Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 54, Issue 1
Displaying 1-21 of 21 articles from this issue
  • Naoki KOBAYASHI, Yoshihiro HIRAMATSU, Yoshiteru KONO, Fumiaki TAKEUCHI
    2001 Volume 54 Issue 1 Pages 1-8
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The Fukui Earthquake of M 7.1 was occurred in 26 June, 1948. The epicenter is located at the central part of an alluvial plain (the Fukui plain). In order to understand locations both of the epicenter of the earthquake and active faults surrounding the plain in terms of crustal structure and geology, the basement structure beneath the plain was estimated by using gravity data of about 1, 000 points. Applying 2D-Talwani's method iteratively, we calculated 3D density structure beneath the plain up to a depth of 4km. Three-layers model is assumed in this study: i. e., from top to bottom, each layer corresponds to the Quaternary sediments (density 2.1g/cm3), the Neogene sedimentary layers (2.4) and the lowest Neogene layer and the lower strata (2.67), respectively. An elongated depression of about 3km depth with N-S orientation is found in the central part of the Fukui plain. Epicenter of the Fukui earthquake is located on the deepest part of the depression. The results also imply that active faults in the Fukui plain are situated along boundaries where the basement structure steeply changes.
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  • Yasuhiro YOSHIDA, Kenji KANJO, Satoshi HARADA, Kenji FUJIWARA, Toshiki ...
    2001 Volume 54 Issue 1 Pages 9-16
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The needs for installing broadband seismometers in inconvenient places are increasing. A simple and easy installation method is necessary for this purpose because it is difficult to make seismic observatory consuming much time and money. However only a few researches have been done for this purpose. In this paper we try to find the best way to install seismometers in inconvenient places. To decrease background noises under simple installation, we tested several ways to install broadband seismometers in shallow vaults. We made two vaults with concrete blocks whose sizes are about 1.5m3 and install a broadband seismometer in it. One block was set on the ground, and the other was settled below the ground. We checked background noise level of vertical component for three cases of insulation stuff: sand, styrofoam, and none. The noise level of the seismometers below the ground is lower than that on the ground for all three cases. The noise level is the lowest when the seismometer is installed in the underground vault filled with sand. The order of the amplitude of noise level is in proportion to the temperature fluctuation near the seismometer. Noise levels show higher correlation with temperature than wind speed, precipitation and atmospheric pressure. These suggest that the inhibition of temperature change near seismometers is the key to decrease background noise level. Therefore the installation of seismometer in the underground vault filled with sand achieves the lowest noise level, though the level is about 10dB higher than that in deep tunnel at permanent observatory near the vault.
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  • Satoru YAMAGUCHI, Hideki MURAKAMI, Naoto OSHIMAN
    2001 Volume 54 Issue 1 Pages 17-31
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Many types of luminous phenomena were observed at the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake) of January 17, 1995. A questionnaire survey was done regarding luminous phenomena in order to determine 1) spatial distribution of eyewitnesses of luminous phenomena; 2) origin-times and duration-times of the phenomena; 3) properties of lights, such as color, shape, and brightness.
    Out of 798 people surveyed, 31 answerers (3.9% of the total) reported that they had observed luminous phenomena. Characteristics of the phenomena may be described as follows; a) More than 50% of the eyewitnesses were within 10km of the epicenter of the main shock. b) Almost all observations of the phenomena occurred simultaneously, or at a maximum of 2-3 minutes from the arrival of the seismic wave to the eyewitnesses' location. c) The duration-time of the phenomena was less than 30 seconds. d) The gross form of luminescence was a belt of light or curtain shape. e) The colors of luminescence were white, blue, and orange, while blue-white was predominant.
