Journal of Physics of the Earth
Online ISSN : 1884-2305
Print ISSN : 0022-3743
ISSN-L : 0022-3743
Volume 43, Issue 5
Displaying 1-7 of 7 articles from this issue
  • Kiyoo Mogi
    1995 Volume 43 Issue 5 Pages 533-561
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    The first 5-year earthquake prediction project in Japan started in 1965. Now, the sixth 5-year project (1989-1993) is in progress. Various kinds of fundamental observations have been continued and the density of the observation network has increased with time, but there have been no successful cases of earthquake prediction. However, reliable data useful for long-term and short-term prediction are being steadily accumulated, and several cases have suggested the feasibility of earthquake prediction. In the Tokai district of central Honshu, where a major earthquake is expected to occur in the near future, a program for constant monitoring and assessment was formally inaugurated with the goal of forecasting an M8 class earthquake. The results of research on recent major earthquakes in Japan are reviewed as well as the Tokai earthquake problem.
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  • Masakazu Ohtake, Yuzo Ishikawa
    1995 Volume 43 Issue 5 Pages 563-584
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    More than five hundred permanent stations are included in seismic observation networks in Japan. Those observation stations belong to a numbers of networks; the nation-wide network operated by the Japan Meteorological Agency (JMA), and subregional to local networks operated by national universities and other institutions, including the National Research Institute for Earth Science and Disaster Prevention (NIED). Most of the non-JMA networks were established under the national program for earthquake prediction, which began in 1965. The JMA network has history of more than 100 years, and it now includes about 190 stations which are equipped with both short- and/or intermediate-period seismometers. The other networks, for the most part, are instrumented by highly sensitive short-period seismometers typically with a natural period of 1.0s. Almost all of the networks are linked by digital telemetry, and are supported by a computer-aided data processing systems which execute seismogram reading and hypocenter determination automatically or semi-automatically. The networks of national universities are basically independent of one another, but they carry out on-line and off-line exchange of observation data to some extent. Currently, the earthquakes occurring in Japan are uniformly located down to magnitude 3.0 except for the Ryukyu region by the JMA network, and down to magnitude 2.0 except for northern Hokkaido and Kyushu-Ryukyu arc by integrating data from the university networks. A total of about ninety thousand hypocenters per year are located by Japanese seismic networks.
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  • George Igarashi, Hiroshi Wakita
    1995 Volume 43 Issue 5 Pages 585-598
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Several research groups have made efforts to accumulate reliable data on various geochemical and hydrological parameters for periods longer than 10 years. The data accumulation has enabled them to characterize background fluctuations in the data and consequently to examine their relation to earthquake occurrences without prejudice. As a result, a number of earthquake-related anomalies have consistently been identified in various geochemical and hydrological observations, suggesting that they are sensitive indicators of earthquake occurrence. Comprehensive monitoring based on multicomponent and multi-station observations has begun to result in the detection of more reliable precursory phenomena which appeared simultaneously in two or different kinds of observations at several stations.
