Language：English   Publishing type：Research paper (scientific journal)   Publisher：CSIRO PUBLISHING  

To simulate the observation of the radiation pattern of an earthquake, the direct simulation Monte Carlo (DSMC) method is modified by implanting a focal mechanism algorithm. We compare the results of the modified DSMC method (DSMC-2) with those of the original DSMC method (DSMC-1). DSMC-2 shows more or similarly reliable results compared to those of DSMC-1, for events with 12 or more recorded stations, by weighting twice for hypocentral distance of less than 80km. Not only the number of stations, but also other factors such as rough topography, magnitude of event, and the analysis method influence the reliability of DSMC-2. The most reliable result by DSMC-2 is obtained by the best azimuthal coverage by the largest number of stations. The DSMC-2 method requires shorter time steps and a larger number of particles than those of DSMC-1 to capture a sufficient number of arrived particles in the small-sized receiver.

DOI： 10.1071/EG14116

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

The amplitudes of high-frequency seismic waves generated by local and/ or regional earthquakes vary from site to site, even at similar hypocentral distances. It had been suggested that, in addition to local site effects (e.g., variable attenuation and amplification in surficial layers), complex wave propagation in inhomogeneous crustal media is responsible for this observation. To quantitatively investigate this effect, we performed observational, theoretical, and numerical studies on the characteristics of seismic amplitude fluctuations in inhomogeneous crust. Our observations of P-wave amplitude for small to moderately sized crustal earthquakes revealed that fluctuations in P-wave amplitude increase with increasing frequency and hypocentral distance, with large fluctuations showing up to ten-times difference between the largest and the smallest P-wave amplitudes. Based on our theoretical investigation, we developed an equation to evaluate the amplitude fluctuations of time-harmonic waves that radiated isotropically from a point source and propagated spherically in acoustic von Karman-type random media. Our equation predicted relationships between amplitude fluctuations and observational parameters (e.g., wave frequency and hypocentral distance). Our numerical investigation, which was based on the finite difference method, enabled us to investigate the characteristics of wave propagation in both acoustic and elastic random inhomogeneous media using a variety of source time functions. The numerical simulations indicate that amplitude fluctuation characteristics differ a little between medium types (i.e., acoustic or elastic) or source time function durations. These results confirm the applicability of our analytical equation to practical seismic data analysis.

DOI： 10.1186/s40623-015-0366-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

We performed a detailed analysis of seismograms obtained during intraslab earthquakes beneath the Kanto region and revealed a strong lateral variation in the waveforms of high-frequency trapped P and S waves propagating through the subducting crust of the Philippine Sea plate. Significantly distorted spindle-shaped trapped P and S waves with large peak delays were observed in areas where the Philippine Sea plate is at shallow depths beneath the Kanto region. In order to interpret these seismic observations, in relation to the structural properties of the crust of the Philippine Sea plate, we conducted finite difference method simulations of high-frequency seismic wave propagation using various possible heterogeneous velocity structure models. Our simulation successfully reproduced the observed characteristics of the trapped waves and demonstrated that the propagation of high-frequency P and S waves is significantly affected by small-scale velocity heterogeneities in the subducting crust. These heterogeneities can be traced to a depth of approximately 40 km, before disappearing at greater depths, a phenomenon that may be related to dehydration in the subducting crust at shallower depths.

DOI： 10.1093/gji/ggv423

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Frequency and distance changes in the apparent P-wave radiation pattern (0.75-12 Hz) are investigated using velocity seismograms of shallow strike-slip earthquakes occurring in Chugoku region, southwestern Japan. Data from a dense seismic monitoring network revealed that the four-lobe apparent P-wave radiation pattern was gradually distorted with increasing frequency and propagation distance. Observed features suggest that seismic wave scattering due to small-scale velocity heterogeneity in the crust may be a major cause of this distortion. The effects of seismic wave scattering on apparent P-wave radiation pattern were investigated via 3-D finite difference simulation of seismic wave propagation. Our simulations demonstrated that the scattering of seismic waves modified the apparent P-wave radiation pattern from the original four-lobe shape, and that the small-scale velocity heterogeneity, characterized by the von Karman-type power spectral density function with correlation distance of 1 km, root-mean-square value of 0.03 and decay rate parameter of 0.5, might be adequate for modelling crustal heterogeneity in the target region. It was also found that the scattering attenuation of P wave expected from this heterogeneity is significantly smaller than the apparent P-wave attenuation and S-wave scattering attenuation reported by Multiple Lapse Time Window Analysis of previous studies in Japan. These results might imply that scattering attenuation is not the dominant mechanism of P-wave attenuation in the crust of Chugoku region.

