Journal of Physics of the Earth
Online ISSN : 1884-2305
Print ISSN : 0022-3743
ISSN-L : 0022-3743
Volume 41, Issue 1
Displaying 1-4 of 4 articles from this issue
  • Masayuki Takemura, Tomonori Ikeura, Tomiichi Uetake
    1993 Volume 41 Issue 1 Pages 1-19
    Published: 1993
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Strong motion records for 22 earthquakes from 1982 to 1987 in the subduction zone off Fukushima Prefecture are analyzed. Magnitude M of the earthquakes ranges from 4.0 to 6.7. The source spectrum of each earthquake is obtained from S-waves in horizontal components. Seismic moment Mo of each event is evaluated from spectral amplitudes in a low frequency range. Two corner frequencies are identified on the source spectra for 7 events of M>6, while source spectra of 15 events of M<6 are characterized by one corner frequency. The first corner frequency fc of the M>6 events satisfies a relation of Mo∝fc-3, which is consistent with the data of seismic moment and fault area for the M=7 to 8 class events in the 1938 large earthquake sequence off Fukushima Prefecture. Complicated S-wave forms for M>6 events suggest that the second corner frequency fc* is related to the complex faulting process with small-scale fault heterogeneities. This means that fc* corresponds to the patch corner frequency. It is found that the seismic moment density Mo* at a frequency of fc* is correlative to the excitation of high frequency waves, while the values of fc* are nearly constant, about 1 Hz, for all the M>6 events. We found two types of earthquakes with M>6 off Fukushima Prefecture: high frequency events and low frequency events. High frequency events show larger peak accelerations than a value expected from the attenuation curve, and show larger Mo* values compared with low frequency events with the same seismic moment. High frequency events mostly occurred in the northwestern part, while low frequency events occurred in the southeastern part off Fukushima Prefecture. This indicates that the faulting process is more heterogeneous in the northwestern part than the southeastern part. Concerning the events of M<6 which occurred mostly in the northwestern part, simple S-wave forms suggest that those earthquakes are due to a single fault patch rupture process. These events satisfy another relation of Mo∝fc-3 on the average, which is different from one for larger events. Stress drop deduced from this relation is about ten times as high as the average stress drop from the Mo-fc relation for the event of M>6. Higher stress drop of these events may be due to fault patches with large intensity.
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  • Takuo Maruyama, Tian-You Fan
    1993 Volume 41 Issue 1 Pages 21-39
    Published: 1993
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    A breakdown zone model for a fault of length a on y=0 is constructed in a strip of width 2H, based on two-dimensional elasticity theory. The three-dimensional concept from which this model is derived consists in that a strike slip fault exists in a thin brittle regime overlying a ductile regime or asthenosphere, and that outside of the strip in the brittle regime stresses or displacements will be changed little in the formation of a new shear crack surface. Two types of boundary condition are considered in an idealized form, i.e., zero stress change and zero displacement change on the boundary y=±H. To obtain the stress intensity factor for a semi-infinite crack in a strip, numerical methods are developed for solving integral equations. The results show that the critical shear stress against the ratio a/H for the critical state of fault formation separates in two branches at the point a/H=0, according to those two types of boundary condition.
    What is relevant to our three-dimensional concept of a strike slip fault lies between the two cases. Then an expression for the critical shear stress is suggested, to a first approximation, independent of a/H. This is the same with the formula for an ordinary breakdown zone model in a homogeneous infinite medium. Since in our case, however, material constants are taken in the strip, our model may serve as a breakdown zone model of a strike slip fault related to a structural weak line in the crust.
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  • Shozo Matsumura
    1993 Volume 41 Issue 1 Pages 41-43
    Published: 1993
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
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  • Mitiyasu Ohnaka
    1993 Volume 41 Issue 1 Pages 45-56
    Published: 1993
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
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