巻号一覧
54 巻 (2001 - 2002)
- 4 号 p. 431-
- 3 号 p. 335-
- 2 号 p. 251-
- 1 号 p. 1-
前身誌
54 巻, 3 号
選択された号の論文の7件中1～7を表示しています
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基本モデルの構築とその評価岡田 正実, 伊藤 秀美2001 年 54 巻 3 号 p. 335-345
発行日: 2001/12/25
公開日: 2010/07/01
ジャーナル フリーWe develop a new statistical method to obtain the probability distributions of magnitude of the largest aftershock and the number of major events, using the negative binomial model by Okada and the modified Omori formula. In Gutenberg-Richter's law n (M) dM=a10-bM dM for M<Mm, we suppose that the a-value follows a gamma distribution, Γ(α, σ0), which is justified by the fact that a negative binomial law holds for the number of aftershocks. Here, Mm is the magnitude of main shock. The b-value is considered to be the same for all aftershock sequences. If we observe k aftershocks larger than a threshold, Mm-d, after the main shock, the a-value for the entire aftershock period distributes as Γ{α+k, σ0/(1+qσ0η(d))}. Here, q is portion of aftershocks occurring in the observation period, which is calculated with the modified Omori formula, and η(d)(10bd-1)/(b×In 10). On the basis of this formula, it is easy to derive analytically the distributions of the magnitude difference between main shock and the largest aftershock and the number of major aftershocks. The parameters have been determined from aftershock data of shallow earthquakes with 6.0 in magnitude or larger in inland and coastal area of Japan from 1926 through 1995. Applying the method at the hypothetical times of 0, 24 and 72 hours after the main shock, we see that the present method is more useful just after main shock occurrence and in the early stage of aftershock activity than the conventional probabilistic method.抄録全体を表示PDF形式でダウンロード (1277K) -
宗田 靖恵, 松澤 暢, 長谷川 昭2001 年 54 巻 3 号 p. 347-363
発行日: 2001/12/25
公開日: 2010/03/11
ジャーナル フリーWe estimated the seismic velocity structure of the crust and uppermost mantle beneath the northeastern Japan arc using receiver functions. Since the conventional methods for analyzing the receiver functions have several problems, we improved the methods at first. 1) Although the water level method is frequently used in stabilizing the receiver functions, this method sometimes distorts the results considerably. Moreover there are no objective criteria in selecting the most appropriate water level. In this paper, we adopt smoothing of spectra using Hanning window for the stabilization because its meaning in the frequency-domain FFT analysis is clearer than the water level method and it causes less distortion of the receiver functions. 2) Time-domain stacking of the receiver functions is very popular but this procedure needs many waveforms to obtain good S/N. Moreover, the result obtained from this procedure is not the least-squares estimation. We formulate a new frequencydomain stacking procedure that corresponds to the weighted least-squares method and can treat the data properly according to their S/N. 3) Usually, the structure estimated from the receiver-function analysis has poor uniqueness even if the minimum-roughness constraint is adopted. We have developed an inversion method in which the solution is constrained to the initial model structure. If we use a proper initial structure which satisfies the direct wave travel times, it is expected that the structure estimated from the inversion also satisfies the travel times to a considerable extent. We applied these methods to the seismograms recorded at broadband stations in the northeastern Japan arc in order to estimate the structure beneath the stations. The main results are as follows: 1) In the east of the northeastern Japan arc, the Conrad discontinuity is identified and its depth is consistent with the result of Zhao et al. (1990). 2) There exist low velocity (LV) zones in the lower crust beneath stations HSK and DIT; the LV zones are located just below the Conrad beneath HSK, and just above the Moho beneath DIT. 3) There exists a LV zone in the uppermost mantle beneath station KGJ. This result is consistent with Nakajima et al. (2001b) but our result indicates that the LV zone is as thin as around 5km. 4) Beneath station HOJ the velocity in the surface layer is extremely low.抄録全体を表示PDF形式でダウンロード (2297K) -
西坂 弘正, 篠原 雅尚, 佐藤 利典, 日野 亮太, 望月 公廣, 笠原 順三2001 年 54 巻 3 号 p. 365-379
発行日: 2001/12/25
公開日: 2010/03/11
ジャーナル フリーThe northeastern Japan Arc is a typical arc in the northwestern Pacific. In this area, many geophysical and geological studies have been conducted to resolve the formation of the Japan Sea and the northeastern Japan Arc. Therefore the northeastern Japan Arc is one of the most well investigated trench-arc systems in the world. However, the origin and evolution of the northeastern Japan Arc and the Japan Sea have not been revealed yet. Also, the data for detailed seismic structure in the boundary area are not enough. To understand the formation of the Arc and the Sea, we need data of the seismic structure of the whole arc-trench system from the trench to back-arc basin over the arc. In the autumn of 1997, seismic experiments were carried out in sea and land both on a profile from forearc to back-arc across the northeastern Japan Arc. We obtained a detailed seismic structure beneath the northern Yamato Basin and the eastern margin of the Japan Sea using twenty-six ocean bottom seismographs (OBSs), airguns and explosives as controlled sources on the profile in the Japan Sea. The profile in the Japan Sea is a part of the whole profile to obtain the seismic structure of the northeastern Japan arc-trench