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Volume 51, Issue 4
Displaying 1-9 of 9 articles from this issue
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Estimation of S-wave Profiles in Kawasaki and Yokohama CitiesHiroaki YAMANAKA, Hiroaki SATO, Katsuml KURITA, Kazuoh SEO1999 Volume 51 Issue 4 Pages 355-365
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSArray measurements of long-period microtremors were conducted in Kawasaki and Yokohama cities in the Kanto basin to know S-wave velocity profiles for estimating strong ground motion. We observed long-period microtremors at 8 sites by installing temporarily two arrays at each site. Each array consists of 7 vertical seismometers with station spacing of 0.2 to 3km. Phase velocities of microtremors were estimated from an frequency-wavenumber analysis. They show dispersive features in a period range from 0.5 to 5 seconds, suggesting propagation of Rayleigh wave. The phase velocity data were inverted to S-wave velocity profiles of thick sediments by an inversion based on a genetic algorithm. The S-wave velocity profiles are mainly approximated by four layers with S-wave velocities of 0.4-0.6km/s, 1.0km/s, 1.7km/s and 3km/s, respectively. It was found that the S-wave velocity of the 3rd layer is site-dependent. The depths to the interfaces of these layers in the deduced profiles are in agreement with results of seismic refraction and reflection explorations.View full abstractDownload PDF (1205K) -
Mitsuru YOSHIDA1999 Volume 51 Issue 4 Pages 367-378
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSLove wave propagation for a continental crustal model with a dent is simulated by the two-dimensional finite difference method. Snapshots of displacement clearly show that complex phase and amplitude distributions on the free surface, as well as near the boundaries of the Moho discontinuity, are produced by the interference between incident and scattered waves. Snapshots of scattered waves show that a part of the energy of the incident wave generates secondary waves due to the scattering at irregular Moho discontinuity. The energy of scattered waves is strongest at the edge of a dent in wave's outgoing direction. Defining the propagation from thinner to thicker crust as a downdip propagation and the reverse case as an updip propagation, scattering characteristics of Love waves at a dent indicate that short- and long-period SH waves are radiated in the downdip and updip propagations, respectively. The body waves, which make a constructive interference with surface waves, anomalously raise transmission coefficients of Love waves for short- and long-periods at the edges of a dent in wave's incoming and outgoing directions, respectively.View full abstractDownload PDF (4903K) -
Taku KOMATSUBARA, Kiyohide MIZUNO, Akira SANGAWA, Futoshi NANAYAMA, Hi ...1999 Volume 51 Issue 4 Pages 379-394
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Biwako Seigan active fault system (BSFS) extends for 55km along the western bank of Lake Biwa, Kinki district, southwest Japan. BSFS is one of the most active and longest Quaternary reverse fault systems in the Kinki district. A historical large earthquake (AD 1662 Kambun earthquake, M=7 1/4-7.6) occurred near this fault system. We excavated several trenches at two sites across the Aibano fault, one segment of the northern part of BSFS, in order to reveal the ages of paleoseismic events. On the exposure of the Hirokawa trench, which is near the north end of the Aibano fault, several paleoseismic events are identified. The last two events at the Hirokawa trench are dated after BC 2, 200 (probably BC 2, 200 to AD 800), and BC 7, 300 to BC 5, 600. The last event at the Aiba trench, that locates 5km south of the Hirokawa trench, is dated BC 1, 000 to AD 250 (probably BC 1, 800 to BC 1, 400). The ages of the last event at these two trenches are consistent, and they are thought to be a same event. The interval of the last two events of the Aibano fault is estimated to be 4, 600 to 7, 550 years (probably 4, 800 to 5, 900 years). Ages of these events are consistent with that estimated from morphotectonic survey of the Holocene river terraces. The historical records show the subsidence of the coastal area of Lake Biwa along the southern part of BSFS during the 1662 Kambun earthquake. These facts suggest that BSFS can be divided into two or more segments, and that the southern segment of BSFS might be one of the source faults of the 1662 earthquake, but the northern segment including the Aibano fault did not move during this earthquake. The Aibano fault comprises several fault sections accompanying broad flexure zones, thus the deformation zone of this fault has a width of 1 to 2km. In the last event, 3 to 5m vertical offset might have occurred in the whole deformation zone of the Aibano fault.View full abstractDownload PDF (4133K) -
