地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
63 巻 , 3 号
選択された号の論文の6件中1~6を表示しています
論説
  • 大滝 壽樹, 加野 直巳, 横倉 隆伸
    2011 年 63 巻 3 号 p. 107-121
    発行日: 2011/02/25
    公開日: 2012/03/26
    ジャーナル フリー
    Seismic reflection profiling using P-to-SV converted waves with 3-component seismometers has an advantage for examining deeper S-wave velocity profile compared with other methods such as SH-wave reflection profiling. In spite of this advantage, the reflection profiling with 3-component seismometers has not been popular because it requires three-times more channels than reflection profiling with vertical-component seismometers, which is popular for examining P-wave velocity profile. In our previous paper, we showed a possibility of examining S-wave velocity profile by analyzing P-to-SV reflected waves observed at one 3-component seismometer together with P-wave reflection profiling. Although the method in the previous paper has ambiguity to obtain S-wave velocity, the approach is attractive because one additional 3-component seismometer requires little additional cost. In this paper, we show the validity of the approach using another method. This new method is easy to apply, if P-wave velocity profile was already determined using vertical component reflection profiling. We picked P-to-SV reflected waves on observed radial-component records, and adjusted theoretical travel-time curves to the observed waves at one 3-component seismometer. P-wave velocity profile and depths of discontinuities were fixed to the result of P-wave reflection profiling. When the travel time was calculated, a ratio of P-wave velocity to S-wave velocity was assumed to be the same for all layers. The ratio for well-matched theoretical time was considered as the ratio of vertical travel times of S-wave to P-wave from the surface to the reflected layer. Shear-wave velocity for each layer was calculated from the ratio. Slowness of the P-to-SV reflected wave at large offsets depends on P-wave velocity and a depth of the reflector, but shows little dependence on S-wave velocity at large offsets. This characteristic makes it possible to separate the influence of the reflector depth and of S-wave velocity on travel-time curves of the reflected waves. Tests for simple horizontal layer models and dipping layer models showed that S-wave velocities were obtained within about 10% in error, and that obtained S-wave velocity was stable in errors of P-wave velocity and of an inclination of the layer. We applied this method to a previous seismic reflection survey with 3-component seismometers which was carried out at Fuchu-city, Tokyo. Previous study analyzed P-and S-wave velocity profiles to 2-km depth using P-and converted-wave reflection profiling. Another survey also examined P-and S-wave velocity profiles near the survey line using VSP method. We selected one station near the VSP well among the survey stations, and analyzed the data using the method. S-wave velocity profile that we obtained was consistent with the profiles of the previous studies to 2-km depth. Only one 3-component seismometer was necessary for this method with P-wave reflection profiling. Our results show that this method provides adequate shear-wave velocity profile with little additional cost.
  • 河村 将, 工藤 健, 山岡 耕春, 古本 宗充
    2011 年 63 巻 3 号 p. 123-137
    発行日: 2011/02/25
    公開日: 2012/03/26
    ジャーナル フリー
    We developed a comprehensive statistical validation system of crustal activities with which to easily address spatially and temporally sufficient range of a database of geophysical measures. The system involves carrying out the following four processes: (1) creating the database of geophysical measures with spatially and temporally gridded and other convenient formats, (2) comparing any two geophysical measures, at least one of which is time-variable, (3) classifying the spatiotemporal relationship of these geophysical measures into some types by defining a statistical index, and (4) evaluating and validating the relationships between classification results and the occurrence of target physical events such as large inland earthquakes. With the system, we aim for making a statistical model, or an appropriate rule for monitoring of crustal activities. Formulation of the rule is, in turn, expected to lead to comprehensive understanding of crustal activities. Here, we focused on the relations of seismicity and strain rate to introduce the conception and algorithm of the system. The system requires input of the database and other parameters and leads to output of various spatiotemporal distributions, 2-by-2 contingency tables, and probability gains for prediction and alarm rates for target physical events.