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  • An Evidence of Pre-slip at the Base of the Seismogenic Layer
    Taku KOMATSUBARA, Yasuo AWATA
    2001 Volume 54 Issue 1 Pages 33-44
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The 1970 Southeastern Akita Earthquake occurred just under a precise leveling route, and its source fault was located at the center of dense seismic observation network. Detailed focal mechanism parameters and crustal movements of pre-, co-, and post earthquake were reported. Seismic reflection experiments show that horizontal detachment exists in the deeper extension of the earthquake source fault at the base of the seismogenic layer in a depth of about 13km. Authors reconstruct a series of faulting processes on the above mentioned fault plane, due to vertical crustal movement and observed fault figures. Simple dislocation model assuming uniform slippage shows following faulting processes. 1. Vertical crustal movement in 4 to 14 years before the 1970 earthquake (1966-56) are interpreted as a result of aseismic pre-slip on the horizontal detachment at the base of the seismogenic layer (13km in depth). 2. The aftershock distribution shows coseismic fault slip occurred on the reverse fault and western margin of detachment. The vertical crustal movement from 4 years before to 4 years after the 1970 earthquake were caused of slip on the this fault and detachment which exist its extension, and surface disturbances. 3. The vertical crustal movement from 4 to 10 years after the earthquake (1980-74) is caused of aseismic slip on the horizontal detachment. 4. Net of the pre-slip is comparative to that of the main shock (ca. 25cm). 5. Direction of Pre-slip on the detachment is same to general direction of P-axis in the Northeastern Japan. 6. Length of Pre-slip region is longer than that of earthquake source fault.
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  • Shozo MATSUMURA
    2001 Volume 54 Issue 1 Pages 45-46
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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  • Satoko ODAGIRI, Kunihiko SHIMAZAKI
    2001 Volume 54 Issue 1 Pages 47-61
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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  • Masahiro KOSUGA
    2001 Volume 54 Issue 1 Pages 63-64
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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  • Haruo SATO, Teruo YAMASHITA
    2001 Volume 54 Issue 1 Pages 65-76
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Scattering phenomena of seismic waves give us lots of physical information about the heterogeneous structure of the Earth. Discrete scatterers such as cracks, voids and inclusions are known to be ubiquitous in the Earth's crust. Diverse methods of mathematical analyses have been developed on the scattering of elastic waves for isolated scatterers. These analyses seem to be quite satisfactory while there are both advantages and disadvantages in each method. Recent attention is directed to the analysis of densely distributed scatterers. We have to rely on numerical treatments in these cases and many of the studies are based on the boundary integral equation method. It is still difficult, in spite of advance in computer technology, to assume many scatterers enough for reliable statistical analysis of scattered elastic waves. In addition, detailed investigation of crack response to transmission waves is required under the compressive load condition for quantitative evaluation of the scattering in the crust since most researchers have developed crack models on the assumption of non-compressive load. Random fluctuations of elastic parameters are often used for modeling the heterogeneous Earth medium instead of a distribution of cracks and voids. Not only the coda wave envelopes but also the whole seismogram envelopes of local earthquakes are explained by using the Born approximation for wave scattering through random media. The broadening of S-wave envelopes can be explained by diffraction and/or multiple forward scattering due to velocity inhomogeneities on the basis of the stochastic treatment of wave equation. As a phenomenological approach, the radiative transfer theory has been developed to explain seismogram envelopes of local earthquakes in high frequencies. This theory well describes the energy transport through media having a constant background velocity and a random distribution of scatterers disregarding phase information. From the coda analysis, the scattering coefficient characterizing the scattering power per unit volume has been measured in the world. The inversion method by using the envelope derived from the radiative transfer theory as a Green function is useful especially for the study of highfrequency energy radiation from earthquake faults. It is necessary to develop this theory to adopt the depth dependence of scattering coefficient and intrinsic absorption in addition to the depth dependence of background velocity. We note that stochastic and deterministic approaches are complementary to each other for imaging the heterogeneity of the Earth and the earthquake rupture process in a broad band frequency range.
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  • Kiyoshi YOMOGIDA
    2001 Volume 54 Issue 1 Pages 77-90
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    This paper reviews the application of the boundary integral method to measure scattering and attenuation of high-frequency seismic waves in a deterministic manner. While many numerical approaches such as a finite difference method deal with media with continuous velocity fluctuations, we choose boundary methods because the observed nature of scattering of high-frequency seismic waves such as coda waves is attributed mainly to small-scale heterogeneities characterized by irregular boundaries with strong impedance contrast. First, basic formulations for wavefield in media with many cavities as heterogeneities are presented. Accuracy and numerical stability are checked with simple models, followed by models to simulate wave field in random media. With stationary random media, we investigate frequency dependencies of both the scattering attenuation and the temporal attenuation of coda. Finally, we calculate seismograms in media with localized heterogeneities and point out many new characteristics not observed for stationary random media. For example, there is a spectral peak of scattered waves whose frequency corresponds to the entire spatial extent of heterogeneities, in addition to the original spectral peak for each heterogeneity. In the near future, this approach should be extended into realistic 3-D and layered media, together with any imaging technique including effects of non-isotropic and multiple scattering, in order to make more quantitative comparison with observed seismograms.
