T. Ikawa

Chiba University, Chiba-shi, Chiba-ken, Japan

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Publications (14)6.82 Total impact

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    ABSTRACT: The deep structure and evolution of the crust and upper mantle in the Pan-African terrain of the Lützow-Holm Complex (LHC), Dronning Maud Land, East Antarctica were investigated using Deep Seismic Surveys (DSS). The DSS were conducted on the continental ice sheet of the LHC in the austral summers of 2000 and 2002 by the Structure and Evolution of the East Antarctic Lithosphere (SEAL) program. Processing of the DSS has produced enhanced reflection images of the crust–mantle boundary and of the internal crustal structure. Laminated layering around the crust–mantle boundary is well imaged using coherency enhancement processing after NMO corrections applied to far offset data. The repetitive crust–mantle transition zone imaged by the SEAL-2002 profile suggests the presence of compression stress in a NE–SW orientation during the Pan-African, which occurred during the last stage of formation of a broad mobile belt between the East and West Gondwana super-terrains. Successive break-up processes of the super-continent in mid-Mesozoic could account for the formation of the stretched reflection structure above the Moho discontinuity beneath the LHC as imaged on in the SEAL-2000 profile.
    Tectonophysics 01/2011; 508(1):73-84. · 2.68 Impact Factor
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    ABSTRACT: Deep structure and evolution of the crust and upper mantle viewed from East Antarctic Shield have sufficient significance involving continental growth in Earth's history. In this paper, we demonstrate lithospheric structure of the early-Paleozoic crust of the L¨¹tzow-Holm Complex (LHC), Eastern Dronning Maud Land, East Antarctica. LHC locates among the paleo-collision zones between East- and West- Gondwana super- continent. The "Structure and Evolution of the East Antarctic Lithosphere (SEAL)" project has been carried out since 1990' in a framework of the Japanese Antarctic Research Expedition (JARE). Several geophysical studies including deep seismic refraction / wide-angle reflections have been conducted at the LHC. In the austral summer in 2000, and 2002, deep seismic surveys were conducted on continental ice sheet in northern part of the Mizuho Plateau of the LHC. In both surveys, more than 170 plant-type 2 Hz geophones were planted on the plateau 190 km in length of transect lines. A total of 8,300kg dynamite charge at the fourteen sites on the Plateau gave information involving deep structure of a continental margin of East Antarctica. Wide-angle travel-time analyses revealed the crust-mantle boundary takes values ranging from 38-42 km along the profiles, with velocities of the upper crust, the middle crust, the lower crust and the uppermost mantle, about 6.2, 6.4, 6.5 and 8.0 km/s, respectively. Deepening of the crust-mantle boundary less than 5 km in depth was identified in the profile of SEAL-2000, which almost perpendicular to the coastal line. The velocities in surface bedrock layer in the SEAL-2002 profile, in contrast, have variations in 5.9-6.2 km/s, which corresponds to the metamorphic grade of the surface geology from amphibolite to granulite facies. Additionally, clear reflection phases from the crust-mantle boundary, together with some inner crustal reflections were identified on the same record sections. Laminated layered structure around the crust-mantle boundary was clarified by the coherency enhancement processing after NMO correction applied to very far offset data. The CMP stacking was not so effective due to the following reasons, A. The interval of receiver points is too sparce (1.0km) B. Shot point interval is also very sparce (30km) C. PmP reflection appears only on the very far offset data (more than 50km ) However, it was considered very promising for reflection data processing associated with refraction method, if shorter interval of receiver points, say less than 200m, is adopted in the future. Relatively complicated crustal structure characterized by the reflection section in SEAL-2000 and 2002 indicates the influence of the compression stress in NE-SW direction during the Pan-African; which means the last stage of continent-continent collision between East- and West-Gondwana super-terrains.
