Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 54, Issue 4
Displaying 1-14 of 14 articles from this issue
  • Tsunami of the 1704 Iwadate, the 1793 Ajigasawa, and the 1804 Kisakata Earthquakes
    Yasuko HIWATASHI, Tamao SATO, Fumihiko IMAMURA
    2002 Volume 54 Issue 4 Pages 431-440
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Three earthquakes and accompanying tsunamis of 1704, 1793 and 1804 hit the coastal area of the Aomori-Akita districts in the northern part of Japan Sea, causing several hundreds of fatalities and serious damages to houses and ships. In this study, we simulate the generation and the propagation of these tsunamis by assuming several fault models, and determine a set of fault parameters for each earthquake that best fits the observed distributions of tsunami heights and crustal uplifts. The fault model previously proposed by Sato (1980) for the 1704 Iwadate earthquake generally explains the observed tsunami heights, but a similar model with the fault length 3km longer towards the north is more consistent with the reported crustal uplift at Henashi peninsula. For the 1793 Ajigasawa earthquake, a fault model lying off the coast of Tsugaru peninsula better explains the observed tsunami heights around Ajigasawa. The tsunami simulation also corroborates the observation that tsunami caused by the event might flush into the Jyusan Lake. Among several fault models tested for the 1804 Kisakata earthquake, a fault model located most offshore shows the best agreement with the observed distribution of tsunami heights. The tsunami simulations for the 1704 Iwadate and 1804 Kisakata earthquakes show that the tsunamis are largely amplified at Oga peninsula. The amplification is ascribed to an energy concentration by the reflection in the 1704 Iwadate tsunami and the edge wave trapped in the shallow sea in the 1804 Kisakata tsunami, respectively. Both are important mechanisms affecting the heights of tsunamis along the Japan Sea coast.
    Download PDF (3071K)
  • Tokutaro HATORI
    2002 Volume 54 Issue 4 Pages 441-448
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Many South American tsunamis have been recorded since 1562. Eight of them caused severe damage in Hawaii, Japan and the other Pacific regions. The 1960 Chilean tsunami (m=4.5) is the largest. In the present paper, the distribution of cumulative energy, the sum of tsunami mean hight squares, for each 200km segment from Colombia to Chile is investigated for the periods from 1900-1996 and 1562-1899. Distant tsunamis, such as the 1952 Kamchatka and 1964 Alaska tsunamis, arrived at this region were excluded from the analysis. For the total tsunami energy, ΣHH2, during the whole period, percentages of the received energy in the 1, 000km range were 30% on the South Chilean coast, 21% on the South Peru and Central Chilean coasts and 18% on the North Chilean coast. For the recent 97-years, the arrival energy on the South Chile region is twice larger than the mean rate of energy accumulation. On the contrary, the one in 600km segment in the North Chile region, Arica to Antofagasta, is remarkably small comparing with the expecte value. On the long-term tsunami hazard, it is necessary to take precautions against the tsunami related to large earthquakes off North Chile.
    Download PDF (1171K)
  • Shozo MATSUMURA
    2002 Volume 54 Issue 4 Pages 449-463
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Recently, the microearthquake seismicity observed around the Tokai region has shown several anomalous changes. It is considered that the conditions in this area have been steadily progressing toward the occurrence of the next Tokai earthquake, due to locked subduction of the Philippine Sea plate. The clearest change was recognized in August 1999, when the seismicity rate inside the subducted slab in and around the locked zone decreased. This quiescence continued for more than one year. After October 2000, the seismicity turned active. On the other hand, the seismicity rate inside the crust around the same zone was also found to have been gradually decreasing since the end of 1996. Two moderate-sized earthquakes, the central Shizuoka prefecture earthquakes happened concurrently with these anomalous phenomena, in October 1996 (M4.3), and in April 2001 (M5.1). It seems probable that these events influenced the state of the locked subduction, and caused the seismicity rate change. Spatial pattern analyses on these anomalies revealed that the decrease in the seismicity rate does not always extend over the entire locked zone, but forms a patchwork pattern. As a result, the zone is partitioned into two parts: one is quiescent, and the other is rather activated. Such results resemble previous cases reported by Wyss's group, who suggested that a characteristic seismicity rate change precedes the occurrence of a main shock. Typically this involves a quiescent zone appearing to surround a non-quiescent zone. Wyss's group regard the non-quiescent part as an asperity. Based on their reports, I examined the relationship between the duration of the quiescence (Tq: year) and the area of the asperity (Anq: kmkm2), and derived an equation of Tq=0.27√Anq. This relationship seems reasonable, as it shows the proportionality of Tq to the size of the asperity estimated in linear dimension. If this is applicable to the present case in the Tokai region, we can obtain an evaluation of about 7 years of Tq for the current anomalous stage.
