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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 146 (1996), S. 281-304 
    ISSN: 1420-9136
    Keywords: Energy discriminant ; magnitude ; amplitude ; earthquake ; explosion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Teleseismic observations of explosions tend to be richer in short-period energy than are earthquakes, thus the effectiveness of them b ∶M s discriminant. At regional distances the same basic separation occurs for smaller events in terms ofM L ∶M 0 (Woods et al., 1993) andm b ∶M 0 (Patton andWalter, 1993). While these studies demonstrate the basic differences in excitation, they suffer in practical application because of the detailed information required in the retrieval ofM 0 . In this paper, we introduce a new method of discrimination, based on the energy strength (M E ) from broadband regional records that appears to be effective and efficient. In this method all events are processed as earthquakes, and explosions are distinguished by their stronger energy levels relative to their long-period amplitudes. Results from 29 events recorded by TERRAscope, sampling 15 explosions from NTS and 14 earthquakes from the southwestern United States, are represented, indicating complete separation (45 data points).M L =3.6 is the smallest event examined to date but the method can probably be extended to even smaller levels in calibrated regions.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 128 (1988), S. 157-193 
    ISSN: 1420-9136
    Keywords: Scattering ; finite difference ; Kirchhoff integration ; NTS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A new method for interfacing numerical and integral techniques allows greater flexibility in seismic modeling. Specifically, numerical calculations in laterally varying structure are interfaced with analytic methods that enable propagation to great distances. Such modeling is important for studying situations containing localized complex regions not easily handled by analytic means. The calculations involved are entirely two-dimensional, but the use of an appropriate source in combination with a filter applied to the resulting seismograms produces synthetic seismograms which are point-source responses in three dimensions. The integral technique is called two-dimensional Kirchhoff because its form is similar to the classical three-dimensional Kirchhoff. Data from Yucca Flat at the Nevada Test Site are modeled as a demonstration of the usefulness of the new method. In this application, both local and teleseismic records are modeled simultaneously from the same model with the same finite-difference run. This application indicates the importance of locally scattered Rayleigh waves in the production of teleseismic body-wave complexity and coda.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Saguenay earthquake, 1988 November 25, is one of the first large shield type events recorded by a broadband-high dynamic range instrument, the Streckeisen system, installed at Harvard station (HRV). The event is sufficiently large to be well recorded teleseismically and thus the source characteristics can be determined by independent means and considered known. This allows a detailed study of the propagational effects along this path, at an epicentral distance of 625 km, where the strengths of the surface waves can be compared with the crustal body phases. Broadband modelling using standard analytical techniques and flat layered models works amazingly well over the period range of 0.5–20 s. A detailed strategy for modelling broadband regional phases is given in terms of a decomposition of the vertical and radial seismograms into three segments: Pnl (containing Pn, pPn, sPn, PmP, P coupled PL-waves); Snl (containing Sn, sSn, SmS, etc.); and the fundamental Rayleigh waves. Information about the upper crust is obtained from the fundamental Rayleigh waves while crustal thickness and velocity gradients in the mantle are obtained from Pnl and Snl. This particular crustal model has a thickness of 35 km with a sharp Moho and a substantial gradient in the top 20 km of the mantle, 0.01 km s−1 per km for both P and S velocities. The mantle velocities, α= 8.2 and β= 4.55 km s−1 are slower than expected for a shield environment. Attenuation is not required for waveform modelling or for absolute amplitude estimation.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper addresses the velocity structure of the crust and upper mantle beneath southern China with special emphasis on the Tibet region. Waveform data from 48 earthquakes as recorded on the WWSSN and GDSN are used in this detailed forward modelling study. Constraints on the upper crustal section are derived from modelling local Love waves in the time domain applying the mode-sum modelling technique. Lower crustal constraints are derived by modelling the Pnl-wavetrain with the reflectivity method. An average crustal thickness of 70 km is obtained beneath the Tibetan Plateau with a modest increase of velocity with depth. The lithospheric and upper mantle structure is deduced from modelling S and SS triplication waveform data and relative traveltimes by applying a combination of WKBJ and generalized ray methods. S-SS seismograms chosen with bounce-points directly under Tibet allow remote sensing of this inaccessible region. The resulting model is an averaged 1-D model where corrections for lateral variation have been applied. We conclude that the upper mantle structure in the entire region is basically shield-like below 200 km (SNA). However, the velocity of the lithosphere is abnormally slow, roughly 5 per cent beneath Tibet. The model for Tibet derived does not have a distinct lid, and has a positive velocity gradient in the crust, suggesting crustal shortening. A preliminary velocity model for southeastern China is also suggested.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Three-component broad-band displacement seismograms with paths sampling the Basin and Range province are studied to constrain the crustal structure. to find an average model that fits the data in both absolute time and waveform, we generate broad-band reflectivity synthetics and conduct sensitivity tests on different parts of a layered crustal model, where only a few layers are involved. Generalized rays are used to help identify the various phases. It proves useful to decompose a regional seismogram into segments so that the impact of model parameters on each segment can be clearly identified. Thus, for mid-crustal earthquakes, it is established that the top crustal layer controls the Rayleigh wave, the Airy phase, in shape over the range from 300 to 600 km, and the crustal layer just above the source depth controls its timing. the Pn waves, the Pn and PL portion, are controlled in broad-band character by the mid-crust while the top layer contributes to their long-period motion. These crustal parameters control the tangential motion similarly. the SV wave, the segment between the Pn wave and the Rayleigh wave, is mostly controlled by the shear velocity of the lower crust. In judging the goodness of fit between the array observations and synthetic waveforms, we allow individual data segments to shift relative to the 1-D synthetics by a few seconds to account for some lateral variation. the amount of time shift is found by the cross-correlation in displacement between the data segment and the synthetics. Applying these tests in a forward modelling approach, we find that a simple two-layer crustal model is effective in explaining this data set. In this model, the main crustal layer has P and S velocities of 6.1 km s-1 and 3.6 km s-1, similar to those found by Langston & Helmberger (1974). A surface layer of thickness 2.5 to 3.5 km is required to fit the Rayleigh waves. the refined model can be used as a reference model for further studies in this region.
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 1985-12-01
    Print ISSN: 0031-9201
    Electronic ISSN: 1872-7395
    Topics: Geosciences , Physics
    Published by Elsevier
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  • 7
    Publication Date: 1978-04-10
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 8
    Publication Date: 1973-04-10
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 9
    Publication Date: 1974-09-10
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 10
    Publication Date: 1973-10-09
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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