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  • Kiyoshi Suyehiro, Toshihiko Kanazawa, Naoshi Hirata, Masanao Shinohara
    1995 Volume 43 Issue 5 Pages 599-618
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    In the fall of 1989, a broadband digital seismic sensor system (OBDS: Ocean Broadband Downhole Seismometer) was successfully emplaced in an ODP drill hole in the Japan Sea. The site is located in the northern part of the Yamato Basin about 160km west from the northern Japan west coast. The instrument was clamped within a basalt section beneath 543m of sediments at 715m below seafloor and 3, 523m below sea surface. We obtained a continuous real-time seismic recording of 60h length and an off-line and intermittent sea-floor recording of 30 days operation. The dataset includes airgun signals for studying the local crustal structure, several local earthquakes, and a teleseismic event. An array of OBS's was also deployed during the airgun shooting to study detailed local crustal structure including anisotropy. The results of our experiment may be summarized as follows. We obtained for the first time a digital long-period seismic record from beneath the seafloor. The structural environment of OBDS could be constrained from results of drilling and airgun profiling; the crust is nearly identical to that of the southern Yamato Basin, which is about 14km thick, and the upper crust possesses anisotropy in agreement with the present E-W compressive state of stress. Noise is generally lower than island or seafloor stations, and is as low as about 104 (nm/s)2/Hz at about 0.2Hz, where normally a noise peak is found. Signals from local events and airguns exhibit less reverberations as compared with OBS records; i.e. they are less affected by signal-generated noise, thus allowing better phase identifications. Surface wave dispersion from an event (Mb=5.4) at 39° distance was clearly observed. Thus, our results indicate that downhole emplacement of a seismometer beneath an ocean provides higher quality data than alternative seafloor or island observations. At frequencies lower than 0.1Hz, however, our OBDS did not have sufficient sensitivity to detect the background noise level.
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  • Junzo Kasahara, Hiroshi Utada, Hajimu Kinoshita
    1995 Volume 43 Issue 5 Pages 619-628
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Cooperative efforts by Japanese and U.S. researchers have been initiated to reuse a former submarine telephone cable ("TPC-1, " retired in 1991) for undersea scientific measurements. TPC-1 runs from Ninomiya, Japan to Guam, U.S.A., and has a length of ca. 2, 700km with a strike almost in the N-S direction. A prototype instrument package was successfully installed in northern Japan at depth of 100m using a similar coaxial cable. An instrumental package has been built and will be deployed near 30°N in 1996. The instrument has three component accelerometers with 24bit A/D converters, a hydrophone using analog telemetry, and a quartz thermometer and a quartz pressure sensor with a data length of 16bits. The data will be transmitted to the Ninomiya station in real-time. Other plans to reuse submarine telephone cables between Naoetsu, Japan and Nakhodka, Russia and between Okinawa, Japan and Guam are being considered.
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  • Teruyuki Kato, Michito Imae, Minoru Sasaki, Masaaki Murata, Hiroo Kuni ...
    1995 Volume 43 Issue 5 Pages 629-655
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    VLBI, SLR, and GPS have been introduced and used to study geodesy and geodynamics in Japan as well as elsewhere in the world. The present article briefly overviews Japanese recent activities concerning the applications of these techniques. VLBI and SLR have been used mainly in international experiments such as the Monitor Earth Rotation and Intercompare Techniques (MERIT) campaign and the Crustal Dynamics Project (CDP). These projects have succeeded in determining the relative plate motions around the Japanese Islands. GPS, on the other hand, has been used mostly to monitor local crustal deformations for earthquake prediction research. The combination of these techniques may lead us to a unified view of crustal deformation and the dynamics of the upper mantle.
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  • Hitoshi Mizutani
    1995 Volume 43 Issue 5 Pages 657-670
    Published: 1995
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    The Institute of Space and Astronautical Science (ISAS), Japan, plans to undertake a lunar mission, named as LUNAR-A, which is to be launched in 1997. The scientific objective of the mission is to explore the lunar interior using seismometry and heat-flow measurement to obtain a better understanding of the origin and evolution of the moon. The M-V, the newest version of the Mu series launch vehicles, will be used to send a 53 kg spacecraft into a lunar transfer orbit. Three penetrators containing ultra-sensitive seismometers and heat-flow probes will be deployed from a spacecraft onto the lunar surface, and will constitute a seismic and heat-flow measurement network of a much larger span than the Apollo ALSEP network. The seismic observations are expected to provide key data on the size of the lunar core and its physical properties, as well as data on deep lunar mantle structure. The heat flow measurements at three penetrator landing sites will also provide important data on thermal structure and concentrations of heat-generating elements in the moon. Combining these data, it is expected that we will be able to obtain much stronger geophysical constraints on the origin and evolution of the moon than has been previously obtained.
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