    Download PDF (3480K)
  • Toshihiro IGARASHI, Toru MATSUZAWA, Akira HASEGAWA
    2002 Volume 54 Issue 4 Pages 465-474
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    In order to interpret various seismic activities in relation to the subduction process of the Pacific plate beneath the northeastern Japan arc, information about the spatio-temporal stress change in the slab is indispensable. In this study, we estimated the stress change by analyzing the activity and focal mechanisms of the intermediate-depth earthquakes in detail. Predominant focal mechanism is of down-dip compression type in the upper plane, while down-dip extension type dominates in the lower plane of the double-planed deep seismic zone. If the overall down-dip compressional stress in the plate increases, it is expected that the upper plane seismicity becomes active while the lower plane activity is suppressed. Thus, we can qualitatively estimate the stress change in the plate from the seismicity. For this purpose, we analyzed Z-value which indicated the spatio-temporal change in the seismicity, and R-value which indicated the ratio of earthquake occurrence frequencies between upper plane and lower plane. Both analyses revealed that the relative frequency of earthquakes in the upper plarle to that in the lower plane increased after large earthquakes that occurred on the plate boundary. Z-value distribution indicates that the upper plane seismicity had been active in the westward extension of the after-slip region of the 1994 Far-off Sanriku earthquake during the period of about 1.5 years after the event. Moreover, normal fault type earthquakes near the slab surface disappeared in the same period. These phenomena suggest that down-dip compressional stress in the plate increased by the large earthquake on the plate boundary.
    Download PDF (2630K)
  • Ryoichi NAKAMURA, Tomiichi UETAKE
    2002 Volume 54 Issue 4 Pages 475-488
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    The three-dimensional attenuation structure of the Japanese islands and site amplification factors are obtained by simultaneous inversion of strong motion records recorded by many acceleration seismometers, i. e. JMA87-type seismometers and the Kyoshin-Net (K-NET) seismometers, almost uniformly distributed in Japan.
    The result shows that the Pacific slab is high-Q and that the frequency dependence of Q values is expressed as f0.75.
    We classified all stations into 5 groups by ground conditions mainly on the basis of S-wave logging data and obtained the site amplification factor for each group. The result shows that the frequency band where large amplification factors are obtained by inversion well agrees with the predominant frequency calculated from S-wave velocity data.
    Download PDF (4923K)
  • Eiji KURASHIMO, Masako TOKUNAGA, Naoshi HIRATA, Takaya IWASAKI, Shuich ...
    2002 Volume 54 Issue 4 Pages 489-505
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Large earthquakes occur along the Nankai trough, SW Japan, where the Philippine Sea plate is subducting beneath southwestern Japan arc. To understand tectonics related to the occurrence of these large earthquakes, we need to elucidate crustal and upper mantle structures of the subducting Philippine Sea plate and the overlying southwestern Japan arc. In the summer of 1999, we conducted a highly dense onshore-offshore integrated seismic experiment in the eastern part of Shikoku Island and the adjacent Nankai trough, SW Japan. Controlled seismic signals both from land explosives and air-gun sources were recorded by the land seismic stations. We obtained high signal-to-noise ratio data along the entire length of the land profile.
