Researcher Database

Kazunori Yoshizawa
Faculty of Science Earth and Planetary Sciences Earth and Planetary Dynamics
Associate Professor

Researcher Profile and Settings

Affiliation

  • Faculty of Science Earth and Planetary Sciences Earth and Planetary Dynamics

Job Title

  • Associate Professor

Degree

  • Ph.D.(The Australian National University (Australia))
  • M.Sc.(Hiroshima University)

J-Global ID

Research Interests

  • asthenosphere   lithosphere   anisotropy   tomography   surface waves   地球内部構造   地震学   Seismic Tomography   Earth Structure   seismology   Physics of Earth's Interior   

Research Areas

  • Natural sciences / Solid earth science / seismology

Academic & Professional Experience

  • 2016/04 - Today NIED Visiting Researcher
  • 2007/05 - Today Hokkaido University Faculty of Science Associate Professor
  • 2018/10 - 2018/10 Tohoku University Graduate School of Science Part-time Lecturer
  • 2015/04 - 2016/03 The University of Tokyo Earthquake Research Institute Visiting Associate Professor
  • 2015/08 - 2015/08 Kobe University Graduate School of Science Part-time Lecturer
  • 2012/04 - 2013/03 The University of Tokyo Earthquake Research Institute Visiting Associate Professor
  • 2011/06 - 2011/07 The Australian National University Research School of Earth Sciences Visiting Fellow
  • 2008/02 - 2010/02 Columbia University Lamont-Doherty Earth Observatory Visiting Scientist
  • 2007/04 - 2007/04 Hokkaido University Faculty of Science Assistant Professor
  • 2006/04 - 2007/03 Hokkaido University Faculty of Science Instructor
  • 2002/04 - 2006/03 Hokkaido University Graduate School of Science Instructor
  • 1998/04 - 1998/12 JSPS Research Fellow (DC1)
  • [Other Visiting Positions]
  • (以下,客員・非常勤等)

Education

  • 1999/03 - 2002/03  Australian National University  Research School of Earth Sciences (Ph.D. course)
  • 1996/04 - 1998/03  Hiroshima University  Graduate School of Science  Department of Earth and Planetary Systems Science (Master course)
  • 1993/04 - 1996/03  Hiroshima University  Faculty of Science  Department of Earth and Planetary Systems Science (Skipped)

Association Memberships

  • Society of Exploration Geophysicists   Japan Geosciences Union   Seismological Society of America   American Geophysical Union   Seismological Society of Japan   

Research Activities

Published Papers

  • Masaki Yoshida, Saneatsu Saito, Kazunori Yoshizawa
    Tectonophysics 787 228476 - 228476 2020/07 [Refereed][Not invited]
  • Geophysical Journal International Oxford University Press ({OUP}) 2019/03 [Refereed][Not invited]
  • Isse T, Kawakatsu H, Yoshizawa K, Takeo A, Shiobara H, Sugioka H, Ito A, Suetsugu D, Reymond D
    Earth and Planetary Science Letters 510 116 - 130 2019 [Refereed][Not invited]
  • Hamada K, Yoshizawa K
    GEOPHYSICAL JOURNAL INTERNATIONAL 212 (2) 795 - 797 0956-540X 2018/02 [Refereed][Not invited]
  • B. L. N. Kennett, K. Yoshizawa, T. Furumura
    TECTONOPHYSICS 717 193 - 213 0040-1951 2017/10 [Refereed][Not invited]
     
    Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5-3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a 'mid-lithospheric discontinuity' can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity. (C) 2017 Elsevier B.V. All rights reserved.
  • Yuki Nakashima, Kosuke Heki, Akiko Takeo, Mokhamad N. Cahyadi, Arif Aditiya, Kazunori Yoshizawa
    EARTH AND PLANETARY SCIENCE LETTERS 434 112 - 116 0012-821X 2016/01 [Refereed][Not invited]
     
    Acoustic waves from volcanic eruptions are often observed as infrasound in near fields. Part of them propagate upward and disturb the ionosphere, and can be observed with Total Electron Content (TEC) data from Global Navigation Satellite System (GNSS) receivers. Here we report TEC variations after the 13 February 2014 Plinian eruption of the Kelud volcano, East Java, Indonesia, observed with regional GNSS networks. Significant disturbances in TEC were detected with six GNSS satellites, and wavelet analysis showed that harmonic oscillations started at similar to 16:25 UT and continued for similar to 2.5 h. The amplitude spectrum of the TEC time series showed peaks at 3.7 mHz, 4.8 mHz and 6.8 mHz. Long-wavelength standing waves with a wide range of wavelength trapped in the lower atmosphere are excited by the Plinian eruption. Amplitude spectra of the ground motion recorded by seismometers, however, had frequency components at discrete wave-periods. The condition for the resonant oscillations between the atmosphere and the solid Earth is satisfied only at these discrete wave-period and horizontal wavelength pairs, therefore efficient energy transfer from the atmospheric standing waves to the solid Earth Rayleigh waves occurred at discrete periods and resulted in the harmonic ground motion. (C) 2015 Elsevier B.V. All rights reserved.
  • K. Hamada, K. Yoshizawa
    GEOPHYSICAL JOURNAL INTERNATIONAL 202 (3) 1463 - 1482 0956-540X 2015/09 [Refereed][Not invited]
     