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  • Jun KAWAHARA
    2001 Volume 54 Issue 1 Pages 91-108
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    A large amount of cracks exist in the Earth's crust, especially in fault zones with high distribution densities. They can scatter seismic waves and influence their propagation. This paper reviews theoretical studies on elastic wave propagation in such cracked media. The focus is on the effects of crack scattering on coherent, or mean, waves. Inside regions with randomly distributed cracks, the scattering causes attenuation and dispersion of passing waves in a stochastic sense. For dilute distributions, they are analytically and approximately evaluated based on a traditional theory of multiple scattering. The theory cannot, however, treat scattering of higher than second order in practice. The effects of higher order scattering, that will be obvious for denser distributions, can be taken into account by numerical approaches. When the distributions of cracks are restricted on planes or within zones, reflection and transmission of waves occur. They are also evaluated using analytical as well as numerical approaches, in a stochastic and approximate sense except when the distributions are perfectly periodic. In the long-wavelength limit, the cracked media behave effectively as homogenous and, in general, anisotropic ones.
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  • Mitsuyuki HOSHIBA
    2001 Volume 54 Issue 1 Pages 109-125
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    It is widely accepted that coda waves are composed of incoherent scattered waves for high frequencies. Using the random phase approximation, approaches based on the radiative transfer theory (energy transport theory) have been adopted instead of wave propagation theory itself for the construction of models which describe the envelopes of coda waves. The radiative transfer theory deals directly with the transport of energy through a medium containing scatterers. Since the first paper in 1960s on the single scattering model for cases of weak scattering and that on the diffusion model for cases of strong scattering, a number of improved models have been proposed to expand these models into more complicated cases, e. g., non isotropy in scattering, non spherical source radiation, and the case of separated locations of source and receiver. The single scattering model satisfies causality but does not energy conservation law; on the other hand the diffusion model satisfies the energy conservation law but does not causality. Multiple scattering models have been proposed in 1980s to consider an interpolation between the weak and strong scattering cases. The multiple scattering model can be applicable without assuming the strength of scattering, and satisfies both energy conservation law and causality, but has no simple analytical expression in 3 dimensional space. In parallel with the analytical development, several approaches using Monte Carlo technique were conducted to simulate multiple scattering processes and synthesize the coda wave envelope. Using Monte Carlo simulation the coda wave envelope can be synthesized even in the case of complicated structure having depth dependence on both scattering strength and velocity, beyond mathematical difficulties in analytical studies. The radiative transfer approach has provided a framework for measurements of scattering attenuation and intrinsic absorption, and the comparison of the theory with observation has led us to understand the stochastic inhomogeneity in the real Earth.
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  • Masahiro KOSUGA
    2001 Volume 54 Issue 1 Pages 127-145
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Random inhomogeneities in the lithosphere have been investigated through the analysis of coda waves that are thought to be generated by the inhomogeneities. The power spectral density of upper crustal inhomogeneities measured directly using borehole logs obeys power law for a wide range of wavenumber, which is the characteristics of the exponential or the von Kármán autocorrelation function. The methods preferentially used for the coda analysis are phenomenological modeling based on the energy transport theory or stochastic simulation of wave propagation based on the Born approximation in random media. The parameters estimated from the methods are the scattering coefficient and coda attenuation (Qc-1) of scattering media or the fractional fluctuation of seismic wave velocity and the correlation length of random inhomogeneities. Although the frequency and lapse time dependence of Qc-1 has been widely recognized from the numerous works conducted in the last two decades, the physical meaning of Qc-1 and its lapse time dependence have not yet been clarified completely. Some recent works tried to understand it by considering layered structures with different values of scattering coefficient and intrinsic attenuation. The estimated range of fractional velocity fluctuation and correlation length is 1-8% and 0.15-1.0 [km] in the crust and 2-8% and 1-20 [km] in the upper mantle, respectively. The magnitude of fractional fluctuation shows little difference between the crust and the upper mantle, however, the correlation distance exhibits large difference that reaches one order of magnitude. In addition, the horizontal correlation length is much longer than the vertical one, forming the anisotropic inhomogeneities in the lithosphere. The recent observations of high-frequency teleseismic Pn gave an additional evidence of random inhomogeneities in the upper mantle. Thus the coda methods have great advantage to get stochastic insight into realistic image of the lithosphere.
