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  • 550 - Earth sciences  (6)
  • Seismology  (4)
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  • 2015-2019  (3)
  • 2005-2009  (7)
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  • 1
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    The lithosphere-asthenosphere boundary in the North-West Atlantic region (2005)
    In:  Earth planet. Sci. Lett., Basel, Elsevier Science Publishers, vol. 236, no. 1-2, pp. 249-257, pp. L24312, (ISSN: 1340-4202)
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    Publication Date: 2005
    Keywords: Seismology ; Lithosphere ; Asthenosphere ; Rheology ; earth mantle ; Plate tectonics ; Iceland ; EPSL
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    BIBLIOGRAPHY SEISMOLOGY
  • 2
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    Mapping the source distribution of microseisms using noise covariogram envelopes (2016)
    Oxford University Press
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    Publication Date: 2016-04-16
    Description: We introduce a method for mapping the noise-source distribution of microseisms which uses information from the full length of covariograms (cross-correlations). We derive a forward calculation based on the plane-wave assumption in 2-D, to formulate an iterative, linearized inversion of covariogram envelopes in the time domain. The forward calculation involves bandpass filtering of the covariograms. The inversion exploits the well-known feature of noise cross-correlation, that is, an anomaly in the noise field that is oblique to the interstation direction appears as cross-correlation amplitude at a smaller time lag than the in-line, surface wave arrival. Therefore, the inversion extracts more information from the covariograms than that contained at the expected surface wave arrival, and this allows us to work with few stations to find the propagation directions of incoming energy. The inversion is naturally applied to data that retain physical units that are not amplitude normalized in any way. By dividing a network into groups of stations, we can constrain the source location by triangulation. We demonstrate results of the method with synthetic data and one year (2012) of data from the Swedish National Seismic Network and also look at the seasonal variation of source distribution around Scandinavia. After preprocessing and cross-correlation, the stations are divided into five groups of 9–12 stations. We invert the envelopes of each group in eight period ranges between 2 and 25 s. Results show that the noise sources at short periods (less than 12 s) lie predominantly in the North Atlantic Ocean and the Barents Sea, and at longer periods the energy appears to have a broader distribution. The strongly anisotropic source distribution in this area is estimated to cause significant biases of velocity measurements compared to the level of heterogeneity in the region. The amplitude of the primary microseisms varies little over the year, but secondary microseisms are much weaker in summer than in winter. Furthermore, the peak period of the secondary microseisms shifts from 5–6 s in winter to 4–5 s during the summer.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    PAPER CURRENT
  • 3
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    Joint relative location of earthquakes without a pre-defined velocity model: an example from a peculiar seismic cluster on Katla volcano's south-flank (Iceland) (2016)
    Oxford University Press
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    Publication Date: 2016-10-09
    Description: Relative location methods are commonly used to precisely locate earthquake clusters consisting of similar waveforms. Repeating waveforms are often recorded at volcanoes, where, however, the crust structure is expected to contain strong heterogeneities and therefore the 1-D velocity model assumption that is made in most location strategies is not likely to describe reality. A peculiar cluster of repeating low-frequency seismic events was recorded on the south flank of Katla volcano (Iceland) from 2011. As the hypocentres are located at the rim of the glacier, the seismicity may be due to volcanic or glacial processes. Information on the size and shape of the cluster may help constraining the source process. The extreme similarity of waveforms points to a very small spatial distribution of hypocentres. In order to extract meaningful information about size and shape of the cluster, we minimize uncertainty by optimizing the cross-correlation measurements and relative-location process. With a synthetic test we determine the best parameters for differential-time measurements and estimate their uncertainties, specifically for each waveform. We design a location strategy to work without a pre-defined velocity model, by formulating and inverting the problem to seek changes in both location and slowness, thus accounting for azimuth, take-off angles and velocity deviations from a 1-D model. We solve the inversion explicitly in order to propagate data errors through the calculation. With this approach we are able to resolve a source volume few tens of metres wide in horizontal directions and around 100 metres in depth. There is no suggestion that the hypocentres lie on a single fault plane and the depth distribution indicates that their source is unlikely to be related to glacial processes as the ice thickness is not expected to exceed few tens of metres in the source area. Our method is designed for a very small source region, allowing us to assume a constant slowness for the whole cluster and to include the effects of 3-D heterogeneity such as refraction. Similar circumstances may arise in other volcanic regions with a high level of heterogeneity and where densely clustered earthquakes are often recorded.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
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    A double-correlation tremor-location method (2017)
    Oxford University Press
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    Publication Date: 2017-01-07
    Description: A double-correlation method is introduced to locate tremor sources based on stacks of complex, doubly-correlated tremor records of multiple triplets of seismographs back projected to hypothetical source locations in a geographic grid. Peaks in the resulting stack of moduli are inferred source locations. The stack of the moduli is a robust measure of energy radiated from a point source or point sources even when the velocity information is imprecise. Application to real data shows how double correlation focuses the source mapping compared to the common single correlation approach. Synthetic tests demonstrate the robustness of the method and its resolution limitations which are controlled by the station geometry, the finite frequency of the signal, the quality of the used velocity information and noise level. Both random noise and signal or noise correlated at time shifts that are inconsistent with the assumed velocity structure can be effectively suppressed. Assuming a surface wave velocity, we can constrain the source location even if the surface wave component does not dominate. The method can also in principle be used with body waves in 3-D, although this requires more data and seismographs placed near the source for depth resolution.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Earthquake seismology in Greenland - improved data, better results. (2006)
