Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 47, Issue 2
Displaying 1-12 of 12 articles from this issue
  • Masajiro IMOTO
    1994 Volume 47 Issue 2 Pages 137-142
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The terms of “validity”, “reliability” and “probability gain” are usually used to estimate performance of earthquake prediction. Based on information theory, a new scale is proposed in terms of a log likelihood ratio of an uniform Poisson process to an alternative process of earthquake occurrence. As the alternative process, two Poisson rate model is adopted on the assumption that the large earthquakes may be modeled by two Poisson processes with different rates: the higher rate process simulates large earthquakes that occur in the time-space volume when an earthquake alarm has been declared and the lower rate process simulates earthquakes that occur in a non-alarm time-space volume. The performance of an proposed algorithm can be measured in terms of the likelihood ratio of the two Poisson rate model to a uniform Poisson model. A Monte Carlo simulation experiment shows that correction of the likelihood ratio is required depending on both number of adjusted parameters and number of target earthquakes. The proposed scale is powerful not only for judging statistical significance of precursors but also for comparing effectiveness of precursors.
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  • Hisatoshi BABA, Susumu IIZUKA, Toshi ASADA
    1994 Volume 47 Issue 2 Pages 143-153
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    We studied the seismic activity in and around Iriomotejima Island, SW Ryukyu Arc, Japan, during the period from January 1992 through May 1993. In particular, two earthquake swarms, namely, the swarm which occurred off the SW coast of Iriomotejima Island late in August (SW swarm) and that occurred in the NW region of the Island in the middle of September 1992 (NW swarm), were investigated in detail in this paper. The data used in this study were restricted to “felt earthquakes” whose amplitudes were larger than 1gal. As a result, the number of events was not so large, but homogeneous in time and space.
    The spatial and temporal characteristics of these seismic activities are summarized as follows;
    1) A high swarm activity in the SW region continued several days. The focal zone extended to the NW-SE direction.
    2) The focal region of NW swarm migrated from off the NW coast of the Island to the inland area and then expanded to the surrounding areas.
    3) The focal zones extend in the NW-SE direction. And the vertical distribution of hypocenters shows a sharp seismic boundary plane. This is consistent with the predominant direction of the fault strikes and lineaments in this region.
    4) Some of the remarkable features observed in the later phases appear to represent reflected waves. These features suggest a possibility of the existence of sharp reflectors such as a magma reservoir beneath the region. However, no signs were seen for any surface volcanic activity.
    In consequence, it is considered that these earthquake swarms occurred under the stress field acting in the wide area of the SW Ryukyu Arc.
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  • Tokutaro HATORI
    1994 Volume 47 Issue 2 Pages 155-162
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Many tsunamis which were generated in the SE. Asia region hit the neighboring coasts of the source areas and some tsunamis were observed at the tidal stations of Japan. Tsunami magnitudes on the Imamura-Iida scale are investigated by the author's method 1 (HATORI, 1979, 1986) using the data of inundation heights near the source areas and the tide-gauge records in Japan. The regional characteristics of tsunami magnitudes are discussed in relation to earthquake magnitudes during the period from 1960 to 1993.
    Comparing with the statistical relation of earthquake magnitudes based on the data near Japan and great earthquakes in the world, the magnitude values of the Taiwan tsunarmi sunamis showed relatively to be small. On the contrary, the magnitudes of tsunamis in the vicinities of Philippine and Indonesia exceed more than 1-2 grade (tsunami heights: 2-5 times) compared with the same earthquake magnitude of other regions. The relation between tsunami magnitude, m, and earthquake magnitude, Ms, is expressed as m=2.66Ms-17.5 for these regions. For example, the magnitudes for the 1976 W. Mindanao tsunami (Ms=7.8, 3702 deaths) and the 1992 N. Flores Is. tsunami (Ms=7.5, 1713 deaths) were determined to be m=3 and m=2.5, respectively.
    The focal depth of tsunamigenic earthquakes is shallower than d<36km, and the detectivity of tsunamis is small for deep earthquakes being d>40km. For future tsunamis, it is indispensable to take precautions against shallow earthquakes having the magnitudes Ms<7.0.
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  • Hiroaki YAMANAKA, Masayuki TAKEMURA, Hiroshi ISHIDA, Tomonori IKEURA, ...
