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  • 1
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    In:  Geophys. J. Int., Jena, Scientific American, vol. 150, no. 1, pp. 91-108, pp. RG4001, 29 pp., (ISSN: 1340-4202)
    Publication Date: 2002
    Keywords: Seismology ; Velocity depth profile ; Receiver functions ; GJI
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  • 2
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    In:  Geophys. Res. Lett., Amsterdam, 4, vol. 30, no. 6, pp. 67-1 to 67-4, pp. 1334, (ISSN: 1340-4202)
    Publication Date: 2003
    Keywords: Seismology ; hot ; spot ; Plate tectonics ; Deep seismic sounding (espec. cont. crust)
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  • 3
    Publication Date: 2015-04-01
    Description: The evolution of the Southern Granulite Province of India has remained a contentious issue due to its complex tectonic history and sparsely preserved surface geologic features. This terrain has attracted global attention because of its central role in Gondwanaland tectonics. The crustal structure and composition of this province are examined using passive seismological data recorded by a network of broadband seismic stations sited in the region. Our results clearly show that the composition and seismic structure of the crust across the northeast arm of the Karur-Kambam-Painavu-Trichur shear zone are distinct. This is pronounced even beyond Karur across the eastern and western segments of the Madurai block along its meridional arm as well as across the Namakkal block. These vivid depth images with differing crustal compositions across the blocks, together with occurrence of alkali syenites and other rock types reminiscent of collision, enable us to demarcate the unambiguous presence of a suture in the region.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
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  • 4
    Publication Date: 2016-09-30
    Description: We present a critical analysis of the observations and interpretation of VLF sub-ionospheric measurements related to the main Nepal Gorkha earthquake which occurred on April 25, 2015 (M w 7.8) and its major aftershock on May 12, 2015 (M w 7.3). The VLF narrowband signal used is from NWC (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (lat. 25.41° N; long. 81.93° E). Allahabad is located very close (~360 km) to these earthquake (EQ) epicenters. Two widely used analysis, viz., (1) terminator time and (2) night time fluctuation techniques are applied to extract seismic related effects in the NWC narrowband VLF data. The terminator time analysis yields statistically significant shifts of ~45 and ~26 minutes respectively in evening terminator time in the NWC VLF amplitude signal, one day before both the earthquakes. The nighttime fluctuation method shows a consistent, statistically significant, increase in three parameters one day before the earthquake. The observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity, other earthquakes on the signal path, meteorological disturbances such as lightning activity, wind speed, temperature etc. along the transmitter-receiver great circle path. The VLF sub-ionospheric signal analysis results unambiguously point towards the presence of seismically excited atmospheric gravity waves (AGWs) during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Publication Date: 2017-06-21
    Description: The amplitude asymmetry and initial polarity of seismic induced ionospheric perturbations around the epicenter are considered to be important in providing information about the rupture propagation and related vertical surface deformation. To comprehend this, we study ionospheric perturbations related to the 12 May 2015, M w 7.3 Nepal earthquake. We model the co-seismic slip associated with the event using the InSAR derived surface deformation data. The ionospheric perturbations associated with the obtained surface deformation are explained in terms of rupture propagation, favorable geomagnetic field-wave coupling and satellite geometry effects. We discuss the effects of phase cancellation on the perturbation evolution for various receiver satellite line-of-sight configurations invoking an elementary version of satellite geometry factor (SGF). The present study thus elucidates further the role of non-tectonic forcing mechanisms while identifying ground source pattern using the associated ionospheric perturbations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
    Publication Date: 2017-07-12
    Description: The presence of ionospheric disturbances associated with Sumatra 2004 tsunami that propagated ahead of tsunami itself has previously been identified. However their origin remains unresolved till date. Focusing on their origin mechanism, we document these ionospheric disturbances referred as Ahead of tsunami Travelling Ionospheric Disturbances (ATIDs). Using TEC data from GPS Aided GEO Augmented Navigation (GAGAN) GPS receivers located near the Indian east coast, we first confirm the ATIDs presence in TEC that appear ~90 minutes ahead of the arrival of tsunami at the Indian east coast. We propose here a simulation study based on Tsunami-Atmospheric-Ionospheric coupling that considers tsunamigenic Acoustic Gravity Waves (AGWs) to excite these disturbances. We explain the ATIDs generation based on the dissipation of transverse mode of the primary AGWs. The simulation corroborates the excitation of ATIDs with characteristics similar to the observations. Therefore, we offer an alternative theoretical tool to monitor the offshore ATIDs where observations are either rare or not available and could be potentially important for the tsunami early warning.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
    Publication Date: 2012-07-07
    Description: The interglacial Marine Isotope Stage (MIS) 11 received special attention due to its remarkable resemblance with present-day climate. Based on synchronicity of marine, ice sheet and terrestrial proxy responses, warm episodes with intervening cool phase(s) at MIS 11 are qualitatively established. Here we quantitatively evaluate 15 climate proxies during 368–552 kyr intervals adopting a novel long-range cross-correlation approach and information theory based similarity measures. We also estimate the information flow rate and dominant flow direction between these climate variables using transfer entropy and the related directionality index. Our results unequivocally establish that atmospheric CO2 (pCO2) is the driving signal while all other proxies used in this study are the responses. The climate forcing greenhouse gas, the atmospheric CO2 (pCO2) and the response signals like sea surface temperature (SST) and carbon isotope composition of total organic carbon (δ13CTOC) proxies are strongly correlated (∼1 or −1) without significant observable time lag (less than 1 kyr). Various substages of MIS 11 are recognizable in the SST data alone based on normalized similarity measures. Additionally, eight more proxies from lacustrine sediments are identified as primary. During the cooler substages these proxies bear more similarity with ambient atmospheric pCO2. Thus, the information theory–based similarity measures suggest that atmospheric CO2 fluctuations are best captured by at least 9 climate proxies during cooler interglacial events. Based on the results related to interglacial MIS 11 and 13 obtained in this study, an important implication relevant to anthropogenic CO2 input to the present-day atmosphere can be distilled. It is that sensitive and better-coupled response proxies such as SST and MAT, which already show an increasing trend, are likely to behave in a more dissimilar manner in future. That is, they tend to behave more independently in the near future (∼0.75 kyr).
