地震　第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
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63 巻, 4 号
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論説
  • 弱震時における表層地盤のグリーン関数に基づく強震時の非線形応答に関する研究
    ―2008年岩手・宮城内陸地震におけるIWTH25観測点の記録―
    田中 友香里, 木下 繁夫
    2011 年 63 巻 4 号 p. 197-206
    発行日: 2011/03/25
    公開日: 2012/03/26
    DOIhttps://doi.org/10.4294/zisin.63.197
    ジャーナル フリー
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    Two noteworthy points shown in strong motion seismograms recently recorded in near source regions, are the transient response of tilt motion and the nonlinear response of near-surface layer. In the present study, we investigated the nonlinear response of near-surface layer by means of borehole array recordings. First, we estimated the Green’s function of near-surface layer for SH waves assuming that response of near-surface layer is linear for weak motion. Two Green’s functions are estimated for two different data sets which are constructed from the earthquakes occurred before the 2008 Iwate-Miyagi Inland Earthquake and its aftershocks, respectively. Furthermore, we estimated another Green’s function of near-surface layer using the coda part of the main shock of the 2008 Iwate-Miyagi Inland Earthquake. This Green’s function estimated using the coda part of main shock is signicantly different from the other two Green’s functions, although these two Green’s functions are practically identical. This fact reveals that the rigidity of surface layer reduced during the main shock and recovered after that. The coda part of main shock showed the reduced state of near-surface layer’s rigidity. Next, for explaining the strong motion part of main shock, we constructed a time-series model using the Green’s functions estimated using weak motion data. The nonlinear response of near-surface layer in the strong motion part is represented by the time-dependent pole conguration involving in the time-series model, which is constructed by an exponential auto-regressive model of second order. Hence, our constructed model is applicable to the whole process of main shock. Finally, we validated the practicability of model to the data involving the nonlinear response of near-surface layer by using borehole array recordings obtained at the IWTH25 station.
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  • 震度データによる1914年秋田仙北地震の短周期地震波発生域と地震規模の推定および 1896年陸羽地震との比較
    神田 克久, 武村 雅之
    2011 年 63 巻 4 号 p. 207-221
    発行日: 2011/03/25
    公開日: 2012/03/26
    DOIhttps://doi.org/10.4294/zisin.63.207
    ジャーナル フリー
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    The 1914 Akita-Senboku earthquake was investigated using seismic intensity data estimated from damage records to clarify their magnitudes and source areas related to short-period seismic waves. We evaluated the most appropriate fault model of the Akita-Senboku earthquake on the basis of grid search analysis and other existing research works such as hypocenter distribution of micro earthquakes and S-wave velocity perturbations [Okada et al. (2010)]. The seismic intensity inversion analysis for the Akita-Senboku earthquake assuming an east-dipping fault model (depth: 6-13 km) indicated that the short-period radiation zone was located in the north and deep part of the fault plane and there was a low S-wave velocity zone in the lower crust beneath it. The magnitude of 6.5.6.6 yielded the least evaluation error. The magnitude was also estimated from the area of seismic intensity .ve-lower or more using empirical relationship [Muramatsu (1969)]. It resulted in M=6.6.∼6.7 and reinforced the accuracy of the intensity inversion result. It is concluded that the magnitude of the Akita-Senboku earthquake should be considerably less than M=7.1 after Utsu (1979) even if considering the estimation error. The 1896 Rikuu earthquake was also analyzed to be compared with the Akita-Senboku’s results and to consider their dieffrences. The fault location of the Rikuu earthquake was already identi.ed from surface faults. We assumed two conjugate faults for the northern fault zone of the eastern margin of the Mahiru mountains and the northern fault zone of the eastern margin of the Yokote basin. The most appropriate magnitude was estimated to be M=7.1, which is equivalent to that after Utsu (1979). Therefore, our results indicate that the Rikuu earthquake was considerably larger than the Akita-Senboku earthquake. The short-period radiation zone of the Rikuu earthquake, which might be less accurate due to the lack of intensity data compared to the Akita-Senboku earthquake, was located in shallow zone at the intersection of two conjugate faults. This location contrasts with that of the Akita-Senboku earthquake without surface faults.
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  • 近地強震記録を用いた海溝型繰り返し地震の震源過程の推定と比較
    ―茨城県沖で1982年と2008年に発生したM7の地震を対象として―
    瀧口 正治, 浅野 公之, 岩田 知孝
    2011 年 63 巻 4 号 p. 223-242
    発行日: 2011/03/25
    公開日: 2012/03/26
    DOIhttps://doi.org/10.4294/zisin.63.223
    ジャーナル フリー
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    At the Ibaraki-ken-oki region where the Paci.c plate subducts beneath the North American plate, repeating M7 class subduction-zone earthquakes have occurred with an interval of approximately 21 years. We estimated the source rupture processes of two M7 Ibaraki-ken-oki earthquakes from broadband strong motion records and compared them to investigate the source characteristics of repeating subduction-zone earthquakes. First, the source model of MJ 7.0 earthquake occurring on May 8, 2008 (2008MS) was estimated by the forward modeling through strong motion simulations using the empirical Green’s function method. Strong ground motions were simulated in the broad.band frequency range (0.3-10 Hz). A single rectangular Strong Motion Generation Area (SMGA) was assumed. The synthetic waveforms at stations within 150 km from the hypocenter reproduced the observed ones well. Then, we estimated the source model of another MJ7.0 earthquake occurring on July 23, 1982 (1982MS). We compared the observed waveforms of the 1982MS and the 2008MS at the same stations and identi.ed the initial rupture phase for the 1982MS. We assumed a single rectangular SMGA away from the hypocenter of the 1982MS. The SMGA was located about 33 km westward from the hypocenter and ruptured about 11s after the origin time; this location was just 7 km northward to the 2008MS hypocenter and so inside its SMGA. We analyzed the spectral ratio of the strong motion records of the 1982MS to the 2008MS and estimated the size, stress drop, and slip amount of the SMGA of the 1982MS. The estimated size was the same as that of the 2008MS, but the stress drop and slip amount were approximately 1.5 times larger. Synthetic waveforms reproduced the observed ones well when we assumed the rupture propagation characteristics were the same as that of the 2008MS. This simulation did not have the resolution to several kilometers’change of the location of the SMGA. Our results indicate that the two SMGAs may overlap. They were the same size, but had different stress drops and slip amounts. The hypocentral locations and initial rupture processes were also different. These results show the variety of the repeating earthquakes occurring in this region.
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