DOI： 10.1093/gji/ggv263

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SEISMOLOGICAL SOC AMER  

Simultaneous use of numerous events in the multiple lapse time window analysis (MLTWA) method is the most effective way to obtain Q(i)(-1) and Q(S)(-1) values. Such simultaneous use of numerous events normally shows a large observational scatter, which may be caused by different radiation pattern and focal depth of each earthquake, and regional alteration of the local structure. To avoid these scattering factors, the current study attempted to use single event source as the input data in MLTWA for South Korea. The direct simulation Monte Carlo (DSMC) method was used to simulate the observations, which showed realistic results when the number of recorded stations was eight or more. The obtained values of seismic albedo (B-0) were less than 0.5, which were in accordance with the same in the previous results. Three envelopes constructed by combination of three regional values of Q(Lg)(-1) at 1 Hz produced regional variations, of which the ones at high frequencies of Q(i)(-1) and at low frequencies of Q(S)(-1) were remarkable. This result, showing the local variation indicated an advantage of the study using single source event over the previous studies in South Korea that involved extensively distributed data. Because DSMC was known to be applicable to 3D structure due to its simple algorithm, the current study also tried to correct radiation pattern of the observations by inserting focal mechanism algorithm to the code of the DSMC method. Such insertion, however, generated extremely low values of Q(S)(-1) because the geometrically shrunk receiver could not capture most of the scattered particles. This receiver problem is most likely to be a major obstacle of the 3D study of DSMC. In addition, the significant depth effect of Q(-1) was identified for the prior MLTWA studies resulting from different measurement range and focal depth.

DOI： 10.1785/0120140259

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

To conduct practical evaluations of the long-period ground motions (period of 4 to 8 s) in a laterally inhomogeneous large sedimentary basin, we constructed a three-dimensional (3D) sedimentary velocity structure model for the northern Kanto Basin in Japan using a simple velocity gradient function, where strong lateral variations of seismic velocities in the sediments were expected. The model construction employs waveform analysis and geophysical data from vertical seismic profiling and microtremor surveys in the target region. To validate the velocity structure model, we conducted large-scale 3D finite-difference method simulations of the long-period ground motions for two shallow moderate earthquakes: the northern Tochigi earthquake and the northern Ibaraki earthquake. The simulation results for both earthquakes accurately reproduced the observed long-period ground motions in terms of arrival times, amplitudes, and durations of surface waves. By detailed comparisons of the seismograms for observational and simulated data, we demonstrated that the lateral variation of the seismic velocities in the sediments determines the characteristics of the surface wave propagation in the northern Kanto Basin. Such analyses can provide a better understanding of the complex propagation characteristics of surface waves in laterally inhomogeneous, large sedimentary basins.

DOI： 10.1186/s40623-015-0201-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

To investigate the detailed structural properties of the oceanic crust of subducting oceanic plate, we analyzed high-frequency (1 to 16 Hz) trapped P waves during earthquakes that occurred near the oceanic crust of the Philippine Sea plate. The distinct trapped P waves observed by the dense seismic network of the Kanto-Tokai region, Japan, did not show any apparent peak delay and frequency-dependent dispersion. These observations suggested that the oceanic crust around the source depths was characterized by a homogeneous velocity structure, rather than an inhomogeneous multiple-layered structure. This interpretation was examined by finite difference method simulations of seismic wave propagation using possible velocity structure models. The simulations demonstrated that a uniform velocity oceanic crust of the subducting Philippine Sea plate, which may result from the velocity increase in this layer at 30 to 40 km depth due to metamorphic-dehydration reactions, effectively trapped seismic energy as a short-distance waveguide and developed distinct pulse-like trapped P waves.

DOI： 10.1186/s40623-015-0210-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

We analyzed seismic waveforms of moderate to large shallow earthquakes recorded in the Kanto Basin, Japan, to clarify the characteristics of long-period ground motions in deep sedimentary basins. Our analysis revealed a tendency of long-period ground motions to develop in limited periods up to around 7 s. In addition, we found a specific relation between the predominant period of long-period ground motions and the depth of bedrock. The predominant period tends to increase in proportion to bedrock depth until it reaches approximately 7 s; however, it becomes almost constant for bedrock depths greater than 2 km. These observations, in conjunction with numerical experiments performed in this study using realistic sedimentary basin structure models, indicate that the characteristic of long-period ground motions, excited by the basin-induced fundamental Love waves, is controlled strongly by the shallow sedimentary structure, if the bedrock depth exceeds approximately 2 km. This result might also be important in understanding and evaluating the characteristics of long-period ground motions in other deep sedimentary basins of the world for moderate to large shallow earthquakes.

