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Volume 57, Issue 1
Displaying 1-8 of 8 articles from this issue
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Tsuyoshi WATANABE, Takao TABEI2004 Volume 57 Issue 1 Pages 1-10
Published: August 25, 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Ryukyu Arc is a NE-SW trending island arc that connects southwest Japan and Taiwan. The Philippine Sea plate subducts at the Ryukyu Trench, the southeastern boundary of the Ryukyu Arc. While the plate model predicts a rather high subduction rate (60-80mm/yr), plate coupling has been estimated so small from the earthquake data. Another factor characterizing tectonic features of the region is an active backarc opening at the Okinawa Trough that forms the northwestern boundary of the region. Japanese nationwide continuous GPS array has illustrated a trenchward motion of the Ryukyu Arc. However, it is difficult to quantify the effects of the subduction and the backarc opening because there is little deployment of the GPS network in the direction perpendicular to the strikes of those two boundaries. To model tectonic movement of the region, at first we divide the region into four crustal blocks based on horizontal GPS velocities and geological conditions. As the first approximation, plate coupling is neglected. Crustal velocities predicted from the block motions are in good agreement with the observed ones with an average discrepancy of about 3mm/yr. Next, we introduce moment tensor data of shallow earthquakes to calculate crustal strain rates in several segments and further estimate spatial variation of the plate coupling. The results show that the estimated plate coupling is smaller than 10% in most segments except 50% in the northernmost part. We interpret that the plate coupling at the Ryukyu Trench is so small and the trench can behave as a free boundary. Thus if a backarc opening occurs at the Okinawa Trough, the Ryukyu Arc can easily move trenchward without significant internal deformation. However, detailed mechanism of the backarc opening remains still unknown and is an important problem to be resolved in the future.View full abstractDownload PDF (1306K) -
Noriko KAMAYA, Akio KATSUMATA2004 Volume 57 Issue 1 Pages 11-28
Published: August 25, 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn recent years, improved seismic networks in Japan have detected low-frequency events near the Mohorovicic discontinuity, but away from volcanoes. These can be classified as two types: low-frequency earthquakes (LFE) and low-frequency tremors (LFT). Source regions for LFE are distributed in isolated locations throughout Japan, while those for LFT are distributed within a belt in western Japan. Although LFE immediately below volcanoes have been previously recognized, LFE away from volcanoes represent new observations. These LFE are found specifically both in the back-arc of the volcanic front in northeastern Japan, and to the north of the LFT belt in western Japan. LFE and LFT have long durations, suggesting the involvement of fluids in their generation. This study considers the source processes that trigger LFE and LFT, using the results of high pressure and temperature experiments on water-saturated oceanic basalt and peridotite as a model. Such materials are representative of those in and around the descending slab. In the case of LFT in western Japan, the model provides, as a possible trigger, the water released by the dehydration of chlorite and forming clinopyroxene in the basalt of the descending Philippine Sea Plate. On the other hand, in the case of LFE throughout Japan, not only water from the slab but also magma generated in the slab and mantle wedge and high temperature fluid derived from the magma are possible triggers. Because LFE have clear P-wave onsets, some sort of fracture is likely to be involved in the generation of LFE. LFE and LFT are expected to be indicators useful in earthquake prediction, but the relationships between large earthquakes or earthquake swarms and the occurrence of LFE and LFT are yet to be determined.View full abstractDownload PDF (2427K) -
Zhongyuan YANG, Hideji KAWAKAMI, Hamid R. HADDADI, Yaonan SUN2004 Volume 57 Issue 1 Pages 29-43
Published: August 25, 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe methods for extracting the relationships between seismic waveforms are often used to investigate ground wave propagation properties because of the complexity of actual ground motions. In this paper, the method of normalized input-output minimization (NIOM) is modified so that the predominant frequency of the modeled ground motions is similar to that of the observed ground motions and so that the band-pass filter can be used. This modified NIOM method is applied to the vertical and horizontal components of seismographs observed at the ground surface during a distant earthquake occurred in Tibet-India border region and recorded worldwide, and the average motion of soil particles is studied. We estimate the direction of wave propagation from that with the largest amplitude obtained by the NIOM method. We find that the estimated direction is similar to the great-circle direction. We also find that the average particle motion modeled by the modified NIOM method is similar to the theoretical particle motion of Rayleigh waves in a semi-infinite medium.View full abstractDownload PDF (1609K) -
Naoto INOUE, Yasuyuki TANAKA, Hiroyuki ITOH, Sachiko IWANO, Naoko KITA ...2004 Volume 57 Issue 1 Pages 45-54
Published: August 25, 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Horikawa-Oguraike and Kuzebashi seismic reflection and refraction profiles were obtained in the Kyoto Basin from 1998 to 1999. The gravity measurements were conducted at 50-300m station intervals along the seismic profiles in order to estimate the density of sediments in the basin. The N-S and E-W gravity profiles correspond to the Horikawa-Oguraike and the Kuzebashi seismic profiles, respectively. The Kyoto Basin was composed of basement rocks covered by unconsolidated sedimentary layers. The top surface of the basement of 2D two-layers models were constructed with the constraints of seismic profiles. The inferred density contrast between the basement and the overlaying sediments in the N-S two-layer model was 0.3g/cm3 and the difference between observed and calculated gravity anomalies was within 1.5m gal. The results of the density loggings and the distribution of the seismic velocities along the Horikawa-Oguraike seismic section indicate the density variation. The multi-layered N-S model was constructed based on the seismic profile and the borehole data. On the other hand, the density contrast between the sedimentary cover and the basement in the E-W two-layer model was 0.7g/cm3 and the difference between observed and calculated gravity anomalies was within 1mgal. The regional gravity trend inferred from this study tilts toward the northeast in the Kyoto Basin.View full abstractDownload PDF (1388K) -
Shoji SEKIGUCHI, Yoshihisa IIO, Shiro OHMI, Hisao ITO, Shigeki HORIUCH ...2004 Volume 57 Issue 1 Pages 55-61
Published: August 25, 2004
Released on J-STAGE: March 11, 2010
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Yukihiro SAKAGAWA, Koji UMEDA, Mototaka SUZUKI, Tatsuya KAJIWARA, Youh ...2004 Volume 57 Issue 1 Pages 63-67
Published: August 25, 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (976K) -
2004 Volume 57 Issue 1 Pages 69-70
Published: 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (74K) -
2004 Volume 57 Issue 1 Pages 71
Published: 2004
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (139K)
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