  • 五十嵐 俊博, 飯高 隆, 宮林 佐和子
    2011 年 63 巻 3 号 p. 139-151
    発行日: 2011/02/25
    公開日: 2012/03/26
    ジャーナル フリー
    Recently, several researchers have elucidated crustal structures over a large area of the Japanese Islands from travel time inversion analyses. However, very few studies have paid attention to velocity discontinuities due to the limitations of spatial resolution. In this study, we apply a receiver function analysis to estimate seismic velocity structure and seismic velocity discontinuities of the crust in the Japanese Islands. We search for the best-correlated velocity structure model between an observed receiver function at each station and synthetic ones using a grid search method. Synthetic receiver functions are calculated from many assumed simple velocity structures that consist of a sediment layer to compensate for the effects of the low-velocity sediment layer and two velocity discontinuities. As a result, we clarified the spatial distributions of the crustal S-wave velocities and the tops of mantle depths. Several plain and basin areas are covered with thick low-velocity sediment layers. There are low-velocity layers corresponding to volcanoes in the upper crust (5-15 km deep). In the lower crust (15-25 km deep), our results show low-velocity structures in the eastern part of the Niigata-Kobe Tectonic Zone (NKTZ) and the Median Tectonic Line. Around the crust-mantle boundary, we see clear low-velocity zones beneath volcanoes, the western part of the NKTZ, and in the occurrence regions of the non-volcanic low-frequency tremor. High velocities near the southern coastline of the Japanese Islands correspond to the crust-mantle discontinuity of the subducting Philippine Sea plate. The crustal structure beneath the Itoigawa-Shizuoka Tectonic Line (ISTL) shows relatively low velocities in the shallower part and high velocities in the deeper part compared to neighborhood areas. The ISTL is also the boundary of the velocity structure of the upper crust in the Japanese Islands. The northeastern Japan region has heterogeneities of velocity perturbations, whereas the southwestern Japan region has the relatively stable high-velocity zones. The tops of mantle depths tend to increase in mountain regions with some undulations. The discontinuity of the subducting Philippine Sea plate at a depth of more than 40 km is extracted in several areas from the Kanto district to the Kyushu district. This suggests that the velocity discontinuity of the subducting Philippine Sea plate is larger than that of the overriding plate.
  • 平井 敬, 福和 伸夫
    2011 年 63 巻 3 号 p. 153-163
    発行日: 2011/02/25
    公開日: 2012/03/26
    ジャーナル フリー
    Listening to a sound representing ground motion is valuable to understand an earthquake intuitively compared with only seeing a waveform. In this paper, we propose a new method to make a sound of earthquake, in which several rules different from existing methods are used. In our new method, a sound of earthquake is generated by modifying the frequency information of generating function based on the theory of symmetric Fourier analysis. Applying the method to the JMA Kobe seismogram in 1995 South Hyogo Prefecture Earthquake and predicted waveform at Sannomaru, Nagoya, in future Tokai and Tonankai earthquake, three features of the sound generated by this method are confirmed: the sound has same duration time as the seismogram, high sound is heard when the seismogram has high frequency, and loud sound is heard when the seismogram has large amplitude.
  • 野口 科子, 笹谷 努
    2011 年 63 巻 3 号 p. 165-187
    発行日: 2011/02/25
    公開日: 2012/03/26
    ジャーナル フリー
    We investigate nonlinear soil responses based on strong motion records at a large number of observation sites during the 2003 Miyagi-oki intraslab earthquake (Mw 7.0). First, we examine the efficiency of S-H/V (horizontal-to-vertical spectral ratio for the S-wave portion at the ground surface) method to identify the nonlinearity by comparing the results with those by the standard S-wave surface-to-borehole spectral ratio method. In the examination we propose a new quantitative index to measure the degree of the nonlinear soil response, DNL, which evaluates the gap between the spectral ratio for strong ground motion and that for weak ground motion. The DNL values by the S-H/V method as well as those by the surface-to-borehole spectral ratio method increase with observed peak ground acceleration (PGA) values at surface (100∼1000 cm/s2), reflecting the increase of the degree of nonlinearity. However, both DNL values at sites with large S-wave velocity of the surface layer (>300 m/s) do not show any increase even for large PGA values (∼800 cm/s2), indicating linear site response. From these facts we conclude that the S-H/V spectral ratio method is also efficient to identify the nonlinearity. Second, in order to examine the effects of nonlinear soil response on strong ground motions, we make the broadband strong motion simulation for the 2003 Miyagi-oki earth.quake by means of the empirical Green’s function (EGF) method. The synthetic waveforms for the horizontal components at the ground surface significantly overestimate the observed ones at stations with large DNL values; the synthetic PGA values are about two times greater than the observed ones. We confirm that the overestimation is attributed to the ignorance of the nonlinear soil response in the EGF simulation. Finally we briefly discuss the potential of the DNL value for studying the nonlinear soil response.
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