    A new method of fully nonlinear waveform fitting to measure interstation phase speeds and amplitude ratios is developed and applied to USArray. The Neighbourhood Algorithm is used as a global optimizer, which efficiently searches for model parameters that fit two observed waveforms on a common great-circle path by modulating the phase and amplitude terms of the fundamental-mode surface waves. We introduce the reliability parameter that represents how well the waveforms at two stations can be fitted in a time-frequency domain, which is used as a data selection criterion. The method is applied to observed waveforms of USArray for seismic events in the period from 2007 to 2010 with moment magnitude greater than 6.0. We collect a large number of phase speed data (about 75 000 for Rayleigh and 20 000 for Love) and amplitude ratio data (about 15 000 for Rayleigh waves) in a period range from 30 to 130 s. The majority of the interstation distances of measured dispersion data is less than 1000 km, which is much shorter than the typical average path-length of the conventional single-station measurements for source-receiver pairs. The phase speed models for Rayleigh and Love waves show good correlations on large scales with the recent tomographic maps derived from different approaches for phase speed mapping; for example, significant slow anomalies in volcanic regions in the western Unites States and fast anomalies in the cratonic region. Local-scale phase speed anomalies corresponding to the major tectonic features in the western United States, such as Snake River Plains, Basin and Range, Colorado Plateau and Rio Grande Rift have also been identified clearly in the phase speed models. The short-path information derived from our interstation measurements helps to increase the achievable horizontal resolution. We have also performed joint inversions for phase speed maps using the measured phase and amplitude ratio data of vertical component Rayleigh waves. These maps exhibit better recovery of phase speed perturbations, particularly where the strong lateral velocity gradient exists in which the effects of elastic focussing can be significant; that is, the Yellowstone hotspot, Snake River Plains, and Rio Grande Rift. The enhanced resolution of the phase speed models derived from the interstation phase and amplitude measurements will be of use for the better seismological constraint on the lithospheric structure, in combination with dense broad-band seismic arrays.
  • K. Yoshizawa, B. L. N. Kennett
    GEOPHYSICAL RESEARCH LETTERS 42 (10) 3839 - 3846 0094-8276 2015/05 [Refereed][Not invited]
     
    A new 3-D S wave speed model for the Australian region derived from multimode surface waves allows us to examine the nature of the lithosphere-asthenosphere transition (LAT) and its relation to radial anisotropy. In eastern Phanerozoic Australia the estimated depths of the LAT tie well with those from receiver functions. However, in the Archean and Proterozoic lithosphere in western and central Australia, the LAT derived from the surface wave model is generally much deeper than the discontinuities recognized from receiver functions and shows a smooth transition. There is significant radial anisotropy (SH>SV) in the upper lithosphere as well as in the LAT and the underlying asthenosphere. Strong anisotropy in the asthenosphere reflects the effects of present shear flow in the mantle beneath the continent. The lateral variation of lithospheric anisotropy correlates well with the suture zones between cratonic blocks, representing frozen anisotropy associated with the ancient tectonics of Australia.
  • K. Yoshizawa
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 235 33 - 48 0031-9201 2014/10 [Refereed][Not invited]
     
    A new radially anisotropic shear wave speed model for the Australasian region is constructed from multi-mode phase dispersion of Love and Rayleigh waves. An automated waveform fitting technique based on a global optimization with the Neighbourhood Algorithm allows the exploitation of large numbers of three-component broad-band seismograms to extract path-specific dispersion curves covering the entire continent. A 3-D shear wave model is constructed including radial anisotropy from a set of multi-mode phase speed maps for both Love and Rayleigh waves. These maps are derived from an iterative inversion scheme incorporating the effects of ray-path bending due to lateral heterogeneity, as well as the finite frequency of the surface waves for each mode. The new S wave speed model exhibits major tectonic features of this region that are in good agreement with earlier shear wave models derived primarily from Rayleigh waves. The lateral variations of depth and thickness of the lithosphere-asthenosphere transition (LAT) are estimated from the isotropic (Voigt average) S wave speed model and its vertical gradient, which reveals correlations between the lateral variations of the LAT and radial anisotropy. The thickness of the LAT is very large beneath the Archean cratons in western Australia, whereas that in south Australia is thinner. The radial anisotropy model shows faster SH wave speed than SV beneath eastern Australia and the Coral Sea at the lithospheric depth. The faster SH anomaly in the lithosphere is also seen in the suture zone between the three cratonic blocks of Australia. One of the most conspicuous features of fast SH anisotropy is found in the asthenosphere beneath the central Australia, suggesting anisotropy induced by shear flow in the asthenosphere beneath the fast drifting Australian continent. (C) 2014 Elsevier B.V. All rights reserved.
  • Variability of megathrust earthquakes in the world revealed by the Tohoku-oki megathrust earthquake on 11 March, 2011
    Koyama J, Tsuzuki M, Yomogida K, Yoshizawa K
    Geophy Bull Hokkaido Univ 76 129 - 146 2013/03 [Not refereed][Not invited]
     
