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  • 1
    Electronic Resource
    Electronic Resource
    Parallel 3-D Simulation of Ground Motion for the 1995 Kobe Earthquake: The Component Decomposition Approach (2000)
    Springer
    Pure and applied geophysics 157 (2000), S. 2047-2062 
    Details
    ISSN: 1420-9136
    Keywords: Key Words: 1995 Kobe earthquake, parallel computing, pseudospectral method, strong ground motion, 3-D simulation.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —A new approach to parallel pseudospectral simulation of 3-D seismic wave propagation is developed based on component decomposition of the wavefield. Field quantities and equations of motion are distributed over three processors according to their relationship to the x-, y- and z-coordinates, and computation is carried out concurrently on each processor with inter-processor communications. The efficiency of this approach is evaluated by a theoretical estimate and actual benchmark computations. We then conduct a 3-D simulation of strong ground motion for the 1995 Kobe (Hyogo-ken Nanbu) earthquake in order to show the feasibility of the parallel pseudospectral method. The results of the simulation demonstrate that the complex 3-D structures of the subsurface medium and source fault greatly affect the strong ground motion on the surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00001074
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    Paper (German National Licenses)
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  • 2
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    On the nature of regional seismic phases-II. On the influence of structural barriers (1997)
    Oxford University Press
    Details
    Publication Date: 1997-05-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 3
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    High-frequency Po/So guided waves in the oceanic lithosphere: II--heterogeneity and attenuation (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-24
    Description: In the western Pacific, high-frequency seismic energy is carried to very great distances from the source. The Po and So phases with observed seismic velocities characteristic of the mantle lithosphere have complex and elongated waveforms that are well explained by a model of stochastic heterogeneity. However, in the eastern part of the Pacific Basin equivalent paths show muted Po and weak, or missing, So . Once established, it is hard to eliminate such guided Po and So energy in the mantle lithosphere by purely structural effects. Even sharp changes in lithospheric thickness or complex transitions at fracture zones only weaken the mantle ducted wave trains, but Po and So remain distinct. In contrast, the effect of attenuation is much more severe and can lead to suppression of the So phase to below the noise level after passage of a few hundred kilometres. The differing characteristics of Po and So across the Pacific can therefore be related directly to the thermal state via the enhanced attenuation in hotter regions, such as the spreading ridges and backarc regions.
    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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    Ocean-influenced Rayleigh waves from outer-rise earthquakes and their effects on durations of long-period ground motion (2016)
    Oxford University Press
    Details
    Publication Date: 2016-03-31
    Description: We investigate the cause of anomalously long duration (〉100 s), long-period ( T  = 13–14 s) phase packets following the Rayleigh wave, which are observed widely in eastern Japan during outer-rise earthquakes. As this phase is not seen in records of interplate earthquakes, the late long-period phase is assumed to develop as a result of propagation across the Japan Trench. Using 3-D finite-difference simulations of seismic wave propagation with a detailed structural model that includes seafloor topography and water layers, we demonstrate that the late phase could be generated by a fundamental-mode oceanic Rayleigh wave propagating in the deep Pacific (at a depth of 6 km), which converts to a fundamental-mode continental Rayleigh wave as it crosses the Japan Trench. Our models suggest that the conversion is caused by the change in bathymetry. Based on analysis of surface wave dispersion, we confirm that the first arrival of the Rayleigh wave on land can develop from the first higher mode oceanic Rayleigh wave. The successive arrivals of the two types of Rayleigh waves on land produce ground motions with very long durations after outer-rise earthquakes.