Relationship between the Questionnaire and Measured Seismic Intensity in High Intensity RangeNaoto INOUE, Motohiko SHIBAYAMA, Kenji TSUJIE, Kunihiro RYOKI, Satoshi ...1999 Volume 51 Issue 4 Pages 395-407
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe seismic intensity of JMA (Japan Meteorological Agency) is widely recognized to be providing the important information regarding the damage and intensity of the seismic ground motion. Several questionnaire surveys have been made to detect the essential differences consequent to not only the attribute of the distance from the epicenter or the earthquake fault but also the characteristics of the underground structure. The questionnaire survey based on the method of OHTA et al. (1979) has been applied for major earthquakes in Japan and it has been substantiated the validity of the survey method. After the 1995 Hyogo-ken Nambu earthquake, some groups have carried out questionnaire survey based on this method. The determination of questionnaire seismic intensity as calculated using the formula by OHTA et al. (1979) seems to be available sufficiently in the range of seismic intensity from II to V in JMA scale. The validity of the seismic intensity over V in JMA scale, however, has not been verified because we have not experienced the applicable larger earthquake in Japan to the present. One of the purposes of this study is to confirm the validity of the conventional method by the questionnaire survey of seismic intensity. Site conditions such as geology, topography and so on were examined to compare more strictly both values of seismic intensity obtained from the questionnaire surveys and the in situ measurement. The data selected considering site conditions could give a mean value with the lower fluctuation. The absolute values of the mean questionnaire intensity, however, are somewhat lower than the measured intensity. An equation to transform the questionnaire seismic intensity into measured one is recurred. The new questionnaire seismic intensity obtained from this study is in good agreement with the measured value.View full abstractDownload PDF (4896K) -
Nobuyuki YAMADA, Hiroaki YAMANAKA, Katsumi KURITA, Kazuoh SEO1999 Volume 51 Issue 4 Pages 409-418
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe 1995 Hyogoken-Nanbu earthquake caused heavy damage in the Kobe-Hanshin area and Awaji Island, Japan. Explosion seismic observations were conducted in December, 1995 to understand the cause of the damage distribution in the Kobe-Hanshin area.
We investigate characteristics of later phases in observed explosion records. The observed horizontal particle motions suggest that the major later phase arrived after arrival of direct P-wave is caused by laterally reflection of direct P-wave at the basin-edge near the foot of the Rokko mountains. The three-dimensional simulation of the explosion seismic waves is conducted by a finite difference method using a realistic basin model. The laterally reflected waves can be identified in the synthetic seismograms. In particular, the amplitudes of the reflected waves are significant at the sites in Port Island in the Kobe city, because of steep basin-edge in the Higashinada-ward.View full abstractDownload PDF (4187K) -
An Analysis of the Data from the Dense Microseismic Networks Installed in the Western Nagano Prefecture RegionJunko TAKEDA, Yoshihisa IIO, Yoji KOBAYASHI, Kiyohiko YAMAMOTO, Haruo ...1999 Volume 51 Issue 4 Pages 419-430
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe relationship between seismicity and fluid existing in the crust has been discussed by several experiments since the middle of the 1960's. It is considered in these experiments that earthquakes occur at regions when and where pore pressure is high, since pore pressure has been thought to decrease the frictional strength. In this paper, the relationship between seismicity and fluid (degree of saturation) existing in the crust was investigated by analyzing the data sets of the seismic activity and tomography obtained by the dense networks installed in the Western Nagano prefecture. The distributions of degree of saturation and crack density were inferred from the three-dimensional velocity tomography obtained from the 1986 Joint Seismological Research in Western Nagano Prefecture, by using the theory of new self-consistent scheme (NSC) or differential scheme (DS). Comparing these distributions with the hypocentral distribution in the same region, we found that earthquakes occur at the regions where the degree of saturation is low, namely, at little saturated rocks. Further, it appears that the blocks of low degree of saturation are surrounded by more saturated blocks. This result suggests that earthquakes possibly occur when fluid begins to flow into cracks which are little saturated.View full abstractDownload PDF (5940K) -