    AGU Fall Meeting Abstracts. 12/2006;
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    ABSTRACT: The Median Tectonic Line (MTL) is the most significant inland fault in SW, Japan, which extends more than 1000 km and divides SW Japan into the inner and outer zones. The western part of the MTL shows late Quaternary right-lateral strike-slip movements at several mm/y. Geometry of seismogenic source fault is important for more precise estimation of strong ground motions. According to the shallow (less than 1 km in depth) seismic reflection profile by Yoshikawa et al. (1992), the MTL shows northward dipping at 30 degrees. However, the geometry of its deeper extension is poorly understood. To reveal deeper geometry of fault surface, shallow to deep seismic reflection profiling was performed in August, 2006, across the Izumi Mountains south of Osaka plain. A 20-km long seismic line was deployed perpendicular to the MTL and major geologic boundaries. 10-Hz vertical geophones, connected by a digital telemetry cables, were deployed at a 40 m spacing. To obtain a high-resolution seismic image of the shallow part of MTL, a 10-m spacing receiver array was formed using off-line recorders for 5 km interval. The seismic source was vibroseis trucks. Three shooting patterns were employed: a 10-m spacing shooting by single truck for the toe part, a 80-m spacing shooting by four vibroseis trucks for a whole seismic line and 100 stationary vibroseis sweeps at six sites. The obtained shot gathers by high-energy shots commonly show the reflectors at 4 and 6 seconds (Two-way travel time). Theses reflectors are almost horizontal. The reflection from 4 s. (TWT) is interpreted form the base of seismogenic zone. The high-energy shot gathers at the middle of the seismic line clearly demonstrate the reflectors at 1.5, 2-3, 4 and 6 sec. (Two-way travel time). The shallowest reflection is from the probable fault surface of the MTL and shows northward gentle dipping. The reflectors from 2-3 s (TWT) are interpreted as northward dipping Sambagawa Metramorphic belt. It is highly probable that the deeper extension of MTL shows northward gentle dipping same as in Sikoku (Ito et al., 1994).
    AGU Fall Meeting Abstracts. 12/2006;
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    ABSTRACT: Seismic surveys of the crust and the upper mantle structure have been conducted under the special project for earthquake disaster mitigation in urban areas (DAIDAITOKU). In 2004 deep seismic profilings were carried out in the Kinki district southwest Japan. In the area great earthquakes have been repeatedly occurred every 100 years on the Pacific Ocean side due to the subduction of the Philippine Sea plate (PSP). In addition, active faults are densely developed on shore of Japanese Islands, where large historical earthquakes with M7 or greater attacked large cities, such as Kobe, Kyoto and Osaka. The surveys aim at determining of source faults, propagation paths and site responses for big earthquakes. More specifically, purposes of the surveys are to determine the accurate positions and geometry of source faults, subducting plates and mega-thrust faults, crustal structure, seismogenic zone, sedimentary basins, and 3D velocity structures. These information leads to more realistic 3D estimation of strong motion for large earthquakes. In this study, a seismic refraction and wide-angle-reflection survey was carried out along the Shingu-Maiduru line, which crosses Honshu Island from the Pacific side to the Japan Sea side nearly coincident with the direction of the subduction direction of PSP. In the profile, 13 shots (100-700kg) of dynamite and 3 multivisweeps (a few hundreds of sweeps) with 4 vibroseis trucks were used as sources of refraction and wide-angle reflection surveys. More than 2000 seismometer stations were set at intervals of 50-100m all along the line. Clear first arrivals and reflections from PSP and from reflectors in the crust were obtained at almost all the stations. In particular, waves from dynamite were well recorded at all stations. As for the stacked data of multisweeps, waves can be recorded in the offset range of 30-40km from the source. The multivibration method is particularly useful for the surveys in the urban areas, where dynamite cannot be used as a source. The reflections from PSP are very clear beneath Kii Peninsula and are still visible under the northern Kinki district at depth of about 50-70km, where no earthquakes occur. The reflectors seem to indicate aseismic slab in the northern Kinki district. A clear parallel reflectors associated with PSP are seen beneath Kii Peninsula. The lower reflector seems to coincident with the earthquake distribution, which has been considered as the plate boundary. The upper reflector is determined to be about 8-10km shallower than the lower one. Low frequency earthquakes and/or tremors occur at 30-40km deep in the upper reflector, which is less distinct at the low frequency events. The layer between the two reflectors is thought to be the oceanic crust subducting in the mantle. In the inland area, many clear reflections were obtained at depth of about 15 and 25km. These reflectors decline towards north, which seem to relate with the large active faults.