    1994 Volume 47 Issue 2 Pages 163-172
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    An array measurement of long-period microtremors was done in the western part of the Kanto plain, Japan, in order to extract subsurface structure down to the top of the Earth's crust. The area studied is characterized by the existence of deep sediments with thicknesses of around 2km. The subsurface structure near the array is well-investigated by seismic surveys as well as velocity loggings. We, therefore, can examine the applicability of the array technique of microtremors to the exploration of deep sedimentary layers.
    Ten sites with station spacings of 0.3 to 2km were used for the array measurement, and a vertical seismometer with a natural period of 10s was set at each site. From the f-k spectral analysis, the phase velocities of microtremors in a period range from 0.7 to 5s are found to be in normal dispersion, indicating the large contribution of Rayleigh waves to microtremors. We, then, deduced the 1-D subsurface structure by an inversion of phase velocity dispersion curve of the Rayleigh wave. A reasonable structure for the deep sediments could be obtained, when the thicknesses of the sediments with fixed velocities were parametrized in the inversion. The resultant structure is in good agreement with the results of a seismic reflection survey.
    From these results, we concluded that the array technique of microtremors can be used for the extrapolation of well-investigated structural data at a site to other sites in a basin. Furthermore, we can estimate S-wave velocities of deep sediments at any site by using the microtremor array technique with P-wave data from seismic refraction survey.
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  • Shinji KAWASAKI, Shigeru TODA, Koichi NAKAGAWA, Yoshimasa KOBAYASHI
    1994 Volume 47 Issue 2 Pages 173-182
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    A seismic reflection exploration was conducted in the eastern part of the Arima-Takatsuki Tectonic Line (ATL), Kinki district, central Japan. A survey line with 2700m length was spread across the extension of the ATL that strikes from west to east on the northern border of the Osaka plain. The main aim of this survey is to investigate subsurface structure up to the depth of 1km, which possibly reveals buried faults along the ATL.
    A weight-dropping type wave generator was used as a source apparatus. Through data processing by the CMP profiling method, following results were obtained:
    (1) The depth of the basement rock is about 150m near the Otokoyama hill (northern section) and about 350m on the Osaka plain side (southern section). There is a large-scale buried fault zone below the survey line. The main fault along the ATL seems to extend eastward to the southern part of the Otokoyama hill beyond the survey location.
    (2) Two flexures are discerned in this buried fault zone and they form a graben structure. This structure probably is the topographic expression of ATL (see text for detail).
    (3) The movement producing the southern flexure is presumed to have begun after sedimentation of the 50m depth layer in the Osaka plain, based on parallel dipping of each layer at the depth.
    (4) The rate of vertical movement mentioned above is estimated to be equal to or greater than those of B class active faults, based on the difference of the basement depths across the southern fault.
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  • Tokuji UTSU
    1994 Volume 47 Issue 2 Pages 183
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Takao EGUCHI
    1994 Volume 47 Issue 2 Pages 185-188
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Naoki SHIRAI, Yutaka HIRATA, Hideo TSUNAKAWA, Toshi ASADA
    1994 Volume 47 Issue 2 Pages 189-192
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Masayuki TAKEMURA, Tomonori IKEURA, Kazuyoshi KUDO, Hirohito OHNUMA
    1994 Volume 47 Issue 2 Pages 193-200
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Hiromichi HIGASHIHARA
    1994 Volume 47 Issue 2 Pages 203-224
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The logical structure of the earthquake-resistant design procedures of civil engineering structures is introduced and discussed so that a frame of reference may be established where civil engineers and strong motion seismologists can exchange their knowledge and opinion. Interdisciplinery problems which are to be dealt with in the near future are then identified, and several topics in which the seismologists are expected to take interest are overviewed with representative works achieved recently of the civil engineering in Japan, including the modeling of the ground dynamics, the dynamic soil-structure interaction, the specification methodology of input seismic disturbances and the engineering aspects of strong ground motions.
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  • Present Status and Prospects
    Kazuyoshi KUDO
    1994 Volume 47 Issue 2 Pages 225-237
    Published: August 14, 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    We review the progress of strong earthquake motion observations in Japan since 1980 emphasizing the development of instrumentation and array deployment. The first part of this paper is concerned with the improvements of dynamic range, frequency band, data storage and telemetry used in strong motion observations. The second part discusses the precision of analog and digital recordings and the processing of strong motion records. Thirdly, the strong motion seismograph network and orgnizations for strong motion observations in Japan are briefly summarized and they are compared with those of other countries. As a future prospect of strong motion observation in Japan, we expect mobile array observations to be popular and note necessity for quick and open data exchange.
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  • 1994 Volume 47 Issue 2 Pages 239
    Published: 1994
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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