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  • Kazuo YOSHIMOTO
    2001 Volume 54 Issue 1 Pages 147-158
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Scattering of elastic waves including mode conversions between P- and S-waves is quite nonisotropic compared to that of acoustic waves. Thus, it is important to the analysis of coda waves of local earthquakes to use a fully realistic scattering model of seismic waves. In this paper, we summarize briefly a theory (Born approximation) on the scattering of elastic waves due to local spatial fluctuations of elastic parameters. The directional dependence of scattering amplitude for all scattering modes is evaluated for a random inhomogeneous medium characterized stochastically by using an autocorrelation function. We also review past seismological studies to confirm the usefulness and limitation of single scattering models (approximations), which have been used for the analysis of coda waves to investigate short-wavelength random heterogeneities of the lithosphere. It is shown that three-component seismogram envelope including both P- and S-coda waves is successfully modeled by using a single scattering model adopting the Born approximation for the evaluation of seismic scattering.
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  • Kazushige OBARA
    2001 Volume 54 Issue 1 Pages 159-170
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Long duration or spindle-like shape of S wave seismogram is well known phenomena in the case of long travel distances. This is characterized by the broadening of seismogram envelope. The broadening has been studied based on the scattering in random media. The diffraction and multiple forward scattering is modeled using the parabolic approximation for the wave propagation through the randomly inhomogeneous structure whose correlation distance is much longer than the seismic wavelength. The hypocentral distance dependence and frequency dependence of the envelope broadening are affected by the type of the auto-correlation function, correlation distance, and velocity fluctuation of the random medium. Recently, the model of envelope broadening is formulated by using spherically outgoing scalar-wave instead of plane wave. During the last decade, many observations for envelope broadening are carried out in order to investigate the random inhomogeneity. The envelope broadening characterized by a frequency dependence have sometimes recognized as a dispersion. At some subduction zones, the dispersion has been observed and studied based on the deterministic approach. It must be important to combine the stochastic approach with the deterministic approach for the analysis of dispersion phenomena.
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  • Osamu NISHIZAWA, Xing-lin LEL, Chadaram SIVAJI
    2001 Volume 54 Issue 1 Pages 171-183
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    To study seismic wave propagation in the Earth's interior, a number of laboratory physical model experiments have been conducted so far. However, most of the previous experiments could provide only qualitative results because no effective tools have been available for detecting actual elastic vibrations of solid surface. Recently, a laser Doppler vibrometer (LDV) has been used as an elastic wave transducer. The absolute particle velocity value at solid surface can be obtained directly from the Doppler shift of the He-Ne laser. LDV provides exact waveforms in a very small area with a diameter of 50 microns. Taking the advantages of LDV, we can experimentally study seismic wave propagation in random heterogeneous media by using laboratory physical models. We can reveal effects of random heterogeneity on seismic wave propagation by analyzing the waveform data obtained for model materials whose characteristics are well known. In this review, we first describe about physical model experiments by using LDV, and then present examples of elastic wave field in random heterogeneous media revealed by LDV. As one of quantitative studies of elastic waves in heterogeneous media, we present relationship between travel time fluctuations of P wave and characteristic scale lengths of heterogeneity. We further dsicuss about applications of physical model experiments in seismology and geophysical explorations.