DOI： 10.1186/1880-5981-66-100

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Analyses of dense seismic records in Kanto, Japan, revealed distinct pulse broadening and peak delay of high-frequency S waves at central Chiba. These phenomena are observed at frequency range of 1-8 Hz and exist only for ray paths passing through the low-velocity (LV) zone at depth of 20-40 km beneath northwestern Chiba. To obtain a more detailed understanding of these phenomena, we conducted 2-D and 3-D finite difference method simulations of seismic wave propagation using a realistic heterogeneous structure model. Through numerous simulations we demonstrated that strong seismic scattering, due to localized strong small-scale heterogeneities in the LV zone and in the oceanic crust, is a major cause of strong pulse broadening and peak delay of high-frequency S waves. After comparing simulation results with observations, the most preferable small-scale velocity heterogeneity in the LV zone is characterized by a Gaussian power spectral density function (PSDF) with correlation distance a of 1-2 km and rms value epsilon = 0.07-0.09, superposed on a background exponential PSDF (a = 3 km, epsilon = 0.07). Assuming strong velocity heterogeneities, observed amplitude decay at Chiba is also well explained by strong scattering attenuation in the LV zone. Because the LV zone, which has been reported by seismic tomography studies, is interpreted as being constructed by the dehydration of the subducting oceanic crust of the Philippine Sea Plate, strong small-scale velocity heterogeneity in the LV zone may be related to the random distribution of fluid in this volume.

DOI： 10.1093/gji/ggu031

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

In this paper, we analyze the seismograms of a local earthquake (M-W = 5.8) and find a clear surface wave excitation at the northern edge of the Kanto Basin, Japan. To conduct three-dimensional (3-D) finite difference method simulations for this observation, we investigate the characteristics of the S wave velocity in the Kanto Basin and construct a realistic sedimentary structure model. The S wave velocities derived from the vertical seismic profiling measurements in the Kanto Basin show smooth depth gradients rather than step-like increases and can be successfully modeled by an exponential asymptotically bounded velocity function. Our simulation using the modified Japan integrated velocity structure model by incorporating a sedimentary layer with S wave velocity-depth gradients well reproduces the excitation and dispersion of the observed surface waves. This result indicates that realistic modeling of the impedance contrast at the sediment-bedrock interface is indispensable for the precise evaluation of surface wave excitation at the basin edge.

DOI： 10.1186/1880-5981-66-16

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SEISMOLOGICAL SOC AMER  

For the separation of intrinsic (Q(i)(-1)) and scattering attenuation (Q(s)(-1)) in South Korea, coda observations represented by the multiple-lapse time windows method were numerically simulated using the direct simulation Monte Carlo method. The simulation showed that the depth-dependent velocity model separated by crust and mantle fit better than the uniform velocity model. Under a uniform attenuation assumption, the Q(i)(-1) and total Q(t)(-1) in South Korea represent the lowest values among current global measurements. These low attenuation values can be explained by the inactive seismicity of South Korea corresponding to those of the seismically stable area such as a shield.

DOI： 10.1785/0120100054

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Mount Fuji is the focus of intense study because of its potential hazard signaled by seismic, geologic and historical activity. Based on extensive seismic data recorded in the vicinity of Mt Fuji, coda quality factor (Q(C)(-1)) using a single scattering model hypothesis, and intrinsic and scattering quality factor (Q(i)(-1) and Q(S)(-1)) using the Multiple Lapse TimeWindow (MLTW) method were measured. To focus the study on the magmatic structure below Mt Fuji, the data were separated into two groups: a near-Fuji region of rays traversing an area with radius 5 km around the summit (R < 5 km), and a far-Fuji region of rays beyond a radius of 20 km around the summit (R > 20 km). This classification shows the largest discrepancy of Q(C)(-1) at a range of sampling volumes corresponding to overlapped sampling depth of about 80 km. Further, the spatial division shows significant difference of Q(i)(-1) and Q(S)(-1) at hypocentral distance of 80 km. The large difference of Q(S)(-1) in bandwidths 2-4, and 4-8 Hz indicates lithospheric heterogeneity beneath Mt Fuji with a characteristic heterogeneity scale length of about 1 km. The results have a small error range due to the large data sample, showing that all Q-1 values in the near-Fuji area are greater than those of the far-Fuji area, and Q(i)(-1) for both the near and far-Fuji areas is higher than Q(S)(-1) at high frequencies. The Q(i)(-1) and Q(S)(-1) values for far-Fuji are in the range of values for typical non-volcanic areas. The Q(i)(-1) values of the near-Fuji area are lower than those of other volcanic areas considered, where as values of Q(S)(-1) are not. The low Q(i)(-1) for the volcanic region of near-Fuji suggests that the magmatic activity, indicated by percent partial melt, in the vicinity of Mt Fuji is not as active as hot spot volcanoes, such as Kilauea, Hawaii.