    AGU Fall Meeting Abstracts. 01/2005;
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    ABSTRACT: Large devastating earthquakes sometimes occur on a mega-thrust source fault which underlies the Tokyo metropolitan region. The source faults are located on the upper boundary of the subducting the Philippine Sea plate (PSP), which was so far estimated mainly from seismicity distribution. To get better prediction of the strong ground motion, we need to precisely characterize the source fault of those large earthquakes. A depth and geometry of the PSP is a key for characterization. We deployed four controlled source seismic lines in the Kanto area from 2002 to 2003: The 150-km long Boso line in 2002, the 80-km Sagami line in 2003, the 80-km Tokyo Bay line in 2003, and the 165-km Eastern part of the Kanto Mountain line in 2003. We, for the first time, directly identify the source fault using deep seismic reflection profiling images to be the upper surface of the PSP, which is found to be much shallower than previous indirect estimation based on seismicity distribution by 10 to 20 km. This geometry serves as a new constraint for studies of Kanto seismotectonics and seismic imaging using earthquakes such as high resolution 3D tomography and receiver function analysis. In the seismic profiles, we also found that highly reflective zones correspond to regions of silent slip plus areas outside the asperity zones of the 1923 Kanto earthquake. We propose that an asperity on a plate boundary can be detected by an evaluation of its reflectivity and represents possible regions of rupture for future earthquakes. Together with the obtained velocity structure, our result provides the essential control for the research on crustal deformation, modeling of source fault and strong ground motion estimation.
    AGU Fall Meeting Abstracts. 01/2004;
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    ABSTRACT: The location and geometry of the source fault, and crustal velocity structure, provide the basic information for more precise estimation of strong ground motions with devastative earthquakes. The deep seismic profiling around Metropolitan Tokyo (Kanto area) began from 2002 under the project named `Regional Characterization of the Crust in Metropolitan Areas for Prediction of Strong Ground Motion' as five year's project. Deep seismic profiling was performed along the Sagami (Sagami 2003) and Tokyo Bay (Tokyo Bay 2003), to obtain an image of the source fault of the Kanto earthquake of 1923 (M7.9), upper surface of the Philippine Sea plate, and deeper extension of inland active faults. In Sagami 2003, seismic reflection data were acquired along a 75-km-long seismic line from the flank of the Hakone volcano to Tokyo Bay through the coast of Sagami Bay. The seismic source was four vibroseis trucks and air guns (1500 cu. inch). The seismic signals were recorded by geophones (10 Hz) on land along the coast with 20 33-km-long spread. The seismic data was processed by standard CMP-reflection method. The obtained seismic section portrays the east dipping reflectors beneath Odawara at depth ca. 4 km to Kamakura at depth ca. 13 km for 40-km-distance forming a narrow (< 1 km) concentrated zone of reflectors. The location and geometry of reflectors are almost coincidence with the source fault model (model II) proposed by MatsuOura et al. (1980) for the Kanto earthquake using a inverse method from geodetic data. Thus, it is interpreted that the source fault of the earthquake is in the narrow zone of the concentrated reflectors. The deeper extension of the Kozu-Matsuda Fault, showing the one of the highest slip rates (3 mm/y: vertical component) among active faults in Japan and was assessed high seismic risk, merges to the east dipping reflectors at ca. 6.5 km in depth beneath the Oiso hills. In the Tokyo Bay 2003, seismic reflection data were acquired along a 71-km-long seismic line from the Miura Peninsula to Tokyo Bay. Same seismic sources were used as in Sagami 2003. Beneath the Tokyo Bay seismic signals were recorded by ocean bottom cables with hydrophones (OBC). The obtained shot gathers show the deepest reflections from 5.5 sec (TWT).
    AGU Fall Meeting Abstracts. 11/2003; -1:01.