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  • Kin'ya NISHIGAMI
    2001 Volume 54 Issue 1 Pages 185-191
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Deterministic analysis of coda waves has become one of the effective approaches to estimate inhomogeneous structure in the crust and uppermost mantle. Seismic array observation gives coherent wave trains, which are suitable for estimating scattering properties beneath the array. Local or regional seismographic network offers distribution of coda wave energy in space and time, which has an advantage of estimating scattering properties in a wide area below the station network. In this paper, several studies of the latter analysis are introduced. One of those analyses, proposed by Nishigami (1991), is to invert coda envelopes observed by a local seismographic network into spatial distribution of scattering coefficients. An observational equation is derived under the assumption of S to S single isotropic scattering and spherical source radiation, where observed data are the deviations of coda envelope from a master curve decaying smoothly. The unknown parameters are the relative scattering coefficients assigned to small blocks that divide the three-dimensional space. The observational equation can be solved by a recursive stochastic inversion. This analysis method applied to the central California data delineates deep structure of the San Andreas fault system as a structure with strong scattering. Several regions in central Japan were also analyzed by this method and detailed distribution of S wave reflectors was estimated below active volcanoes, Mt. Ontake and Mt. Nikko-Shirane. Weak scattering correlated with a seismic gap area was also recognized in the Hokuriku region. Revenaugh (1995a) proposed another method, callled Kirchhoff coda migration, which stacks the energy of single-scattered waves within the coda of teleseismic P waves observed by a local seismographic network. This method was applied to southern California and the following images were obtained, strong P to P scattering from the slab subducting beneath the Transverse Ranges at depths of 50-200km; P to Rg scattering correlated with topographic roughness; and P to S scattering from the aftershock area of the 1992 Landers earthquake. These two methods analyzing the seismic network data seem to be effective to estimate the heterogeneous structure in the crust and uppermost mantle. From the viewpoint of imaging the deep structure of active faults, the inversion analysis of coda envelopes from local earthquakes may be more effective. The variance reduction of the observed data, however, is low, for example, about 14% for the central California analysis. In the future, the simple models of scattering and source radiation assumed in the inversion analysis should be improved into more realistic ones to increase the model fitting. Forward modeling such as proposed by Obara (1997) will also be important to understand the whole shape of coda envelopes related to inhomogeneous distribution of scattering and attenuation intensity.
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  • Satoshi MATSUMOTO
    2001 Volume 54 Issue 1 Pages 193-201
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Seismic array analysis is one of the most effective techniques in order to estimate inhomogeneous structure of the Earth. Especially, inhomogeneities with shorter wavelength than typical wavelength of seismic waves behave as scatterers for propagating seismic wave. There are two categories for the analysis method of array observation data. One is the determination of location and strength of scatterer by beam forming the array data. FK and semblance analysis are adopted to estimate ray directions coming to the array. Slant stack method is a technique for back projecting scattered wave field to the Earth's interior. Scatterer distribution obtained by these methods relates to seismic activity and existing fracture zone. Another category is estimation of statistical parameters of inhomogeneity. Inhomogeneous structure is characterized by correlation function of fluctuation of velocity and density. The function can relate to variation of amplitude and phase observed by a seismic array. Thickness of inhomogeneous layer and both of strength and correlation distance of the structure have been estimated in continental crust and upper mantle, . Many regional studies have been carried out and suggested an image of randomly inhomogeneous structure in the Earth. In order to construct more realistic model of the Earth's interior, we need to estimate short wavelength structure in more detail. Therefore, it is necessary to develop new techniques by merging the methods of both categories.
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  • Special Interest to the Petrophysical Properties of Rocks
    Hitoshi MIKADA
    2001 Volume 54 Issue 1 Pages 203-213
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Seismolithological interpretation is of great importance in the geophysical exploration. Brite spot processing, seismic property mapping, and amplitude versus offset (AVO) or versus angle (AVA) have been developed to obtain lithological, poroelastic parameters of formations using seismic methods. The recent development in AVO/AVA has revealed that seismic anisotropy plays an important role in the estimation of the formation parameters. In this review paper, we first show some basic equations for relating seismic AVO/AVA methods with formation parameters. The advantage of using direct measured properties in a seismic property mapping method and the importance of anisotropy analyses are also reviewed. Finally, we discuss the direction of litho-seismic property estimation. It should be noted that empirical relations from well log data and lab-experimental data to seismic properties of rocks, especially for combining seismic data with real lithological properties of unknown underground materials are indispensable. It would also be quite important to have lithological/rock-physical parameter estimation schemes of deep materials in the Earth when using seismic methods.