DOI： 10.1111/j.1365-246X.2009.04121.x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Apparent, scattering, and intrinsic S-wave attenuations (Q(S)(-1), Q(scat)(-1), and Q(int)(-1)) of the upper lithosphere in the Kanto region of Japan were measured in the 1- to 32-Hz frequency range using Multiple Lapse Time Window Analysis (MLTWA) for 115 borehole seismograms of local earthquakes. A new set of time windows for MLTWA, in which multiple isotropic scattering is assumed, was proposed and employed to estimate the frequency dependence of S-wave attenuation parameters. Scattering attenuation was found to dominate intrinsic attenuation in the S-wave attenuation mechanism at low frequencies (<2 Hz), whereas the opposite relation was observed at high frequencies. The transition is caused by the different frequency dependences of Q(scat)(-1) (alpha f(-1.5)) and Q(int)(-1)(alpha f(-0.7)) at this frequency. Interestingly, Q(scat)(-1), is almost frequency independent at frequencies >8 Hz, which implies the self-similar nature of short-wavelength heterogeneities in the upper lithosphere. In terms of the upper lithosphere of the Kanto region, these results may indicate that the random heterogeneities characterized by ;the Gaussian autocorrelation function with a fractional fluctuation epsilon approximate to 10% and a correlation length a approximate to 2 km are superimposed on the weak background self-similar heterogeneity.

DOI： 10.1186/BF03352957

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Language：Japanese   Publisher：SEISMOLOGICAL SOCIETY OF JAPAN  

We relocated hypocenters around active faults to investigate the relation between the hypocenter distribution and the subsurface geometry of the faults. We selected two regions. One is the source region of the 2004 Mid-Niigata Prefecture Earthquake. 179 earthquakes which occurred in the source region during about 2.5 years before the main shock were relocated by applying the double-difference earthquake location algorithms (the DD method). The hypocenters gathered in a narrow area after relocation. The lower limit of the relocated hypocenters corresponds to that of the aftershock distribution. The hypocenter of the main shock was located near the lower limit of the relocated hypocenters. We could not find the configuration of the earthquake source faults from the relocated hypocenters, because the hypocenters were distributed over the entire region. The other region that we studied is the southern part of the Yamagata Basin fault zone, where seismicity is occasionally active in small clusters. We relocated 206 hypocenters by the master event method and the DD method. The hypocenters relocated by the DD method gathered in very small clusters with a size of about 400 m. Neither clusters nor hypocenters within each cluster formed planes. Therefore, we could not guess the geometry of the subsurface faults. If we assume that the subsurface fault extends along the outer rim of the hypocenter distribution, the fault plane inclines by about 75 degrees in the shallow part and is nearly horizontal at 10 km in depth.

DOI： 10.4294/zisin.61.1

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JLC/20016765489?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

In Japan, the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded. The longest seismic profiles are analyzed. A remarkable feature, a long-duration coda wave after the PmP wave (reflected wave at the Moho boundary), is observed on the record sections. Several possible models are considered to explain the long-duration coda wave. The model with many scatterers located in the uppermost mantle explains the observed data well while the undulating Moho and continuous layering models do not account for some aspects of the observed data. The scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent. We estimate the scattering coefficient of the uppermost mantle and crust using simulations. The scattering coefficients obtained for upper crust, lower crust, and uppermost mantle are 0.01, 0.02, and 0.025, respectively. The scattering coefficient of the uppermost mantle is slightly larger than that of lower crust, which is characterized as being reflective. The many scatterers in the uppermost mantle might be related to magmatism in Japan. This will be one of the important observations for understanding formation processes of the Moho boundary and uppermost mantle in the island-arc environment. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2006.01.013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SEISMOLOGICAL SOC AMER  