Hiroe MIYAKE, Tomotaka IWATA, Kojiro IRIKURA1999 Volume 51 Issue 4 Pages 431-442
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn 1997, two magnitude 6 class earthquakes occurred in the northwestern part of Kagoshima prefecture in Japan. One of them occurred on March 26 (MJMA6.5) and the other on May 13 (MJMA6.3). We examined the source models of these two events using the observed seismograms by K-NET at four stations surrounding the source areas. We proposed an objective estimation method to obtain the parameters N and C which are needed for the empirical Green's function method by IRIKURA (1986). This method, we call source spectral ratio fitting method, gives estimates of seismic moment ratio between a large and a small event and their corner frequencies by fitting the observed source spectral ratio with the ratio of source spectra which obeys the ω-2 model. This method has an advantage of removing site effects in evaluating the parameters. The best source model of the March mainshock was estimated by comparing the observed waveforms with synthetics using empirical Green's function method. The size of the asperity is about 7km length in the E-W direction by 6km width in the dip direction. The rupture started at the eastern-bottom of the asperity (8.2km) and extended radially to the western-upper direction. We noticed that the source model with additional small asperity to the eastern extension of major asperity improved the waveform fitting of the east-side station. In case of the May event, the aftershock distribution is ‘L’ shaped, and the mainshock waveforms on the rock site are composed of two clear pulses. The hypocenter of the mainshock is located close to the bottom (7.7km deep) of the intersection of the ‘L’ shape. Therefore, we considered two fault planes, the N-S plane and the E-W plane. The seismic moments of the two pulses were estimated nearly the same. We tested which fault plane ruptured first by comparing the initial part of the observed seismograms with synthetics. The matching was fairly good only when the N-S plane ruptured first and the E-W plane did later. The best source model of the May mainshock consists of two asperities. The first asperity has a size of about 3km length by 4km width in the N-S plane, where the rupture propagated radially from the northern-bottom. The other asperity with almost the same size but trending E-W ruptured radially from the western-bottom about two seconds after the first rupture.View full abstractDownload PDF (1404K) -
Sou NISHIMURA, Masataka ANDO, Shin'ichi MIYAZAKI1999 Volume 51 Issue 4 Pages 443-456
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSInter-plate coupling on the convergent plate boundary along the Nankai Trough is analyzed on the basis of GPS measurement data of horizontal displacements in southwest Japan for a period between March 1996 and April 1998. During this period, five moderate-size earthquakes occurred in the study area: M6.6 event offshore Tanegashima in October 1996, two M6.6 events in Hyuganada in October and December 1996, and two M6.1 events in Kagoshima in March and May 1997. Co-seismic displacements associated with these earthquakes are first removed from the GPS displacements. Then, a back-slip model is applied to estimate slips on the coupling interface. High ratios of back-slip relative to plate motion (approximately 100%) are obtained along the eastern part of the Nankai Trough. On the contrary, offshore Kyushu, the ratios are negative, suggesting southeastward motion of the southern part of Kyushu relative to the Philippine Sea plate. We regard these negative values of the inter-plate coupling as a result of the post-seismic deformation associated with two Hyuganada earthquakes in 1996. The stress relaxation model of the asthenosphere is tested using a 3-D finite element method, and it is found that the effect of the relaxation is too small to explain the observed displacements during the post-seismic period. A feasible model adopted to explain the displacements is the after-slip, which possibly occurred on the focal region or its neighboring extension on the plate boundary. The post-seismic slips in Tanegashima and Hyuganada region thus obtained amount to 145 and 208mm, respectively. Generally, the after-slip can explain the observed post-seismic displacements in Kyushu. However, in the western part of Kyushu, the eastward displacement residuals still remain. To elucidate these displacements, different mechanisms such as a back-arc spreading of the Okinawa Trough, or an internal deformation of the continental plate, should be considered.View full abstractDownload PDF (1606K) -
Yoshiko YAMANAKA, Masayuki KIKUCHI1999 Volume 51 Issue 4 Pages 457-458
Published: March 26, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (3177K)
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