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Volume 43, Issue 3
Displaying 1-16 of 16 articles from this issue
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Masaki KANAO, Kiyoshi ITO1990 Volume 43 Issue 3 Pages 311-320
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSAttenuation property of lithosphere has been studied in various regions by analyzing decay rate of coda amplitude. In this paper, Qc values were determined for intra-plate earthquakes recorded by the telemetered network in the middle and northern Kinki district, southwest Japan. All events are shallower than 20km having magnitudes of 1.8-4.0. Qc was determined by fitting a back scattering model to band-pass filtered coda decay curves in five frequency bands from 2 to 32Hz, with 8 lapse times ranging 20 to 85 s. By fitting Qc to the power law, Qc=Qofn, average values of Qc and n in this district were obtained as Qo=74 and n=0.77. These are typical values in the active tectonic region such as island arcs. Although the Qc value depends on the lapse time used for the analysis, n is almost independent of the lapse time. The radius (r) of scattering area contributing coda wave is roughly estimated as r=Vst/2, where Vs is S-wave velocity and t is lapse time. Therefore, Qc values can be derived as a function of depth from the relationship between Qc and lapse time. Qc increases rapidly with increasing lapse time and becomes nearly constant at large lapse times. This shows that Qc increases rapidly in the shallow crust and becomes nearly constant in the upper mantle. The increasing of Qc with depth is larger in the northern part than in the middle part of Kinki district. This suggests that the depth dependence of Qc is related to the structure of the lower crust and the upper mantle.View full abstractDownload PDF (1699K) -
Masajiro IMOTO, Nobuo HURUKAWA, Yoshihiko OGATA1990 Volume 43 Issue 3 Pages 321-326
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThree dimensional spatial variations of Gutenberg Richter b-values are studied for the microearthquakes which occurred in the Kanto area, central Japan, during the period from January 1983 to December 1988. In order to derive precise variations of b-values, hypocenters of these earthquakes are relocated using station corrections for travel times. The introduction of a prior distribution of gentle spacial variations of b-value makes estimated b-values stable and reliable. Parameters of the prior distribution are chosen to minimize the Akaike's Bayesian Information Criterion (ABIC). As a result, it is found that the b-value is low in the subducting plates and high in the Eurasian plate. However, a high b-value appears at the lower plane of the double seismic zone in the Pacific plate.View full abstractDownload PDF (1050K) -
Yasuaki MURATA1990 Volume 43 Issue 3 Pages 327-339
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSA new method is proposed for the automatic and simultaneous estimation of Bouguer density and anomalies. This method solves and reduces the significant biases and estimation error of Bouguer density especially when the correlation between Bouguer anomalies and terrain is strong. The Bouguer density is estimated by fitting a smooth surface to observed Bouguer anomalies. This is carried out by an objective trade-off between the minimizations of the sum of the square of the residuals and a penalty to the surface roughness. Roughness of the surface is evaluated by integration of squares of the first and the second order derivatives. Specifically, the object function to be minimized is given as
where ρ is the unknown Bouguer density, f is a surface function estimating the Bouguer anomalies, s is a vector of unknown parameters for the surface function, Fi, i=1, ..., N are observed free-airanomalies, Hi, i=1, ..., N are coefficients for a Bouguer density, xi and yi are observed coordinates, Δ [1] f and Δ [2] f are the first-order and the second-order derivatives of f, and ω1 and ω2 are hyperparameters controlling the trade-off between the residuals and the roughness. Once the suitable trade-off parameters are given, Bouguer density ρ and parameter vector s can be estimated by a standard least square algorithm such as Householder decomposition. Determination of suitable trade-off parameters is done by an objective Bayesian procedure minimizing ABIC (Akaike Bayes Information Criterion) [AKAIKE (1980)] . Analysis of three data sets of gravitational observation demonstrates the utility of the ABIC minimization method. One data set covers a large area (50km×70km) over granitic rocks and metamorphic rocks in Abukuma, next data set covers a middle area (25km×20km) over andesite in Kirishima volcano area, and the other covers a small area (750m×850m) in Oya rock region. Other automatic methods, like the F-H relationship method, determine unreasonable Bouguer densities, while the ABIC minimization method estimates reasonable density values.View full abstractDownload PDF (2703K) -
