding the source budget of SPD. We further argue on theoretical and observational ground that such behavior reflects that of the overall plate tectonic motion and in fact accounts for a fraction of the latter over long term. The exact amount of the fraction is indeterminate until mass transport models of plate tectonics prove adequate. This viewpoint is in contrast to that of Cambiotti et al. (2016, https://doi.org/10.1093/gji/ggw077) which required the coseismic effect to get annihilated completely by the interseismic effect under their earthquake cycle decomposition of the velocity field at the faulting system.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Spherical Harmonic Stacking for the Singlets of Earth's Normal Modes of Free Oscillation (2014)
    Wiley
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    Publication Date: 2014-07-19
    Description: We extend the spherical harmonic stacking (SHS) method of Buland et al . [1979] for the radial (vertical) component in the seismogram to the transverse (horizontal) components of the displacement field. Taking advantage of the orthogonality of the spherical harmonic functions (scalar and vectorial), SHS isolates and accentuates the signals of individual singlets of the Earth's normal modes of free oscillation. We apply the SHS on the broad-band IRIS seismograms from up to 97 IRIS seismic stations for the 2004 Sumatra-Andaman earthquake, in experiments targeted to spheroidal as well as toroidal modes -- 2 S 1 , 0 S 3 , 2 S 2 , 3 S 1 , 1 S 3 , 0  T 2 and 0  T 3 . We report the complete resolution of the singlet frequencies of these multiplets, some for the first time, and estimate the singlets' complex frequencies using the frequency-domain AR method of Chao and Gilbert [1980]. The latter contain useful information to be used in inversions for the 3-D structure of the Earth's interior.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Gravity Changes Due to Large Earthquakes Detected in GRACE Satellite Data via Empirical Orthogonal Function Analysis (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract We report on the seven largest earthquakes for which we can detect unequivocal co‐seismic signals in the time‐variable gravity (TVG) data of the GRACE (Gravity Recovery and Climate Experiment) satellite mission during the GRACE era of 2002‐2017. After reducing the land‐hydrological effect according to model GLDAS, we employ the empirical orthogonal function (EOF) analysis to solve for the spatial pattern and time history of the coherent (standing‐oscillation) modes, that provide in an objective manner an overall view of the spatio‐temporal scenario. The solved EOF mode for an earthquake‐induced TVG typically features a spatial pattern (along with polarity) for the co‐seismic jump, in conjunction with the time series showing the jump (plus possible pre‐ and post‐seismic variability). In all cases the earthquake EOF mode solutions match well with the least‐squares co‐seismic jump in both spatial pattern and amplitude, and agree generally with previous reports in the literature. We conclude according to the examined cases that the lowest earthquake magnitude threshold that can be detected by GRACE is the Mw‐8.3 of the 2013 deep‐focus Okhotsk event, which in fact provides a testable case where the TVG we observe versus the seismologically derived source models may reflect different aspects of the source mechanism under different timescales.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Dynamics of axial torsional libration under the mantle-inner core gravitational interaction (2016)
    Wiley
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    Publication Date: 2016-11-20
    Description: The aims of this paper are: (i) Formulating the dynamics of the mantle-inner core gravitational (MICG) interaction in terms of the spherical-harmonic multipoles of mass density. The modeled MICG system is composed of two concentric rigid bodies (mantle and inner core) of near-spherical but otherwise heterogeneous configuration, with a fluid outer core in between playing a passive role. We derive the general equation of motion for the vector rotation, but only focus on the polar component that describes the MICG axial torsional libration. The torsion constant and hence the square of the natural frequency of the libration is proportional to the product of the equatorial ellipticities of the mantle and inner-core geoid embodied in their multipoles (of two different types) of degree 2 and order 2 (such as the Large Low-Shear-Velocity Provinces above the core-mantle boundary). (ii) Studying the geophysical implications upon equating the said MICG libration to the steady 6-year oscillation that are observed in the Earth's spin rate, or the length-of-day variation (Δ LOD ). In particular the MICG torsion constant is found to be = C IC σ z 2  ≈ 6.5 ×10 19  N m, while the inner core's ( B IC  − A IC ) ≈1.08 ×10 31  kg m 2 gives the inner core triaxiality ( B IC  − A IC )/ C IC  ≈1.8 ×10 -4 , about 8 times the whole-Earth value. It is also asserted that the required inner-core ellipticity amounts to no more than ~140 m in geoid height, much smaller than the sensitivity required for the seismic wave travel-time to resolve the variation of the inner-core.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Decadal polar motion of the Earth excited by the convective outer core from geodynamo simulations (2017)
    Wiley
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    Publication Date: 2017-09-30
    Description: Long time geodetic observation records show that the orientation of the Earth's rotation axis with respect to the terrestrial reference frame, or polar motion, changes on a broad range of time scales. Apart from external torques from the luni-solar tides, these changes are excited by interactions among different components of the Earth system. The convective fluid outer core has long been conjectured a likely contributor to the observed polar motion on time scales upwards of decades, such as the ∼30−year Markowitz wobble. We investigated the electromagnetic (EM) coupling scenario across the core-mantle boundary (CMB) via numerical geodynamo simulation for different geodynamo parameters (Rayleigh numbers and magnetic Rossby numbers). Our simulated polar motion varies strongly with the dynamo parameters, while its excitation on decadal time scales appear to converge asymptotically within the adopted range of numerical Rossby numbers. Three strongest asymptotic modes emerge from numerical results, with periods around 30, 40 and 60 years for the prograde excitation, and around 24, 30 and 60 years for the retrograde excitation. Their amplitudes are all larger than 5 × 10 −8 , or approximately 10 milliarcseconds. The results suggest that the electromagnetic core-mantle coupling could explain a substantial portion, if not all, of the observed decadal polar motion. In particular, the predicted 60-year polar motion deserves special attention for future observations and studies.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Typhoon-induced ground deformation (2017)
    Wiley
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    Publication Date: 2017-11-03
    Description: Geodetic instruments now offer compelling sensitivity, allowing to investigate how solid Earth and surface processes interact. By combining surface air pressure data, non-tidal sea level variations model and rainfall data, we systematically analyze the volumetric deformation of the shallow crust at 7 borehole strainmeters in Taiwan induced by 31 tropical cyclones (typhoons) that made landfall to the island from 2004 to 2013. The typhoon's signature consists in a ground dilatation due to air pressure drop, generally followed by a larger ground compression. We show that this compression phase can be mostly explained by the mass loading of rainwater that falls on the ground and concentrates in the valleys towards the strainmeter sensitivity zone. Further, our analysis shows that borehole strainmeters can help quantifying the amount of rainwater accumulating and flowing over a watershed during heavy rainfalls, which is a useful constraint for building hydrological models.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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