system. The Yamato Basin has a layer with a P-wave velocity of about 6km/s. The 6-km/s layer has a thickness of 3-4km. A layer with a P-wave velocity of 6.7-7.1km/s underlies the 6-km/s layer and has a thickness of approximately 10km. The entire crust is about 15-16km thick, and the Moho interface is approximately 18km deep below the sea surface in the Basin. The thickness of the 6-km/s layer and the depth of the Moho interface gradually increase with approaching to the northeastern Japan Arc. The velocities of the lower crust with 6.7-7.1km/s are diminished with increase of a thickness of the 6-km/s layer. The P-wave velocity in the uppermost mantle is 8.0km/s. In the northern Yamato Basin, the crust is twice thicker than usual oceanic crust and we can not confirm a high velocity layer in the lower crust, which were found in the southern Yamato Basin. Beneath some continental margins (volcanic margin), high velocity layers (>7km/s) interpreted as the result of a large igneous activity were found at the bottom of the crust. However, in the boundary region between the Yamato Basin and the northeastern Japan Arc, there is no seismic evidence of such a high velocity layer in a lower crust. Thus, the seismic velocity structure in this area is similar to those of non-volcanic margins rather than the velocity structure in volcanic margins.抄録全体を表示PDF形式でダウンロード (2607K) -
横浜市とその周辺地域における検討三浦 弘之, 翠川 三郎2001 年 54 巻 3 号 p. 381-395
発行日: 2001/12/25
公開日: 2010/03/11
ジャーナル フリーThe effects of 3-D deep underground structures in and around Yokohama City on the characteristics of rather long-period ground motion are examined. The detailed 3-D deep underground structure was modeled by interpreting the time delay between PS converted waves and the direct P waves (PS-P time) calculated from earthquake records. This PS arrival time was determined on the stacked receiver function waveforms. The results show that the basement depth varies between 2.5 to 4.0km. The propagation characteristics of rather long-period ground motions in the southern part of the Kanto basin were examined by analyzing records pertaining to the east off Izu Peninsula earthquake of May 3rd, 1998. The results show that Love wave packets traveling across the basin turn into the main contributors to the observed rather long-period ground motions. The amplitudes of the Love waves appear well correlated with the basement depth. The observed phase velocities for periods between 5 and 10s show good agreement with those of fundamental mode Love waves calculated for the estimated model. However, in cases where steep dipping of the basement is detected, a simple underground model beneath the sites cannot explain the amplitudes and phase velocities. This is caused by the effect of these irregularities of the deep underground structure on the waves propagation.抄録全体を表示PDF形式でダウンロード (3629K) -
佐鯉 央教, 小菅 正裕, 渡邉 和俊2001 年 54 巻 3 号 p. 397-414
発行日: 2001/12/25
公開日: 2010/03/11
ジャーナル フリーWe have investigated the waveform characteristics and source mechanisms of deep low-frequency earthquakes (DLFeqs) that occurred near the Osorezan volcano in the Shimokita Peninsula and near the Esan volcano in the Oshima Peninsula, northern Japan. The seismograms were collected by the temporal observation conducted for seven and six months in 1998 and 1999, respectively, using many broadband (with a flat velocity response up to 20 or 30s) and midband (up to 5s) seismometers. DLFeqs in both areas form clusters with focal depths ranging from the lower crust to the uppermost mantle. The most remarkable feature of seismograms is the dominance of low-frequency oscillation around 2Hz. The amplitude of source displacement spectra also shows a prominent peak at the same frequency and rapid decay with increasing frequency beyond the peak. Thus the source spectra of DLFeqs are different from the ordinary ones approximated by the omega-square model. From the broadband spectra we confirmed that the lower bound of significant seismic energy of DLFeqs is above 1Hz. A particle motion analysis exhibits the change in oscillation pattern with time in the major part of seismograms from well-excited S-wave to long-tailed S coda, suggesting the time variation of radiation characteristics of low-frequency waves from the source. The source type of DLFeqs inferred from the amplitude ratio of S- to P-waves is either single force or fault slip in low frequency range (1-5Hz), while the ratio in high frequency range (5-20Hz) shows good agreement with a type of tensile crack or single force. The waveform of initial part of P- and S-waves from one DLFeq can be relatively well explained by a moment tensor solution of a reverse faulting with little non-double-couple component. Though we could not constrain the source mechanism of DLFeqs uniquely from the results of above analyses, we propose the following model under the assumption of a source driven by magma. The pressurized magma in one reservoir extends the preexisting crack, producing high-frequency seismic radiation by a tensile crack. If the crack connects to the other reservoir, magma flow from one to the other excites low-frequency radiation of single force type, sometimes followed by reversed flow to balance the pressure between the reservoirs.抄録全体を表示PDF形式でダウンロード (2646K) -
大見 士朗, 和田 博夫, 伊藤 潔2001 年 54 巻 3 号 p. 415-420
発行日: 2001/12/25
公開日: 2010/03/11
ジャーナル フリーPDF形式でダウンロード (732K) -
鳥取県西部地震, 芸予地震の強震記録に基づく検討加藤 研一, 山添 正稔, 武村 雅之2001 年 54 巻 3 号 p. 421-429
発行日: 2001/12/25
公開日: 2010/03/11
ジャーナル フリーPDF形式でダウンロード (1224K)
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