    (in Japanese with English abstract)
  • B. L. N. Kennett, A. Fichtner, S. Fishwick, K. Yoshizawa
    GEOPHYSICAL JOURNAL INTERNATIONAL 192 (2) 871 - 887 0956-540X 2013/02 [Refereed][Not invited]
     
    The mantle component of the Australian Seismological Reference Model (AuSREM) has been constructed from Australian-specific sources, primarily exploiting the wealth of seismic sources at regional distances around Australia recorded at portable and permanent stations on the continent. AuSREM is designed to bring together the existing information on Australia, from both body wave and surface wave studies and provide a synthesis in the form of a 3-D model that can provide the basis for future refinement. The model is grid based with a 0.5 degrees sampling in latitude and longitude, and is designed to be fully interpolable, so that properties can be extracted at any point. For the upper mantle the primary source of information comes from seismic surface wave tomography, supplemented by analysis of body wave arrivals and regional tomography which provide useful constraints on the relation between P- and S-wave speeds in the mantle lithosphere. A representative model has been developed to capture the features of mantle structure drawing on a range of studies. The mantle structure is represented by grid values at 25 km intervals in depth from 75 to 300 km. Shallower structure is linked to the AuSREM crust through the recent Moho depth model of Kennett et al., which exploits all available sources of seismological information. Below 300 km depth and in the surrounding area AuSREM is linked to the S40RTS model of Ritsema et al.
  • Taciser Bakirci, Kazunori Yoshizawa, Mithat Firat Ozer
    GEOPHYSICAL JOURNAL INTERNATIONAL 190 (2) 1058 - 1076 0956-540X 2012/08 [Refereed][Not invited]
     
    A 3-D upper-mantle structure beneath Turkey is investigated using phase speeds of fundamental-mode Rayleigh waves employing a conventional two-station method with high-density seismic networks in Turkey. We analyse 289 seismic events with moment magnitude 5.5 and greater, and with focal depth shallower than 100 km between 2006 and 2008. Waveform data are derived from 164 three-component broad-band seismic stations operated by two national seismic networks. At first, Rayleigh-wave phase speed maps are obtained from the inversion of two-station phase speeds using about 10003000 paths, depending on the period of Rayleigh waves. The three-dimensional S-wave model is then obtained in the depth range from 40 to 180 km using the phase speed maps in the period range from 25 to 120 s. Our model reveals the fast anomalies in the north of Cyprus associated with the subducted portion of the African oceanic lithosphere from the Cyprus trench. We identify a vertical discontinuity of the fast anomaly associated with the Cyprus slab starting at 6080 km depth which may represent a minor tear of the Cyprus slab. We observed that the western part of the Cyprus slab is getting closer to the edge of the Hellenic slab beneath the Isparta Angle (IA) and Antalya Basin. Our model also indicates a slow wave speed anomaly beneath the IA and Antalya Basin probably due to hot materials of asthenosphere rising from a tear of the subducted African oceanic lithosphere; that is, a slab tear between the Cyprus and the Hellenic subductions. In the eastern part of Turkey, a widespread slow anomaly appears in the model that corresponds to the Eastern Anatolian Accretionary Complex (EAAC). Our model shows a fast anomaly beneath the EAAC that can be interpreted as the detached portion of the subducted Arabian lithosphere.
  • Junji Koyama, Kazunori Yoshizawa, Kiyoshi Yomogida, Motohiro Tsuzuki
    EARTH PLANETS AND SPACE 64 (12) 1189 - 1198 1343-8832 2012 [Refereed][Not invited]
     