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    ABSTRACT: The Kuril Arc has been colliding against the Northeast Japan Arc around the Hidaka mountain range, Hokkaido, Japan. Furthermore the Pacific plate is subducting north-northwestward beneath the Hidaka Collision Zone (HCZ). Recent seismic reflection experiments in this area have revealed that the delamination-wedge structure is formed by Arc-Arc Collision beneath the HCZ (Ito et.al.,1998,2000,Tsumura,1999). However the relationship between the delaminated lower portion of the lower crust and the subducting Pacific plate has remained unclear. In order to reveal the detailed 3-dimensional structure of the HCZ, super-deep seismic reflection experiments were conducted in the southern part of the HCZ, Samani-cho, Hokkido, in September, 2000. Two seismic lines were deployed: Line1 is parallel to the subducting trend of the Pacific plate (NW-SE), and Line2 is nearly parallel to the trend of the Arc-Arc Collision (NE-SW). Line1 is almost perpendicular to Line2. The length of Line1 and Line2 is about 16km and 8km, respectively. Five vibrators were operated, obtaining seismic data of the both lines simultaneously. Here we report the results from 2D seismic profiles. The main features recognized from this survey can be summarized as follows: For Line1: 1) At shallower portion, southeast-dipping reflections are truncating horizontal reflections, which are seen at 3 sec two-way travel time (TWT) in the northwestern half of the survey line. 2) A northwest-dipping strong reflection at 14 sec TWT descends down from the southeastern end of the survey line to the center, whereas gently southeastern-dipping reflection is seen at 14 sec TWT in the northwestern half. 3) Intermittent events are found at 7 sec. , 9 sec. , 12 sec. , and 16 sec TWT. For Line2: 4) Above 5 sec TWT, horizontal reflections are predominant but complicated. 5) There are gently southwest-dipping reflections at 8 to 9 sec TWT, nearly horizontal reflections at 13 sec to 16 sec TWT, but they are discontinuous and unclear. The northwest-dipping reflector at 14 sec seen in Line1 probably corresponds to the upper boundary of the Pacific plate, which is suggested by the study of seismic activity in this area and shows the subducting trend of the Pacific plate. The existence of the reflector derived from the subducting Pacific plate indicates that seismic waves generated by the vibrators passed through the delaminated lower portion of the lower crust. This provides us good prospects that the further improvement of data processing must reveal the relationship between the delaminated lower portion of the lower crust and the subducting Pacific plate.
    01/2001;
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    ABSTRACT: Photorespiration rates under air-equilibrated conditions (0.04% CO2 and 21% O-2) were measured in Chlamydomonas reinhardtii wild-type 2137, a phosphoglycolate-phosphatase-deficient (pgg1) mutant and a suppressor double mutant (7FR2N) derived from the pgp1 mutant. In both cells grown under 5% CO2 and adapted air for 24 h in the suppressor double mutant, the maximal rate of photorespiration (phosphoglycolate synthesis) was only about half of that in either the wild type or the pgp1 mutant (18-7F) cells. In the progeny, the reduced rate of photorespiration was accompanied by increased photosynthetic affinity for inorganic carbon and the capacity for growth under air whether accompanied by the pgp1 background or not. Tetrad analyses suggested that these three characteristics all resulted from a nuclear single-gene mutation at a site unlinked to the pgp1 mutation. The decrease in photorespiration was, however, not due to an increase in the CO2/O-2 relative specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase of 7FR2N or of any other suppressor double mutants tested, The relationship between the decrease in the rate of photorespiration and the CO2-concentrating mechanism is discussed.
    Plant and Cell Physiology 01/1999; 40(8):792-799. · 4.13 Impact Factor
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    ABSTRACT: The 1995 Kobe (Hyogo-ken Nanbu) earthquake (Mw 6.9) surface rupture appeared along the Nojima fault on the northwest coast of Awaji Island. The aftershock epicentres in northern Awaji Island are not aligned with the surface faulting. suggesting a complicated active fault structure. To reveal the structure of the seismogenic fault and its associated active faults, a perpendicular 41.6-km-long seismic reflection survey was undertaken across northern Awaji Island. The Trans-Awaji seismic reflection profile reveals the fault geometry beneath the area. Awaji Island was uplifted by movement on the Kariya fault along its east coast, which produced an asymmetry in Neogene basin that is shallower in the western part than in the eastern part. The faults beneath Awaji Island show a ‘pop-up’ or a ‘positive flower structure’ and the basement is bounded by two active faults, the Nojima along the west coast and the Kariya along the east coast. Both are high-angle, reverse faults with a right-lateral, strike-slip movement. The cluster of aftershocks in the northern part of Awaji Island has a Y-shaped hypocentral distribution. The Nojima and Kariya faults thus appear to connect in the middle of the upper crust at about 7 km depth, forming a seismogenic master fault. Three major seismic events have been reported on the active faults in northern Awaji Island: a prehistoric event dated 2000 yr BP, the 1596 Keicho-Fushimi earthquake, and the 1995 Kobe earthquake. Based on the structural relationship of the active faults that we observed in this study, we suggest that these events occurred on the proposed master fault at mid-crustal depth.
    Tectonophysics. 01/1998;
  • K. Suzuki, T. Ikawa
    Plant and Cell Physiology. 26(6):1003-1010.
  • K. Suzuki, T. Ikawa
    Plant and Cell Physiology. 25(3):367-375.
  • K. Suzuki, T. Ikawa
    Plant and Cell Physiology. 25(3):377-384.
  • K. Suzuki, T. Ikawa
    Japanese Journal of Phycology. 41(1):19-28.
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