Shozo MATSUMURA1990 Volume 43 Issue 3 Pages 341-357
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe seismic activity in the Kanto-Tokai districts was examined by introducing a new monitoring procedure based on composition of statistical parameters. The parameters selected were b-value, υ-value (the parameter representing an earthquake occurrence pattern), and averaged P- and T-axis directions. These parameters, all of which can be derived directly from seismic data, have specific characteristics, and are considered to represent the state of local crustal activity. The entire Kanto-Tokai area was partitioned into 57 small regions, and each parameter was calculated for every region from the period of March 1986 to October 1989, with use of a seismic database of the National Research Institute for Earth Science and Disaster Prevention. By setting appropriate threshold values to variations for each parameter, anomalous periods were sought. The final result is represented by a weighted summation of anomalous periods. Although each parameter may have its own particular statistical fluctuation, the synthesis of all the parameters should present a statistically reliable measure of anomalous seismic activity. We recognized several anomalous periods which corresponded well with occurrences of moderate sized earthquakes. The result suggests this monitoring may be used in application to earthquake prediction.View full abstractDownload PDF (2895K) -
Analysis of Coda Waves from Artificial Explosions and from a Natural Intermediate-depth EarthquakeYasuto KUWAHARA, Hisao ITO, Masanao SHINOHARA, Hitoshi KAWAKATSU1990 Volume 43 Issue 3 Pages 359-371
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSA temporary seismic observation with a small array was carried out in the Izu-Oshima volcano to detect the heterogeneities causing seismic wave scattering. During the observation, we recorded seismic waves generated by two dynamite sources and by natural intermediate-depth earthquake (M=4.4, depth=71km, epicentral distance=137km). The semblance technique has been used to determine the propagation directions and the apparent propagation speeds of the spatially coherent components in the coda waves. The analysis reveals that neither of the coda waves from two dynamite sources contain any coherent component across the array. This result for the dynamite sources indicates that the scatterers are uniformly distributed in the shallow part of Oshima volcano. The coda waves of the natural earthquake, on the other hand, contain a number of coherent components especially in P-wave coda, while a small number of those are in S-wave coda. These coherent components are continuously found in P-wave coda having an about 14-second duration. The remarkable feature of the coherent components is that those have almost the same apparent propagation speeds and directions. The coherent phases in P-wave coda are interpreted as channel waves propagating in the low velocity layer at the upper part of the subducting Philippine Sea plate.View full abstractDownload PDF (2213K) -
By Hokko Explosions (1988)Isao TORIUMI, Yoshihiro TAKEUCHI, Shintaro OHBA, Masanori HORIKE, Yuta ...1990 Volume 43 Issue 3 Pages 373-378
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSSeismic wave records at the surface of sedimentary plain are mainly composed of main phase and surface waves. The informations of input wave record in base rock and S wave velocity distribution to base rock are enough to consider the main phase. And moreover the informations about underground structure to bed rock of all sections of plain are necessary to discuss surface wave which travels from surrounding rock range to observation point. For this items of the Osaka plain, there were some crumbs of informations. In 1988, we tried two explosions at a manmade “Hokko” island in Osaka bay. This paper shows the results: five ground structures and a countour map of depth to base rock of the Osaka plain.View full abstractDownload PDF (1122K) -
Shigeki HORIUCHI, Sadato UEKI, Azusa NISHIZAWA, Dapeng ZHAO, Ryota HIN ...1990 Volume 43 Issue 3 Pages 379-387