We investigated the spatial distribution of S-coda-wave energy in northeastern Honshu, Japan, where a volcanic front runs from north to south. Seismograms of a local event with a magnitude of 6.4 recorded by the Hi-net were analyzed in this study. The mean square amplitudes of coda waves at 82 seismic stations were calculated for bands with frequencies of 2-4, 4-8, 8-16, and 16-32 Hz. From these measurements, a clear spatial variation of coda-wave energy was observed across the volcanic front from east (forearc) to west (backarc). The coda-wave energy is uniformly distributed in the forearc, whereas an exponential decrease with horizontal offset from the volcanic front is found in the backarc. The decay rate increases with increasing frequency.
We propose a diffusion-absorption model to give a physical insight into the observed regional variation in coda-wave energy. This model explains the observed characteristics successfully and predicts that the exponential variation of coda-wave energy in the backarc is a function of the diffusivity and the absorption coefficient. The magnitude of the intrinsic attenuation factor in the backarc is estimated at about Q(i)(-1) = 0.02 at a frequency of 10 Hz by assuming a value of 0.01 km(-1) for the scattering coefficient. This intrinsic attenuation factor is about twice as large as those reported from previous studies on the forearc.

DOI： 10.1785/0120050085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Berlin  

An M=5 earthquake occurred on September 15, 1998, in the Nagamachi-Rifu fault (NRF), northeastern Japan. In the aftermath of this event, many seismograph stations were constructed temporarily around the fault, forming a dense network of stations with a spatial separation of 5 km. We report here our estimation of the three-dimensional velocity structures of the P and S waves using arrival-time data recorded at these stations with the aim of understanding the heterogeneous structure around the NRF. Low-Velocity and high Poisson's ratio anomalies are imaged in the lower crust beneath the volcanic area, which are probably associated with the partially molten materials conveyed through the upwelling flow in the mantle wedge. A distinct low-velocity anomaly, which is explainable by the existence of H2O-filled pores, is observed in the mid crust at the deeper extension of the NRF. Two low-velocity anomalies that are probably associated with the remnants of magmatic activity that formed the Shirasawa caldera and with the existence of thick late-Cenozoic sedimentary layers are observed at depths shallower than 10 km in the hanging wall of the NRF. Our results successfully characterize the major features of the complex velocity structure around the NRF, with implications for the existence of fluid-rich regions in the mid to lower crust. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS)
The Seismological Society of Japan
The Volcanological Society of Japan
The Geodetic Society of Japan
The Japanese Society for Planetary Sciences
TERRAPUB.

DOI： 10.1186/BF03351989

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Publisher：The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences  

DOI： 10.1186/BF03353350

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DOI： 10.4294/zisin1948.55.2_193

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL SCIENCE LTD  

Temporal variation of the scatterer distribution has been detected around the focal area of an M6.1 earthquake, northeastern Japan, by applying semblance analysis to seismic array data. The variation was found in slant-stacked waveforms of two explosions detonated just above the focal area one month before and two months after the M6.1 earthquake. According to geodetic observations, a pressure source that might be a magma reservoir exists beneath this region and its location changed during the observation period. Slant-stacked waveforms are slightly different for the two explosions, The difference suggests that a scatterer contributing to the coda part is shifted towards the east and to greater depths only after three months. In this region, seismic and volcanic activity was very active during the interval between the two explosions. The volcanic activity of Mt Iwate just north of the M6.1 earthquake, which started several months beforehand, continued during this period. It may be considered that scatterer change relates to seismic and/or volcanic activity in the studied area.

DOI： 10.1111/j.1365-246X.2001.00339.x

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DOI： 10.4294/zisin1948.52.4_407

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DOI： 10.1029/1999JB900437

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.4294/zisin1948.52.2_283

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BIRKHAUSER VERLAG AG  

Borehole seismograms from local earthquakes in the aftershock region of the 1984 western Nagano Prefecture, Japan earthquake were analyzed to measure the frequency-dependent characteristics of P- and S-wave attenuation in the upper crust. The records from a three-component velocity seismometer at the depth of 145 m exhibit high S/N-ratio in a wide frequency range up to 100 Hz. Extended coda normalization methods were applied to bandpass-filtered seismograms of frequencies from 25 to 102 Hz. For the attenuation of high-frequency P and S waves, our measurements show Q(P)(-1) similar or equal to 0.052f(-0.66) and Q(S)(-1)similar or equal to 0.0034f(-0.12), respectively. The frequency dependence of the quality factor of S waves is very weak as compared with that of P waves. The ratio of Q(P)(-1)/Q(S)(-1) is larger than unity in the entire analyzed frequency range.