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  • Effects of Fault Geometry on Transmission Waves
    Naoto YOSHIOKA, Fumitaka FUNAHASHI
    2001 Volume 54 Issue 1 Pages 215-223
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    In this paper, we focus on the effects of “an extensive single fault” on transmission waves. We first briefly review the studies with this concern. The effects of faults on seismic waves have received relatively little attention either theoretically or experimentally. Nevertheless, a number of theoretical and experimental works have been carried out mainly by N. G. W. Cook and his colleagues at the University of California, Berkeley. A theory for the reflection and transmission coefficients has been proposed and it has successfully explained experimental observations. As an example of this kind of study, we present our recent results of experimental work in which P- and S-waves with known wavelength were transmitted across a fault. The fault consists of evenly spaced square contacts, the size and the height of which were artificially controlled. We found that the transmissivity of the waves is solely determined by the ratio of the contact size to wavelength. The smaller the ratio is, the larger the amplitude of first arrival is for both P- and S-waves. We propose a mathematical model based on the stiffness of the fault. By comparing the transmission coefficients calculated by this model with the observed ones, we found that the model is not applicable if the ratio of contact size to wavelength is larger than 0.25. In this case, the amplitude of transmitted waves is found to be proportional to the real contact area.
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  • Shigeki NAKAGAWA, Naoshi HIRATA
    2001 Volume 54 Issue 1 Pages 225-232
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
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    Seismic waves are refracted, reflected, or scattered by heterogeneities in the Earth's crust. Small-scale structures less than wave-lengths are difficult to be observed by conventional refraction and/or reflection methods although scattered wave energy is observed as coda waves. In March 1997, a seismic reflection survey was conducted to reveal the active tectonics in the Japan arc across the Northern Fossa Magna in Japan. The profiling revealed a strongly deformed thick sedimentary layer. At the same time, we carried out a seismic array observation using off-line seismic recorders in the same area to image crustal structure. We used four large vibrators as seismic sources with a spacing of 200m. The array consisted of 39 seismometers distributed two dimensionally with a spacing of 100m. We applied the CSP (common scatterer point) stacking method for all the combination of sources and receivers. We assumed 3-D grid points with a spacing of 100m in a studied area. We calculated a travel time field for the 3-D structure by a finite difference method. To investigate whether a scattered energy was really originated from a particular grid point or not, we calculated cross-correlation coefficients between the CSP stacked waveforms and a source waveform. We interpret that the correlation coefficients represent a distribution of point scatterers. The scatterers are distributed near the Earth's surface and at depths of 800m and 1600m. Compared with the reflection profile, scatterers are distributed in severely deformed strata. Since our method strongly depends on the background velocity structure, it is important to use a realistic background velocity structure to estimate scatterer distributions accurately.
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  • the Median Tectonic Line (MTL), Northwestern Kii Peninsula, Japan
    Tomonori KAWAMURA, Eiji KURASHIMO, Masanao SHINOHARA, Noriko TSUMURA, ...
    2001 Volume 54 Issue 1 Pages 233-249
    Published: July 15, 2001
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    We carried out a seismic survey in the northwestern Kii Peninsula, Japan. A major active fault system, called the Median Tectonic Line (MTL), passes through the study area. The detailed subsurface structure of the MTL in the study area has been investigated by seismic reflection surveys and borehole data of the Geological Survey of Japan. The MTL is composed of high-angle right lateral faults and low-angle reverse-slip faults. Since such a complicated fault system is expected to produce strong scattered waves, the MTL fault system in the study area is an ideal location to study the relationship between distribution of seismic scatterers and the subsurface structure. The survey was designed to record scattered waves as effectively as possible. First, we obtained a seismic velocity structure with spatially high resolution by combining both reflection and refraction methods. Next, we applied a semblance analysis to estimate the distribution of seismic scatterers using the obtained velocity structure. The semblance coefficient at a particular point in the profile is related to the magnitude of scattering there. A high semblance coefficient implies effective excitation of scattered waves. A test with synthetic data, it shows that the spatial resolution for scatterer location is less than 20m. Three regions are detected with high semblance coefficients, and corresponding to the fractured Izumi group (Cretaceous), conglomerate-predominant layers in the Shobudani Formation (Plio-Pleistocene) and rugged unconformity surface at the top of Sambagawa metamorphic rocks. One of the two detected low semblance regions coincides with mud-predominant layers in the Shobudani Formation, and the other with the low-angle MTL fault zone. The rocks in the fault zone have been crushed so homogeneously by repeated faultings that the zone must be recognized as a low semblance region.
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