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSVelocity structure beneath Bandai volcano, the southern part of the Tohoku District, is studied by the use of amplitude data of the P-waves from local earthquakes occurring around the volcano and by the use of the data obtained from a fan-shooting seismic experiment around the volcano using a large capacity marine airgun. Instead of estimating amplitude change of the P-wave in accordance with the difference in event size, focal mechanism, attenuation by Q, site effect beneath observation stations, and geometrical spreading, amplitude of the initial part of the P-wave is normalized by the maximum amplitude of its coda wave to estimate attenuation along the ray path for the initial part of the P-waves. By arranging normalized seismograms, observed at a station near Bandai volcano from local earthquakes around the volcano, by back azimuths measured from the station to epicenters of events, it is found that initial parts of the P-waves become small in case when ray paths, connecting events to the station, pass t rough a zone within about 2.5km from the summit of the volcano. Anomalous attenuation zone found by the fan-shooting seismic experiment is also coincident with the zone determined by the local earthquake data. Theoretical amplitudes of the diffracted P-waves are calculated, and the results indicate that the existence of a melting zone with a diameter of 5km just beneath Bandai volcano explains observations of both the amplitude and the travel time data.View full abstractDownload PDF (1798K) -
Part 2: Characteristics of Tsunami Propagating over the Pacific OceanFumihiko IMAMURA, Nobuo SHUTO, Chiaki GOTO1990 Volume 43 Issue 3 Pages 389-402
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe linear Boussinesq equation should be used in numerical simulations for distant tsunamis in the Pacific Ocean, because the dispersion effect is not negligible. Its difference equation which can not be expressed by an explicit scheme requires a long CPU time. One of the present authors has introduced a new technique, in which the first term of discretization error in the difference equation of the linear long wave theory was used and controlled to replace the physical dispersion term. This method is applied in the present study. The effect of ocean current on the tsunami propagation is confirmed to be negligible in wave direction and wave height. The 1960 Chilean tsunami is simulated. The Coriolis force has not only the effect on the propagation direction but also the dispersion effect, which is examined by comparing with the computed result without the Coriolis force. Effect of the sea bottom topography is examined in detail in terms of wave energy. About 40% of initial wave energy remains on the continental shelf in the neighborhood of Chile, and the rest is radiated to the Ocean. After scattered and trapped by islands and sea mounts, about 25% of the total energy arrives at Japan. The computed results shows a fairly good agreement with tide records after making correction of the effect of the water depth. For further discussions of the tsunami in shallow seas, simulations should be performed using the shallow-water theory and detailed topography.View full abstractDownload PDF (2768K) -
Azusa NISHIZAWA, Shoji TOMITA, Toshihiko KANAZAWA, Takaya IWASAKI, Hid ...1990 Volume 43 Issue 3 Pages 403-411
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe region off Fukushima Prefecture is characterized by high seismic activity. On Feb. 2, 1987, M6.4 and M6.7 earthquakes occurred there sequentially. Thereafter six earthquakes with M≥5 took place from Feb. to Apr., 1987 in this region. On June 10, we deployed five ocean bottom seismometers (OBS) to estimate focal depths accurately and to investigate the nature of the seismic activity. During the observation period (18 days) we obtained high quality seismic data from four of the OBSs, including an M5.7 earthquake and its aftershocks. We determined 289 hypocenter locations with accuracy, especially using by high quality S wave data.
Almost all the earthquakes occurred in the region where the water depths were less than 2km and the region was more than 100km landward from the trench axis. The hypocenter distribution in a profile perpendicular to the trench axis shows a clear dipping plane corresponding to the boundary between the landward Eurasian plate and the subducting Pacific plate. We also observed twoearthquakes which took place at the lower plane of the double seismic zone and confirmed that the lower plane seismic activity existed at about 110km landward from the trench axis.View full abstractDownload PDF (1738K) -
Nobuo HURUKAWA, Masajiro IMOTO1990 Volume 43 Issue 3 Pages 413-429
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe Philippine Sea plate subducting beneath the Japanese Islands is colliding with the Pacific plate at the depth of 50 to 80km beneath southwestern Ibaraki and central Chiba prefectures, the eastern Kanto district. Relocating earthquakes caused by this collision, we investigated focal mechanisms and a fine structure in the source region in detail.