    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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    Spatial- and Frequency-Dependent Properties of Site Amplification Factors in Japan Derived by the Coda Normalization Method (2012)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2012-08-01
    Description: We have estimated broadband ( f =0.5–8 Hz) site amplification factors at 1800 sites in Japan based on the coda normalization method using 4001 seismograms obtained from 48 events. The results show that, in the low-frequency band ( f =0.5–1 Hz), the site amplification factor varies over a wide range from –4 to 24 dB, with larger site amplification factors (〉8 dB) in such major basins as Kushiro, Rumoi, Sapporo, Aomori, Sendai, Niigata, Tokyo, Toyama, Nagoya, Osaka, Oita, and Kagoshima. On the other hand, the site amplification factor in the high-frequency band ( f =4–8 Hz) varies over a relatively narrow range from 0 to 18 dB at each station and has no clear correlation with the surface geology of each station. We then examined the contribution of the site amplification at each station to the anomalous distribution of the intensity pattern during large earthquakes by correcting the observed ground accelerations at each K-NET and KiK-net station with their corresponding estimated site amplification factors. The corrected pattern of intensity shows simple concentric isoseismal intensity contours around the hypocenter, indicating the applicability of our estimates to strong motion predictions. We confirmed that our estimates are superior to those derived from geology, such as those based on the regression of the averaged S -wave velocity from the surface to a depth of 30 m ( ), with a smaller standard deviation of intensity variations based on a standard decay function of intensity versus hypocentral distance.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 6
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    Spatial and Temporal Variations in Coda Attenuation Associated with the 2011 Off the Pacific Coast of Tohoku, Japan (Mw 9) Earthquake (2013)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2013-05-03
    Description: The spatial and temporal variations of coda attenuation ( ) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan ( M w  9) earthquake. The values were determined from the amplitude decay rate of the S -wave coda in narrower overlapping frequency bands in the range f =1.0–24 Hz, based on a single isotropic scattering model for more than 400 earthquakes ( M JMA 3~6.5) in the region recorded in a period from January 2005 to August 2011, including pre- and postseismic period. Our estimates of the spatially averaged value, in f =1.0–24 Hz, is almost stable with small variations (〈10%) during the preseismic period, which is in good agreement with that estimated by former studies in the area of northeast Japan. We found that the value of increases by about 10%–16% after the 2011 Tohoku event in f =1.25–3.5 Hz in some stations of northern Japan, which is confirmed by a statistical t -test at 99.9% confidence level. The change in is spatially limited to the rupture zone, while other paths remain nearly unaffected, suggesting local changes of scattering properties in the vicinity of the Tohoku-Oki source volume. This change may be attributed to an increase in the density of open microcracks in the mainshock source volume (such as due to increase in stress induced by the 2011 event) and probably the fluid content in fractures in the rocks. A model of heterogeneity due to coseismically opened cracks (dominant scale length of a =0.6–1.8 km in f =1.25–3.5 Hz) enhanced by increased stress change possibly controls the increased after the 2011 event in the source rupture region.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 7
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    Seismic- and Tsunami-Wave Propagation of the 2011 Off the Pacific Coast of Tohoku Earthquake as Inferred from the Tsunami-Coupled Finite-Difference Simulation (2013)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2013-05-03
    Description: The 2011 Tohoku earthquake ( M  9.0) caused significant hazards including strong ground motion, coseismic ground deformation, and tsunamis. For a deeper understanding of these phenomena, we conducted a large-scale, parallel computer simulation by means of the finite-difference method (FDM) with newly developed tsunami-coupled equations of motion. This method is based on the equations of motion for an elastic medium, treating seawater as elastic material having zero S -wave velocity. Tsunami waves are included by taking into account gravity and their equilibrium with the pressure gradient. An effective method of FDM simulation using the state-of-the-art massively parallelized K computer is also studied. With the use of a 3D layered velocity structure that includes topography and bathymetry, basement structure, Moho depth, and plate boundaries, and use of the appropriate source-rupture model based on joint inversion of seismic waves and geodetic observations, the simulated seismic waves demonstrate anomalous seismic-wave propagation by the thick 3D basin structure inland. We also found that the sea column acts as a strong absorber of seismic waves for shallow up-dip slip. On the ocean side, coseismic deformation and tsunamis are dynamically generated in this simulation. Tsunami attacks on the subsided Pacific coast are simulated without any additional assumptions. We also modeled the tsunami and seismic records of ocean-bottom pressure gauges above the fault. Because sea-bottom tsunami and seismic observation is powerful but very complex, recording both seismic and tsunami waves, the use of these coupled numerical simulations has great potential for resolving complex source fault rupture processes in a 3D heterogeneous structure. Online Material: Mpeg movies of fault slip, seismic-wave propagation, seafloor and ground displacement, and tsunamic propagation.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 8