    In:  Geophysical Research Abstracts, Vol. 8, 05055, 2006
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    Publication Date: 2020-02-12
    Keywords: 550 - Earth sciences
    Type: info:eu-repo/semantics/conferenceObject
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  • 6
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    Rayleigh wave group velocity tomography in southern Greenland from correlation of ambient seismic coise (2006)
    In:  3rd General Assembly European Geosciences Union (Vienna, Austria 2006)
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    Publication Date: 2020-02-12
    Keywords: 550 - Earth sciences
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  • 7
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    Rayleigh wave attenuation in Greenland (2005)
    In:  Geophysical Research Abstracts, Vol. 7, 04666, 2005
    Details
    Publication Date: 2020-02-12
    Description: Rayleigh wave attenuation in Greenland is studied for the first time ever. Using a twostation method, apparent values of Q are calculated across Greenland in the period range from 25 to 150 seconds. Data are primarily from the GLATIS project (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet, Dahl-Jensen et al, 2003), using temporary and permanent broadband seismographs in Greenland. 969 seismograms were visually inspected and 163 were selected for further analysis. A clear Rayleigh wave arrival was required, as well as great-circle alignment of events with two stations within a 5 degrees tolerance. Selected data was filtered twice, first to remove instrument responses, and then, with phase-matched filters, to reduce effects of noise and multipathing. Measurements of amplitudes were attempted with both spectral estimation and multiple filter analysis, with the later being the more stable method. The seismograms are filtered with narrow bandpass filters peaked at selected frequencies between 0.005 and 0.04 Hz, and the maximal Rayleigh wave amplitudes are measured. Average interstation values of Q are calculated and linearly inverted. The resulting surface maps of apparent Q are compared with Rayleigh wave velocities from the same region.
    Keywords: 550 - Earth sciences
    Type: info:eu-repo/semantics/conferenceObject
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  • 8
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    Variations of shear-wave splitting in Greenland: Mantle anisotropy and possible impact of the Iceland plume (2008)
    In:  Tectonophysics
    Details
    Publication Date: 2020-02-12
    Description: Seismic anisotropy was investigated by measuring shear-wave splitting at 19 broadband stations in Greenland. We examined mostly SKS and SKKS phases, but also some PKS and depth phases of SKS (e.g. pSKS, sSKS) for deep events. Splitting parameters (fast polarization and time delay) were determined for these phases. The fast polarizations at nine sites in southern Greenland are quite uniformly oriented about N–NE. Two sites in central northern Greenland show a similar geometry to southern Greenland. Similar fast polarizations in southern and central northern Greenland suggest continuity of structural fabric beneath large parts of Greenland. This coherent pattern extends across a number of geological provinces of varying age and suggests a common cause of anisotropy not related to the bitwise formation of the Greenland continental block. Four sites in an east–west oriented belt crossing central Greenland show varying fast polarizations and suggest a separate process causing the anisotropy there, which may indicate that these processes are not currently active. The overall pattern of anisotropy in our results, with the exception of variations across central Greenland, is similar to results obtained from Rayleigh waves. The irregular geometry of splitting across central Greenland may be related to the impact of the Iceland plume at ∼ 60 Ma. Reported splitting time delays range from 0.4 to 1.4 s with an average of 0.8 s, which can generally not be explained by crustal anisotropy alone. If confined to a lithosphere of thickness on the order of 100 km, time delays of up to 1.4 s indicate anisotropy of up to about 6%, assuming that the a crystallographic axis of olivine is preferentially contained in the horizontal plane. We suggest that the anisotropy beneath Greenland is located mainly in the upper mantle but some contributions from the crust and lower mantle may be present.
    Keywords: 550 - Earth sciences
    Language: English
    Type: info:eu-repo/semantics/article
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  • 9
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    Shear wave splitting varations in Greenland (2006)
    In:  Geophysical Research Abstracts, Vol. 8, 04195, 2006
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    Publication Date: 2020-02-12
    Keywords: 550 - Earth sciences
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  • 10
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    The lithosphere-asthenosphere boundary in the North-West Atlantic region (2005)
    In:  Earth and Planetary Science Letters
    Details
    Publication Date: 2020-02-12
    Description: A detailed knowledge of the thickness of the lithosphere in the north Atlantic is an important parameter for understanding plate tectonics in that region. We achieve this goal with as yet unprecedented detail using the seismic technique of S-receiver functions. Clear positive signals from the crust-mantle boundary and negative signals from a mantle discontinuity beneath Greenland, Iceland and Jan Mayen are observed. According to seismological practice, we call the negative phase the lithosphere-ashenosphere boundary (LAB). The seismic lithosphere under most of the Iceland and large parts of central Greenland is about 80 km thick. This depth in Iceland is in disagreement with estimates of the thickness of the elastic lithosphere (10-20 km) found from postglacial rebound data. In the region of flood basalts in eastern Greenland, which overlies the proposed Iceland plume track, the lithosphere is only 70 km thick, about 10 km less than in Iceland which is located directly above the proposed plume. At the western Greenland coast, the lithosphere.
    Keywords: 550 - Earth sciences
    Language: English
    Type: info:eu-repo/semantics/article
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