    Analyzing these explosive and air-gun data recorded by the land seismic stations, we could obtain a detailed geometry of the subducting Philippine Sea plate and the crustal and upper mantle structure beneath eastern Shikoku Island. The signals from the air-gun source are especially useful to reveal a structure of an ocean-continental transition zone, which is not obtained using only land explosive data. The uppermost crust beneath eastern Shikoku Island is covered with a surface layer with velocities of 4.0-5.0km/s. The surface velocity shows a remarkable lateral change at the geological boundary between granitic rocks and an accretionary belt. The velocity of the uppermost crust is obtained as 5.7km/s. The island arc Moho is about 33km deep beneath northeastern Shikoku Island and the crustal thickness seems to thin toward the north. Beneath the southern edge of the land profile, the top of the subducting Philippine Sea plate is located at a depth of about 18km with a dip angle of approximately 12 degrees. The subduction angle steepens beneath this point. The top of the subducting plate can be traced to a depth of about 30km, almost parallel to the Wadati-Benioff seismic plane. This subduction angle is shallower than that beneath the Kii peninsula. Hypocenter distribution associated with the underthrusting of the Philippine Sea plate beneath eastern Shikoku Island is located beneath the subducting oceanic Moho. The seismicity underlies a contact zone between the nreanic and island-arc crusts.
    Download PDF (7421K)
  • Aki ITO, Ryota HINO, Minoru NISHINO, Hiromi FUJIMOTO, Seiichi MIURA, S ...
    2002 Volume 54 Issue 4 Pages 507-520
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    In August 1999, we made a marine-land cooperative seismic experiment in the forearc region of the northeastern Japan to clarify seismic structure in the source region of the 1978 Miyagi-oki earthquake. At land seismic stations, we observed prominent later arrivals interpreted as the wide angle reflections from deeper part of the crust of the overriding NE Japan arc and also from the subducting Pacific plate. To locate these reflectors precisely, we inverted first arrival time data obtained at both land and ocean bottom stations for P wave velocity (Vp) distribution. Using the thus estimated Vp structure model, we mapped the seismic reflectors corresponding to the observed reflection arrivals. Among these reflectors, the Moho of the NE Japan arc was found to shallow towards the east, from 34km beneath the coastal line to about 20km at 70km off the shoreline. The estimated Moho depth beneath the coast is larger by about 5km than those in the previously presented crustal models and there is a strong intra-crustal reflector at Moho depths in these previously presented models. We also found two landward dipping parallel reflectors in the upper mantle, at a depth range of about 30-60km. The location of the shallow one coincides with the focal depth distribution of the interplate seismicity in this area, suggesting that this is the plate boundary, the rupture plane of the 1978 Miyagi-oki interplate earthquake. The deeper one, located less than 10 km deeper than the plate boundary, should be the Moho of the subducting oceanic crust.
    Download PDF (4324K)
  • [in Japanese]
    2002 Volume 54 Issue 4 Pages 521
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Download PDF (255K)
  • Takeshi SAGIYA, Takuya NISHIMURA, Yuki HATANAKA, Eiichi FUKUYAMA, L. E ...
    2002 Volume 54 Issue 4 Pages 523-534
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    The 2000 Western Tottori Earthquake occurred on October 6, 2000, in the border of Tottori-Shimane prefectures. Japanese nationwide continuous GPS array recorded coseismic as well as postseismic crustal movements due to the earthquake. The maximum coseismic displacement observed was about 17cm. The coseismic deformation pattern clearly demonstrates a left-lateral strike slip source mechanism, which is consistent with seismic data analysis results. Leveling surveys around the focal region revealed up to 15cm vertical displacement near the source fault. GPS sites northeast of the source region were displaced up to 2cm to the northeast during 3 months after the earthquake, while a GPS site on the other side moved to the south by 2cm. Crustal deformation data are inverted to estimate two types of static fault models. One is a single rectangular fault model which can reproduce the coseismic displacement vectors observed by GPS. The other fault model, whose geometry is derived from precise re-determination of aftershocks, provides detailed information on slip distribution. We need to supplement subfaults in the shallower (depth<4km) parts to the original fault model in order to explain leveling change data, which implies systematic bias in the hypocenter depth or a full relaxation of stress by the main shock. Estimated moment magnitude were 6.6 and 6.7 for the two models, consistent with waveform inversion analysis.