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
    Publication Date: 2018-01-27
    Description: The oblique-thrust Kaikoura earthquake of Mw 7.8 that struck New Zealand on 13 November 2016 at 11:02:56 UTC (local time at 00:02:56 AM on 14 November 2016), was one of the most geometrically and tectonically complex earthquakes recorded over land in modern seismology. The event ruptured in the region of multi-segmented faults and propagated unilaterally northeastward for more than 170 km from the epicenter. The GPS derived co-seismic surface displacements reveal a larger widespread horizontal and vertical co-seismic surface offsets of ~6m and ~2m respectively with two distinct tectonic thrust zones. We characterize the seismic induced ionospheric perturbations based on non-tectonic forcing mechanisms and demonstrate that these acoustic wave originated perturbations are mainly linked to the demarcated two distinct surface thrust directions with horizontal reinforcement trending the rupture rather than only to the displacements oriented in the rupture propagation.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
    Publication Date: 2005-04-01
    Description: Crustal configuration of the central Chugoku region with disposition of the Philippine Sea Plate (PHS) in this area are investigated through the receiver function approach using short-period Hi-net data. Images of the upper mantle discontinuities are also obtained. Restituted short-period receiver functions bring out discernible variations in average composition of the crust and its thickness in the study region. The Vp / Vs values in the study area are generally high, reaching values in excess of 1.85 at a few places. The central part of the study region showing the highest Vp / Vs values is coincidentally a subregion of least seismicity, possibly bestowed with special subsurface structure. Migrated receiver function images, both Ps and Pps images, unambiguously trace the NW subducting PHS taking a steeper plunge in the northwest part of the Chugoku region reaching depths of 70 km from its low dip disposition in the southeast. An excellent correlation of the subducting PHS with the hypocenters is also seen. We demonstrate that short-period data after restitution and application of appropriate low pass filters can indeed detect presence of the global 410-km and 660-km discontinuities and map their disposition reasonably well. Our migrated receiver functions image the deflections in the 410-km and 660-km discontinuities in an anti-correlated fashion on expected lines of Clapeyron slope predictions induced by subduction of the Pacific plate (PAC) beneath Japanese islands, though PAC itself is feebly traced but shows good correlation with slab seismicity. ©2005 The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.〈br /〉〈br /〉〈a href="http://doi.org/10.1186/BF03352563" target="_blank"〉〈img src="http://bib.telegrafenberg.de/typo3temp/pics/f2f773b55e.png" border="0"〉〈/a〉
    Print ISSN: 1343-8832
    Electronic ISSN: 1880-5981
    Topics: Geosciences , Physics
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  • 10
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    Geological Society of America (GSA)
    Publication Date: 2013-07-31
    Description: An understanding of the tectonic makeup of an area through study of surface geological features, together with depth information on the nature of the underlying lithosphere, forms the key to diamond exploration strategy. Although diamonds have been reported from the Indian craton for many centuries, the search for their carrier rocks dates back only to the beginning of the twentieth century. This study focuses on a wide area in southeast India, parts of which are sources of both diamondiferous and nondiamondiferous kimberlites and lamproites. Using Ps (SV and SH) and Sp receiver functions, we recovered depth images of the lithospheric mantle beneath southeast India encompassing the Eastern Dharwar–Bastar cratons and the adjoining Eastern Ghats mobile belt. These images reveal the presence of two significant velocity anomalies of contrasting nature at different depths beneath the study region. High-velocity features are observed between 160 and 220 km depth (Lehmann discontinuity depth, or L-depth), and a complex low-velocity contrast layer is delineated at ~80–100 km depth. Analyses of results from several other studies that include regional geology, geophysics, geochemistry, and geochronology allow us to infer that the positive velocity contrasts at L-depth represent preserved oceanic remnants of a ca. 1.6 Ga paleosubduction event in southeast India. Analysis of selected geothermobarometry data in conjunction with other evidence presented in this study indicates that the craton beneath southeast India is underlain by a thick lithospheric root/keel in excess of 200 km, suggesting an environment conducive to diamond stability. Consequently, we interpret the complex low-velocity contrast layer feature as a midlithospheric discontinuity and not a shallow lithosphere-asthenosphere boundary. The diamond formation potential of the area is discussed in light of a working model that incorporates the Mesoproterozoic paleosubduction scenario (ca. 1.6 Ma event) and subsequent kimberlite and/or lamproite intrusions. Wide regions covering the Godavari graben and adjoining areas are identified as potential zones for diamond exploration.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
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