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  • 1
    Publication Date: 2016-03-09
    Description: We developed an improved method for the separation of intrinsic and scattering attenuation of seismic shear waves by envelope inversion called Qopen . The method optimizes the fit between Green's functions for the acoustic, isotropic radiative transfer theory and observed energy densities of earthquakes. The inversion allows the determination of scattering and intrinsic attenuation, site corrections and spectral source energies for the investigated frequency bands. Source displacement spectrum and the seismic moment of the analysed events can be estimated from the obtained spectral source energies. We report intrinsic and scattering attenuation coefficients of shear waves near three geothermal reservoirs in Germany for frequencies between 1 and 70 Hz. The geothermal reservoirs are located in Insheim, Landau (both Upper Rhine Graben) and Unterhaching (Molasse basin). We compare these three sedimentary sites to two sites located in crystalline rock with respect to scattering and intrinsic attenuation. The inverse quality factor for intrinsic attenuation is constant in sediments for frequencies smaller than $10\hspace{1.00006pt}\mathrm{Hz}$ and decreasing for higher frequencies. For crystalline rock, it is on a lower level and strictly monotonic decreasing with frequency. Intrinsic attenuation dominates scattering except for the Upper Rhine Graben, where scattering is dominant for frequencies below $10\hspace{1.00006pt}\mathrm{Hz}$ . Observed source displacement spectra show a high-frequency fall-off greater than or equal to 3.
    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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  • 2
    Publication Date: 2015-10-22
    Description: In this study, frequency-dependent seismic scattering and intrinsic attenuation parameters for the crustal structure beneath the W-Bohemia/Vogtland swarm earthquake region close to the border of Czech Republic and Germany are estimated. Synthetic seismogram envelopes are modelled using elastic and acoustic radiative transfer theory. Scattering and absorption parameters are determined by fitting these synthetic envelopes to observed seismogram envelopes from 14 shallow local events from the October 2008 W-Bohemia/Vogtland earthquake swarm. The two different simulation approaches yield similar results for the estimated crustal parameters and show a comparable frequency dependence of both transport mean free path and intrinsic absorption path length. Both methods suggest that intrinsic attenuation is dominant over scattering attenuation in the W-Bohemia/Vogtland region for the investigated epicentral distance range and frequency bands from 3 to 24 Hz. Elastic simulations of seismogram envelopes suggest that forward scattering is required to explain the data, however, the degree of forward scattering is not resolvable. Errors in the parameter estimation are smaller in the elastic case compared to results from the acoustic simulations. The frequency decay of the transport mean free path suggests a random medium described by a nearly exponential autocorrelation function. The fluctuation strength and correlation length of the random medium cannot be estimated independently, but only a combination of the parameters related to the transport mean free path of the medium can be computed. Furthermore, our elastic simulations show, that using our numerical method, it is not possible to resolve the value of the mean free path of the random medium.
    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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  • 3
    Publication Date: 2015-04-04
    Description: Hydraulically induced microseismicity is used to study high-frequency attenuation properties (6–72 Hz) in an enhanced geothermal system. Intrinsic and scattering attenuation are separated by jointly inverting seismogram envelopes for structural parameters, source and site effects. Modelling of synthetic envelopes is based on radiative transfer theory. To speed up inversion, an analytical solution of the radiative transfer equation for a 3-D isotropic scattering medium is implemented. In order to compensate for the actual anisotropic scattering, a smoothing algorithm is applied to introduce envelope broadening and peak delay. The approach is tested with seismic data from four fluid-induced earthquakes ( M w  ≤ 1) recorded by a temporary seismic network at the German Continental Deep Drilling (KTB) site at epicentral distances of less than 20 km. Full S -wave envelopes are inverted in 12 overlapping frequency bands with centre frequencies between 1.5 and 72 Hz. With data sampling at 200 Hz and high-frequency S -wave sources, attenuation estimates are obtained for the rarely probed frequency range between 30 and 70 Hz. From the inversion, we infer average values of the transport scattering coefficient g *, and the intrinsic absorption parameter b , as well as corresponding quality factors Q s and Q i . By comparison with attenuation estimates from regions with different tectonic activities, we see that both Q s and Q i for the investigated geothermal region fit best to moderate scattering and intrinsic regimes as obtained in tectonically active regions. A comparison with a regional attenuation model for southern Germany proves that attenuation estimates are scale-dependent. To compare intrinsic and scattering attenuation in the KTB region the transport mean free path (TMFP) and the absorption length ( l a ) are calculated. For both, we find a clear frequency dependence proportional to f –0.8 (TMFP) and f –0.3 ( l a ). TMFP decreases from 340 km at 6 Hz to 60 km at 72 Hz, whereas absorption length drops from 40 to 20 km, respectively. Thus, intrinsic absorption dominates over scattering attenuation by at least one order of magnitude. The influence of scattering becomes more significant towards higher frequencies. Moreover, comparing the apparent attenuation (inverse sum of TMFP and l a ) to values estimated with the spectral ratio technique, achieves a good agreement with mean deviations in the order of 3–5 per cent. From the frequency dependence of TMFP, it can be inferred that a von Karman-type of random medium with a Hurst exponent of  = 0.11 is a good model for representing the stimulated reservoir at the KTB. The fractal distribution of scatterers agrees well with results derived from independent analysis of acoustic logs.