Earthquakes observed by a network of National Research Institute for Earth Science and Disaster Prevention are relocated by the Joint Hypocenter Determination (JHD) method. The JHD method was extended by using a following constraint: A station correction is independent of both a distance and an azimuth from a center of a studied region to a station. This prior information makes results by the JHD method stable.
Almost all earthquakes in this region are thrust events of which nodal planes dip westward at a low angle. They occurred inside a boundary layer with a thickness of about 4km dipping westward at an angle of about 30°, that is a boundary of the subducted two plates. Stress state and a seismic velocity in the boundary layer imply an origin of the layer is a subducted oceanic crust in the Pacific plate. Its size is about 90km in the NS direction and 7-16km in the EW direction. It inclines westward at an angle of about 25° in the northern part, and inclines west-southwest at an angle of about 30° in the southern part. Although all events inside the boundary layer are thrust type of which nodal planes are parallel with this layer, many of them may have occurred along planes which are parallel to nodal planes dipping eastward at a high angle.
However, a magnitude 6.1 earthquake with a vertical fault plane of which east side subsided occurred inside the Pacific plate in Oct. 1985. This event implies coupling of the two plates above it is stronger than that of the surrounding region. Furthermore, a few earthquakes occurring at the western edge of the boundary layer show normal faulting with the NE-SW or ENE-WSW tension axes, which are supposed to be caused by stress concentration at the ends of the boundary layer mentioned above. Therefore these two kinds of exceptional events define the boundary between strong- and weak-coupling areas of the two plates very well.View full abstractDownload PDF (3354K) -
Tamao SATO1990 Volume 43 Issue 3 Pages 431-438
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe time-term method has been applied to Pn velocity in northeastern Japan. About 400 Pn arrivals with a distance range from 200 to 1300km were used in the analysis. A Pn velocity of 7.87±0.03km/s was determined from the classical time term method. A reference Pn velocity of 7.55±0.05km/s and a distance dependence of 0.30±0.05km/s per 1000km were obtained from the modified method which allows for the dependence of Pn velocity on distance. Station time-terms derived from the modified method are almost the same as those from the classical one, and also agree with the relative travel-time residuals for teleseismic events. The station time-terms were used to estimate crustal thicknesses across the array after they were corrected for a thick low-velocity sediment beneath the stations around Mt. Iwaki. The crust is estimated to be about 5km thinner near the coasts of both the Pacific Ocean and the Japan Sea than in the inland area.
When the time-term analysis was modified for both the distance dependence and azimuthal anisotropy, the result showed a 3.2 per cent Pn anisotropy with the maximum velocity directed N16° E. This direction is largely different from those of the maximum velocity both in the eastern margin of the Japan Sea and the north western Pacific basin. A characteristic dependence of apparent velocities on azimuth suggests a lateral change in Pn velocity beneath the coast of the Pacific Ocean. A more reliable estimate of Pn anisotropy in northeastern Japan will be possible if the lateral change in Pn velocity can be adequately taken into account in the anisotropic model.View full abstractDownload PDF (1664K) -
Yuichi SUGIYAMA1990 Volume 43 Issue 3 Pages 439-442
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
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Yasunori SUZUKI1990 Volume 43 Issue 3 Pages 443-446
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
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Hitoshi KAWAKATSU1990 Volume 43 Issue 3 Pages 447-450
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
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Akira SANGAWA, Hiromasa OHNISHI, Kiyoshi FUKE1990 Volume 43 Issue 3 Pages 451-452
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
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Shin'ya TSUKADA, Kin'ya NISHIGAMI, Norio HIRANO, Hiroo WADA1990 Volume 43 Issue 3 Pages 453-457
Published: September 24, 1990
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSDownload PDF (1275K)
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