All issues
Volume 51, Issue 3
Displaying 1-7 of 7 articles from this issue
- |<
- <
- 1
- >
- >|
-
Norihito UMINO, Toru MATSUZAWA, Shuichiro HORI, Ayako NAKAMURA, Akira ...1998 Volume 51 Issue 3 Pages 253-264
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSTwo earthquakes with magnitudes 5.9 and 5.7 occurred beneath the Onikobe geothermal area, northeastern Japan, on 11 August 1996. A detailed study of the earthquakes and their aftershocks revealed that the M5.9 earthquake occurred in the region between the Sanzugawa caldera and the Onikobe caldera perhaps along a geological fault. The bi-lateral fault slip motion was stopped at the caldera walls in both ends. The M5.7 earthquake with right-lateral strike slip type focal mechanism took place along the Onikobe caldera rim or a geological fault close to the caldera rim. On 13 August 1996, M4.9 earthquake occurred on the southwestern extension of the M5.7 fault and had a focal mechanism of strike slip type with some reverse fault components. The fault slip of the M4.9 earthquake did not extend into the Mukaimachi caldera. Recent earthquakes with magnitudes about 5 have occurred in this geothermal area almost every ten years. M4.9 earthquake occurred on the southwestern edge of the Onikobe caldera on 5 July 1976, having a focal mechanism of reverse fault-type. On 28 March 1985, a left-lateral strike slip earthquake with magnitude 5.3 took place also along the Onikobe caldera rim. The lengths of earthquake faults estimated from aftershock distributions are at most 10km and seem to be consistent with characteristic lengths of geological heterogeneity, i. e. lengths of geological faults and diameters of calderas in this area. Seismic tomography study shows that low S wave velocity areas are located inside the calderas. Relatively large earthquakes and their aftershocks occurred only within high S wave velocity areas. It seems reasonable to suppose that temperature inside the calderas is too high to generate earthquakes. Thermal structure is one of major factors that govern seismic activities in the crust.View full abstractDownload PDF (2849K) -
Mitsuru ONODERA, Shigeki HORIUCHI, Akira HASEGAWA1998 Volume 51 Issue 3 Pages 265-279
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe developed a new tomography method to estimate 3-D Vp/Vs structure, which solves observation equations composed of unknown parameters about the Vp/Vs structure and origin times of events. The method is not affected very much by the estimation error of earthquake hypocenters, which is serious for a detailed estimation of velocity structure by seismic tomography using arrival time data of local earthquakes. We applied the method to arrival time data obtained by a temporary observation of aftershocks of the 1996 Onikobe earthquake sequence, in order to estimate the seismic velocity structure in and around the focal area of the sequence. Arrival time data of events obtained by the seismic network of Tohoku University around the area are also used to improve the resolution in the wider area. It is found that the method is very effective and that there exist low-velocity areas corresponding to calderas and a high Vp/Vs area beneath Kurikoma volcano. The fault planes of the main shock with M5.9 and the largest aftershock with M5.7 are located in the high-velocity area between the Sanzugawa and Onikobe calderas with low velocities and along the edge of the Onikobe caldera, respectively.View full abstractDownload PDF (11980K) -
Toshikazu YOSHIOKA, Yuichi SUGIYAMA, Takashi HOSOYA, Ken'ichiro HENMI, ...1998 Volume 51 Issue 3 Pages 281-289
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Yanagase fault is a left-lateral strike-slip active fault trending N-S to NNW-SSE in northern Shiga Prefecture and Fukui Prefecture. We excavated seven trenches, namely N, Ma, Mb, Mc, Md, Ms and S trenches, near the Tsubakizaka Pass along the fault in order to know the timing of recent faulting events. An almost vertical fault plane is clearly observed on the walls of the Md, Ms and S trenches. The fault cuts the AT tephra layer (about 25ka) and is covered by the K-Ah tephra layer (about 7ka) in the S trench. According to the radiocarbon dates of samples from the trench walls, the timing of the latest faulting of the northern part of the fault is estimated to be 7000 to 7200 years ago. The previous trench study on this fault at about 4km south of this trench site, indicated that the fault ruptured about 600 years ago. This difference in the timing of the last faulting event may mean that the Yanagase fault is devided into at least two segments.View full abstractDownload PDF (1195K) -
Toshimi SATOH, Hiroshi KAWASE, Shin'ichi MATSUSHIMA1998 Volume 51 Issue 3 Pages 291-318
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe examine the differences of empirical site characteristics among S-wave, P-wave, coda, and microtremors and interpret them theoretically. Data used in this study are seismograms of 43 earthquakes and microtremor records observed at 20 strong motion stations within a 20km×30km region in Sendai, Japan. We calculate horizontal-to-vertical spectral ratios (H/V) for microtremors. Earthquake records are divided into P-wave, early P coda, S-wave, early S coda, and late S coda windows. For each window of seismograms we calculate H/V, horizontal spectral ratios (H/H), and vertical spectral ratios (V/V) with respect to a bedrock station where site effects are sufficiently small. The H/V for early P coda rapidly deviates from H/V for P-wave and quickly converges to H/V for microtremors. The H/V for early S coda gradually converges to H/V for microtremors in the frequency range lower than 3HZ as lapse time is longer. At soft