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    Scattering of high-frequency P wavefield derived by dense Hi-net array observations in Japan and computer simulations of seismic wave propagations (2013)
    Oxford University Press
    Details
    Publication Date: 2013-03-12
    Description: We studied the scattering properties of high-frequency seismic waves due to the distribution of small-scale velocity fluctuations in the crust and upper mantle beneath Japan based on an analysis of three-component short-period seismograms and comparison with finite difference method (FDM) simulation of seismic wave propagation using various stochastic random velocity fluctuation models. Using a large number of dense High-Sensitivity Seismograph network waveform data of 310 shallow crustal earthquakes, we examined the P -wave energy partition of transverse component (PEP T ), which is caused by scattering of the seismic wave in heterogeneous structure, as a function of frequency and hypocentral distances. At distance of less than D  = 150 km, the PEP T increases with increasing frequency and is approximately constant in the range of from D  = 50 to 150 km. The PEP T was found to increase suddenly at a distance of over D  = 150 km and was larger in the high-frequency band ( f  〉 4 Hz). Therefore, strong scattering of P wave may occur around the propagation path (upper crust, lower crust and around Moho discontinuity) of the P -wave first arrival phase at distances of larger than D  = 150 km. We also found a regional difference in the PEP T value, whereby the PEP T value is large at the backarc side of northeastern Japan compared with southwestern Japan and the forearc side of northeastern Japan. These PEP T results, which were derived from shallow earthquakes, indicate that the shallow structure of heterogeneity at the backarc side of northeastern Japan is stronger and more complex compared with other areas. These hypotheses, that is, the depth and regional change of small-scale velocity fluctuations, are examined by 3-D FDM simulation using various heterogeneous structure models. By comparing the observed feature of the PEP T with simulation results, we found that strong seismic wave scattering occurs in the lower crust due to relatively higher velocity and stronger heterogeneities compared with that in the upper crust. To explain the observed regional difference, the velocity fluctuation model with 3–4 per cent stronger fluctuation and smaller is required at the backarc side of northeastern Japan.
    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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  • 9
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    High-frequency Po/So guided waves in the oceanic lithosphere: I--long-distance propagation (2013)
    Oxford University Press
    Details
    Publication Date: 2013-11-04
    Description: In many parts of the ocean high-frequency seismic energy is carried to very great distances from the source. The onsets of the P and S energy travel with speeds characteristic of the mantle lithosphere. The complex and elongated waveforms of such Po and So waves and their efficient transport of high frequencies (〉10 Hz) have proved difficult to explain in full. Much of the character can be captured with stratified models, provided a full allowance is made for reverberations in the ocean and the basal sediments. The nature of the observations implies a strong scattering environment. By analysing the nature of the long-distance propagation we are able to identify the critical role played by shallow reverberations in the water and sediments, and the way that these link with propagation in a heterogeneous mantle. 2-D finite difference modelling to 10 Hz for ranges over 1000 km demonstrates the way in which heterogeneity shapes the wavefield, and the way in which the properties of the lithosphere and asthenosphere control the nature of the propagation processes. The nature of the Po and So phases are consistent with pervasive heterogeneity in the oceanic lithosphere with a horizontal correlation length (~10 km) much larger than the vertical correlation length (~0.5 km).
    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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  • 10
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    Effects of sea water on elongated duration of ground motion as well as variation in its amplitude for offshore earthquakes (2016)
    Oxford University Press
    Details
    Publication Date: 2016-11-14
    Description: We investigated the effects of sea water on the propagation of seismic waves using a 3-D finite-difference-method simulation of seismic wave propagation following offshore earthquakes. When using a 1-D layered structure, the simulation results showed strong S- to P- wave conversion at the sea bottom; accordingly, S -wave energy was dramatically decreased by the sea water layer. This sea water de-amplification effect had strong frequency dependence, therefore resembling a low-pass filter in which the cut-off frequency and damping coefficients were defined by the thickness of the sea water layer. The sea water also acted to elongate the duration of Rayleigh wave packet. The importance of the sea water layer in modelling offshore earthquakes was further demonstrated by a simulation using a realistic 3-D velocity structure model with and without sea water for a shallow ( h = 14 km) outer-rise Nankai Trough event, the 2004 SE Off Kii Peninsula earthquake ( M w = 7.2). Synthetic seismograms generated by the model when sea water was included were in accordance with observed seismograms for long-term longer period motions, particularly those in the shape of Rayleigh waves.
    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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