    Download PDF (4021K)
  • Crustal Deformation Associated with the Western Tottori Prefecture Earthquake in 2000
    Hiroshi YARAI, Hiroyuki NAKAGAWA, Mikio TOBITA, Makoto MURAKAMI, Satos ...
    2002 Volume 54 Issue 4 Pages 535-546
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    The coseismic displacement field on the Earth's surface associated with the Western Tottori Prefecture Earthquake (October 6, 2000) was mapped by spaceborne C-band SAR (Synthetic Aperture Radar) interferometry. We obtained the interferogram from two SAR scenes before and after the earthquake (September 17 and October 11, 2000, respectively) acquired by RADARSAT. We successfully detected not only the coseismic displacement field associated with the main shock on Oct. 6 but also the one associated with the sub-event on Oct. 8. C-band SAR interferometry is effective even on the flat or moderate terrain covered with vegetation in Japan.
    We confirmed that the observed displacement field is consistent with the calculated field by the fault model that is constructed based on geodetic and seismic data. Complex fringe pattern associate with local deformation can be seen in Yumigahama Peninsula sand bar. Remarkable decorrelations in the interferogram may indicate occurrence of subsurface liquefaction along the coastal region. These findings strongly suggested the potential of spaceborne C-band SAR interferometry for seismic hazard monitoring in Japan.
    Download PDF (6112K)
  • Keiichi UETA, Katsuyoshi MIYAKOSHI, Daiei INOUE
    2002 Volume 54 Issue 4 Pages 547-556
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Associated with the western Tottori earthquake of October 6, 2000 (MJMA 7.3), the concrete lining of a headrace tunnel 200m below the surface in the epicentral area was faulted at two locations. In the eastern fracture zone, a NW-trending fault offsets the concrete lining 2cm left-laterally. Detailed total station mapping across the approximately 7-m-wide fracture zone reveals 10 to 20cm of left-lateral warping of the side walls of the tunnel. At the other location about 365m to the west, warping of the tunnel's side walls reveals left-lateral offset of approximately 2cm across the 4-m-wide fracture zone. It may be inferred that strands of the major faults associated with the 2000 western Tottori earthquake ruptured the tunnel 10 to 20cm left-laterally. Hence it appears that the total slip on the fault planes reduced from 1-2m at depth to 10-20cm near the ground surface.
    Download PDF (3606K)
  • Daiei INOUE, Katsuyoshi MIYAKOSHI, Keiichi UETA, Akiko MIYAWAKI, Kazuk ...
    2002 Volume 54 Issue 4 Pages 557-573
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    In order to evaluate the fault activity associated with the 2000 Tottori-ken Seibu earthquake, we carried out aerial photograph interpretations of fault landforms, geological investigation and trenching study in the epicentral area.
    As the results of the aerial photograph interpretation, we found some northwest-southeast trending lineaments with sinistral offsets of valleys and ridges in the epicentral area. Above lineaments are sub-parallel and the total length of the lineaments is about 23km. The fracture zones with fault gouge and horizontal striations on sharp fault planes were found along the lineaments, the fact also suggests that the lineaments coincide with the active faults. According to the outcrop exposure of the fault and trench excavation at Kusumi, we found the evidence of the recent surface-ruptured event that had occurred sometime between 770-1, 260 A.D. Hence there is possibility to estimate the magnitude 7 class earthquake before the 2000 Tottori-ken Seibu earthquake by the lineament length of about 23km.
    Download PDF (7888K)
  • Earthquake by Use af the Station Corrections as a Function of Hypocenter Coordinates
    Shiro OHMI
    2002 Volume 54 Issue 4 Pages 575-580
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Download PDF (1094K)
  • Hiroshi KATAO, Koji YOSHII
    2002 Volume 54 Issue 4 Pages 581-585
    Published: April 25, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    Download PDF (1191K)
feedback
Top