Journal of Physics of the Earth
Online ISSN : 1884-2305
Print ISSN : 0022-3743
ISSN-L : 0022-3743
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Volume 32, Issue 6
Displaying 1-3 of 3 articles from this issue
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  • BAYESIAN ESTIMATION OF HYPOCENTER WITH ORIGIN TIME ELIMINATED
    Mitsuhiro MATSU'URA
    1984 Volume 32 Issue 6 Pages 469-483
    Published: 1984
    Released on J-STAGE: April 30, 2009
    DOIhttps://doi.org/10.4294/jpe1952.32.469
    JOURNAL FREE ACCESS
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    In this study a new approach to hypocenter location is formulated from a Bayesian point of view. Observational data for hypocenter location are the arrival times of phases at seismic stations. Unknown parameters are the spatial coordinates and origin time of a quake focus. In general, we have some prior information about the spatial coordinates, but not about the origin time. Then, according to Bayes' theorem, the probability density function (pdf) of hypocenter parameters posterior to observed data is proportional to a product of the likelihood and the prior pdf of the spatial coordinates. Integrating the posterior pdf over the whole range of origin time, we can eliminate the origin time from the location problem, and obtain the marginal pdf of only the spatial coordinates. The best estimate of a hypocenter is given by a set of the spatial coordinates which maximizes the marginal pdf. Supposing Gaussian errors in both observational and prior data, we obtain a simple and robust algorithm to invert arrival time data for a hypocenter. Estimation errors of parameters are evaluated by an asymptotic covariance matrix, which gives a good approximation to exact covariance when the estimate is linearly close to a true hypocenter. The advantages of this approach are as follows : i) the use of prior data resolves unknown parameters completely within finite errors, and ii) the elimination of origin time suppresses the instability of estimates.
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  • REGIONAL HETEROGENEITIES OF THE EARTH TIDES IN CENTRAL JAPAN AS REVEALED BY TIDAL GRAVITY OBSERVATIONS
    Takeshi ENDO
    1984 Volume 32 Issue 6 Pages 485-510
    Published: 1984
    Released on J-STAGE: April 30, 2009
    DOIhttps://doi.org/10.4294/jpe1952.32.485
    JOURNAL FREE ACCESS
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    Tidal gravity observations were conducted at seven stations in Central Japan with two LaCoste & Romberg gravimeters. Analyses of the obtained data revealed remarkable regional heterogeneities of tidal gravity parameters. These heterogeneities cannot be eliminated by ocean tide corrections. After ocean tide corrections the obtained δ-factors show a tendency to decrease from a coastal zone toward an inland one by 1.5 % for M2 and by 3 % for 0, constituents. Such large lateral variations in 5-factor, occurring within a range of about a few hundred kilometers, have not been clearly obtained in previous studies. We have examined several possible causes of these δ-factor variations such as correction errors of ocean tide effects, meteorological disturbances, and groundwater perturbations, but none of them can explain the δ-factor variations. The earth's laterally heterogeneous structure seems to be a likely cause, but a satisfactorily quantitative explanation cannot be given at present. Improvements of an earth tide theory including effects of lateral heterogeneities are necessary for further quantitative explanations.
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  • SOURCE PARAMETERS RELEVANT TO HETERO-GENEITY OF A FAULT PLANE
    Yasuo IZUTANI
    1984 Volume 32 Issue 6 Pages 511-529
    Published: 1984
    Released on J-STAGE: April 30, 2009
    DOIhttps://doi.org/10.4294/jpe1952.32.511
    JOURNAL FREE ACCESS
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    Accelerograms due to 16 earthquakes with JMA magnitude from 5.5 to 7.4 are analyzed. Second corner frequencies for the earthquakes are in a range from 1 to 8 Hz, and ratios of the rms stress-drop to the global stress-drop are from 2 to 10. The second corner frequency increases with the ratio of the rms stress-drop to the global stress-drop, and it is almost independent of the characteristic length of the fault plane. Based on stochastic source models, the second corner frequency is inversely proportional to an average size of element faults constituting a fault plane, so that the present result suggests that the average size decreases with increase in the heterogeneity of stress-drop on the fault plane. Also, the result implies that the average size of element faults is almost independent of the entire fault size for the earthquakes analyzed in this study.
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