sediment stations H/V for early S coda within 15sec after the S-wave arrival becomes identical with H/V for late S coda. On the contrary, at one rock station and several hard soil stations H/V for early S coda agree with H/V for S-wave. The H/H and V/V for early S coda are larger than those for S-wave at soft sediment stations. We also show that H/H for S-wave are different from H/V for microtremors and H/V for S-wave. If we use shallow S-wave velocity structures only, theoretical S-wave site response (H/H) and H/V for the fundamental mode of Rayleigh wave cannot explain H/V for microtremors except for some higher peak frequencies. When we use whole sediment structures on top of the seismological bedrock, theoretical H/V for the fundamental mode of Rayleigh wave agrees with H/V for microtremors and theoretical H/V for an obliquely incident SV-wave agrees well with H/V for S-wave. Theoretical S-wave site response based on the whole sediment structure agrees well with H/H for S-wave, but does not with H/V for microtremors. Our results mean that so-called Nakamura's technique based on H/V for microtremors is not valid in Sendai as a method to estimate empirical S-wave site responses. Only on the condition that the peak amplitudes of H/V for microtremors be greater than 3 and the peak frequencies be lower than 1Hz, the frequencies of the maximum peak in H/V for either microtremors or S-wave are similar to those in H/H for S-wave. Concerning coda we conclude that Rayleigh wave contamination in coda is significant in the frequency range lower than 3Hz at soft sediment stations.View full abstractDownload PDF (3035K) -
Analyses of the Records from the Earthquakes of East Yamanashi PrefectureTomiichi UETAKE, Kazuyoshi KUDO1998 Volume 51 Issue 3 Pages 319-333
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSuccessive later arrivals forming long duration of strong motion have been a great interest in the field of engineering seismology, since the 1985 Michoacan, Mexico earthquake. We analyze the strong motion data set obtained at Ashigara valley from two earthquakes (MJ 5.3 and MJ 4.5) of east Yamanashi prefecture. The data set consists of ground motion records obtained at seven rock sites and twenty-one sediment sites. Focusing on frequency range between 0.2 and 0.5Hz, direct S-wave and two significant later arrivals are discussed. The one of the later arrivals predominates in horizontal component and the other is apparent in vertical component. These later arrivals are identified at different time and their dominant frequencies are not the same. These later arrivals are significant at sediment sites in southern part of the valley, but they are unclear at the northern part of the valley and rock sites. To clarify the nature of those later arrivals is our major concern in this study. In order to determine the apparent propagation velocity and azimuth of the later arrivals, a semblance analysis is applied to small array data at the southern part of the valley. The later arrivals have relatively low velocity, and the propagation azimuths does not necessarily coincide with the propagation azimuth of direct S-wave. The propagation velocities match with the ones of Love and Rayleigh waves. The results of seismic refraction surveys suggest that the depth to the basement is not uniform in central area of the valley and the depth at southern part is deeper than that at northern part of the valley. The later arrivals are supposed to be basin-induced surface wave generated by oblique basement structure at central area of the valley. The location does not coincide with the surface geological boundary or basin edge (Kannawa fault) at the north of Ashigara Valley.View full abstractDownload PDF (3417K) -
Katsuhiko ISHIBASHI1998 Volume 51 Issue 3 Pages 335-338
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (488K) -
Takao KAGAWA, Kojiro IRIKURA, Masayuki TAKEMURA1998 Volume 51 Issue 3 Pages 339-354
Published: December 25, 1998
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThis paper introduces the present feasibility of strong ground motion prediction technique on the basis of fault rupture propagation. Investigating the several applications of the technique, we point out its advantage and disadvantage against empirical techniques that have been used for engineering purposes.
We consider that the simulation technique itself is applicable enough for real engineering problems. In the case of having a detail source model and sedimentary structure model at a site, we can reproduce even strong ground motion waveform which can be comparable with observed one. However, we do not have enough information of source model for a future earthquake and sedimentary structure model for a target site. Predicting the ground motion caused by future earthquake, the shortage or uncertainty of the simulation models causes small confidence on the result of strong ground motion prediction.
Strong ground motion generated by real earthquake depends strongly on the fault rupture process and site condition. The existing empirical techniques produce mean characteristics of the past ground motion. It is not a source-specific nor site-specific ground motion. We have to estimate strong ground motion using the technique which can explain the aspects of actual site-specific ground motion. To achieve an advantage of the technique against empirical based techniques, it is important to investigate processes of fault rupture and sedimentary structures to reduce the uncertainty of predictions.
We hope that this paper will motivate further study for seismic disaster mitigation through the strong ground motion prediction.View full abstractDownload PDF (2363K)
- |<
- <
- 1
- >
- >|