DOI： 10.1007/s000240050205

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present a method for synthesizing three-component seismogram envelopes of local earthquake in a randomly inhomogeneous semi-infinite medium. The method extends the single scattering model of Sate [Sate, H., 1984. Attenuation and envelope formation of three-component seismograms of small local earthquakes in randomly inhomogeneous lithosphere. J. Geophys. Res. 89: 1221-1241.] by incorporating the effects of a free surface, frequency-dependent non-spherical radiation from a double-couple point source, and non-isotropic scattering including wave-type conversion. We synthesize seismogram envelopes-the root-mean-square (rms) amplitude of seismograms-at a receiver located on the free surface by dividing an inhomogeneous medium into many small cells and summing the energy of scattered waves from the cells on the isochronal scattering shells of different scattering modes for a given lapse time. Our main focus is the free surface effect, which alters the shape of isochronal scattering shells and the amplitudes of incident waves. Synthesizing three-component seismogram envelopes for different source-receiver configurations, we find that these effects on seismogram envelopes are not negligible and are pronounced at early S coda. P coda envelope shape is not very sensitive to free-surface incorporation because of dominant SP scattered waves which come from the source direction. (C) 1997 Elsevier Science B.V.

DOI： 10.1016/S0031-9201(97)00061-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The short-wavelength crustal heterogeneity is investigated in the Nikko area (Central Japan), using a three-component seismogram envelope analysis applied to small local earthquakes. We stochastically model fluctuations in crustal density and seismic wave velocities by using an exponential auto-correlation function. The model parameters are correlation length a and root mean square (rms) fractional fluctuation strength epsilon. Free surface and non-spherical source radiation effects are incorporated in the synthesis of three-component seismogram envelopes. We use 149 short-period seismograms recorded by array stations from 10 local earthquakes with magnitudes 1 similar to 2 and epicentral distances less than 15 km. A best fit minimum residual between observed and synthetic envelopes in the frequency band of 8 similar to 16 Hz gives epsilon(2)/a similar or equal to 8 x 10(-6) m(-1). With the additional information on the strength of direct wave attenuation and the excitation of coda waves at low frequencies, we estimate that a = 300 similar to 800 m and epsilon = 5 similar to 8%. This strong heterogeneity and short correlation length probably reflect the geological condition of the Nikko area which is located in the vicinity of the volcanic front. (C) 1997 Elsevier Science B.V.

DOI： 10.1016/S0031-9201(97)00062-9

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Language：Japanese   Publishing type：Research paper (other academic)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL SCIENCE LTD  

We propose a new method for the simultaneous measurement of P- and S-wave attentuation by extending the conventional coda-normalization method which had been limited to the estimation of S-wave attenuation. Using this method, we measured frequency-dependent attenuation of both P and S waves in the lithosphere beneath the Kanto area, Japan, from seismograms of 174 local earthquakes for the frequency range 1.5 less-than-or-equal-to f less-than-or-equal-to 24 Hz. The values of Q(S)-1 and Q(P)-1 corresponding to peak amplitude decays show strong frequency dependence, and are expressed by using power laws Q(S)-1 = 0.012f-0.73 and Q(P)-1 = 0.031f-0.95, respectively. The ratio Q(P)-1/Q(S)-1 is found to be larger than unity for the whole frequency range. The apparent attenuations of P and S waves with travel distance are almost the same for frequencies higher than 1 Hz. Our results differ from the characteristics of low-frequency wave attenuation reported by other studies for frequencies lower than 1 Hz. This frequency dependence and the ratio may suggest that scattering loss due to random heterogeneities in the earth medium plays an important role in seismic-wave attenuation in the lithosphere.

DOI： 10.1111/j.1365-246X.1993.tb01476.x

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Language：English   Publisher：CENTER ACADEMIC PUBL JAPAN  

DOI： 10.4294/jpe1952.41.327

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DOI： 10.4294/zisin1948.55.2_217

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

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Language：Japanese   Presentation type：Poster presentation  

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Language：Japanese   Presentation type：Oral presentation (general)  

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