    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
    Publication Date: 2014-06-21
    Description: We introduce the single-station cross-correlation (SC) technique of processing ambient seismic noise and compare its results with the established cross-correlation (CC) and autocorrelation (AC) techniques. While CC is the correlation of the signals of two seismic stations with each other and AC is the correlation of a signal with itself, SC is the correlation of two different components of a single three-component seismic sensor. The comparison of the three different correlation techniques shows that CCs give the best results at frequencies below 0.5 Hz and that SCs give the best results at higher frequencies. In all three processing techniques, ambient seismic noise is correlated in order to reconstruct the Green's function describing the wave propagation between the first and the second sensor. By relating the coda parts of the daily Green's functions with the long-term reference Green's functions, shear wave velocity changes are determined. Here, we apply this technique to the data of 20 seismic stations in the surroundings of the fault zone of the Iwate-Miyagi Nairiku earthquake ( M W  = 6.9), which occurred on 2008 June 13, UTC (2008 June 14, Japan Standard Time) in the northern part of the Japanese island Honshu. The data range from 2008 January to 2011 June and therefore include the Tohoku earthquake ( M W  = 9.0), which occurred on 2011 March 11, off the coast of northern Honshu. The data are analysed in five different frequency ranges between 0.125 and 4.0 Hz. The data show coseismic velocity changes for both earthquakes followed by a post-seismic velocity recovery. In general, the coseismic velocity changes increase with frequency. For the Iwate-Miyagi Nairiku earthquake, the strongest velocity changes occur close to the fault zone. Quickly recovering coseismic velocity changes can be separated from changes not recovering during the study period. For the Tohoku earthquake, the complete area is affected by coseismic velocity changes. A modelling of the depth of the coseismic velocity changes indicates that the Iwate-Miyagi Nairiku earthquake can be explained either by large shallow velocity changes or by small, but deep changes. For one station, the observations can only be explained by assuming deeper changes. For the Tohoku earthquake, the modelling shows that different parts of the study area are affected in different ways, some showing shallow changes, others deeper changes. Furthermore, seasonal velocity variations occur, which are compatible for the different stations above 0.5 Hz, with velocity maxima in autumn.
    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
    Publication Date: 2016-03-24
    Description: We present a systematic study of seismic velocity changes associated with a megathrust and five strong crustal earthquakes in Japan. We perform both cross-correlation and single-station cross-correlation analysis for station pairs and stations, respectively. The correlation of ambient seismic noise allows us to reconstruct the Green's functions of the wave propagation. By relating the coda parts of the daily Green's functions with the long-term reference Green's functions, shear wave velocity changes are determined. We analyse data from four areas in Japan where large earthquakes occurred: Iwate-Miyagi (2008 M W 6.9 Iwate-Miyagi Nairiku earthquake), Niigata (2004 M W 6.6 Chūetsu, 2007 M W 6.6 Chūetsu-oki and 2011 M W 6.2 Nagano/Niigata earthquakes), Noto Peninsula (2007 M W 6.7 Noto Hantō earthquake) and Fukuoka (2005 M W 6.6 Fukuoka earthquake). In all areas, we analyse time-series which start before the respective earthquakes and last until after the 2011 M W 9.0 Tōhoku-oki earthquake. The analysis in five different frequency ranges between 0.125 and 4.0 Hz yields time-series of the velocity changes for the different station pairs or stations. At the time of the respective earthquakes, we observe coseismic velocity drops in all areas which are followed by a partial post-seismic recovery process. For the Tōhoku-oki earthquake, coseismic velocity drops can also be observed in all regions. There is a general trend of increasing coseismic velocity drops with frequency in all four areas. The largest coseismic drops are observed close to the fault zones. Over the observed time range, the post-seismic recovery is only partial and around half of the coseismic velocity drops do not recover. The characteristic recovery times for the recovering part are similar in all areas and frequency ranges, with an average value of 0.55 yr. We model the volumetric strain changes for the different earthquakes and find that the observed pattern of the coseismic velocity drops cannot be explained by these models. The coseismic velocity drops at the different stations are better related with the peak ground velocities and the associated dynamic strain than with the peak ground accelerations, but the correlation is still poor. This suggests that non-linear effects caused by the strong ground motion during the earthquake can explain at least part of the coseismic velocity drops.
    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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  • 6
    Publication Date: 2008-04-01
    Print ISSN: 0377-0273
    Electronic ISSN: 1872-6097
    Topics: Geosciences
    Published by Elsevier
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  • 7
    Publication Date: 2009-06-13
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 8
    Publication Date: 2012-09-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 9
    Publication Date: 2004-07-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 10
    Publication Date: 2006-01-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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