    The seismicity of the Pacific coast of Tohoku, Japan, has been investigated in detail and characterized into regional seismic segments. The 2011 megathrust earthquake of M-w 9.0 on 11 March ruptured almost all of the segments in that area, causing devastating tsunamis. The prime factor that had not been recognized before is the double segmentation along the Japan trench: The apparent absence of earthquakes in the trench-ward segments as opposed to the Japan Island-ward segments that have repeated smaller earthquakes. We term this pattern of seismic activity 'along-dip double segmentation (ADDS)'. The 2011 Tohoku megathrust earthquake is typical of a class of great earthquakes different from that of the 1960 Chile earthquake, in which a young and buoyant plate is subducting rapidly under the continental plate. In the 1960 Chile case, the seismic activity is characterized by 'along-strike single segmentation (ASSS)', where there is weak seismic activity before the main event all over the plate interface of the subduction zone. We study the greatest earthquakes around the world and find that there is a variety of megathrust earthquakes characterized by ASSS to ADDS, where the 2004 Sumatra-Andaman, the 1960 Chile, the 1964 Alaska and the 2011 Tohoku-oki earthquakes are typical end-members.
  • Andreas Fichtner, Stewart Fishwick, Kazunori Yoshizawa, Brian L. N. Kennett
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 190 44 - 50 0031-9201 2012/01 [Refereed][Not invited]
     
    Advances in seismic tomography lead to increasingly detailed models of the Earth that are often represented on irregular and resolution-adaptive grids. To take full advantage of such models, their assessment must progress beyond a purely visual analysis, and tools must become available for their quantitative comparison. We present a method for the spectral analysis and comparison of multi-scale tomographic models. The method is applicable to irregular grids on the sphere, and is more efficient that filters based on spherical-harmonic expansions or convolution integrals. The combination of a spherical spline representation of tomographic information with Abel-Poisson scaling enables the construction of targetted spatial filters by solving a nonlinear inverse problem for appropriate weighting coefficients. This can be readily achieved with a simulated annealing approach for the limited number of weights. Once suitable filters have been generated they can be employed to address issues such as the patterns of small-scale heterogeneity, transitional structures and comparison of independent models from a region. We illustrate our method in a series of applications where we use different bandpass filters to detect differences in the distribution of small-scale heterogeneity beneath central and eastern Europe, and to compare several recent tomographic models of the Australian region. (C) 2011 Elsevier B.V. All rights reserved.
  • Kiyoshi Yomogida, Kazunori Yoshizawa, Junji Koyama, Motohiro Tsuzuki
    EARTH PLANETS AND SPACE 63 (7) 697 - 701 1343-8832 2011 [Refereed][Not invited]
     
    We present some singular characteristics of the 2011 off the Pacific coast of Tohoku Earthquake in comparison with other megathrust earthquakes, such as the 1960 Chilean and the 2004 Sumatra-Andaman earthquakes. In addition to the conventional along-strike segmentation, along-dip segmentation of the fault area or subduction zone is an important feature for the Tohoku subduction zone, as indicated by the difference in background seismicity: virtually no seismicity in shallow segments but active with large events repeating in deep segments. The interaction between along-dip segments (deep and shallow segments) led to the great 2011 Tohoku earthquake. The along-dip segmentation results in plane or two-dimensional rupture propagation on a coseismic fault. Significant along-strike variability is also important for the 2011 Tohoku earthquake, with segments of both weak (e. g., slow or tsunami earthquakes) and strong plate couplings located adjacent to each other. In contrast, every segment appears to be with strong plate coupling for other megathrust earthquakes. One exception is the 1964 Alaska earthquake that shares a certain degree of common characteristics with the 2011 Tohoku earthquake: two distinct seismogenic zones along the dip direction of the trench, that is, the along-dip segmentation is noticeable. Significant along-strike variability also characterizes the activities in and around the subduction zone of the 1964 Alaska earthquake, including a creeping segment and a tsunami-earthquake segment.
  • K. Yoshizawa, Goeran Ekstroem
    GEOPHYSICAL JOURNAL INTERNATIONAL 183 (3) 1538 - 1558 0956-540X 2010/12 [Refereed][Not invited]
     
    A fully automated method for obtaining multimode phase speed measurements from a single seismogram has been developed and applied to a large data set of three-component long-period seismograms in North America, constructing high-resolution phase speed maps on a continental scale. The method of our phase speed estimation is based on a fully non-linear waveform inversion by Yoshizawa & Kennett working with a global search method (the Neighbourhood Algorithm). The entire process of waveform fitting and the evaluation of the estimated phase speed have been fully automated employing several empirical quantitative measures, assessing the quality of waveform fit and the relative contributions of each mode in a chosen time window. The measured phase speed data undergo automatic screening for quality control, comprising the threshold evaluation of their reliability and outlier detection and removal. This new automated method has been applied to a large data set recorded at North American stations, including the latest transportable stations of USArray. Using long-period three-component seismograms recorded during the past eight years, we have successfully retrieved large numbers of regional surface wave paths, including over 20 000 paths for the fundamental-mode Rayleigh waves over a wide range of frequencies, and over 10 000 paths for the higher mode Rayleigh as well as the fundamental-mode Love waves. The consistent results of the automated measurement procedure suggest that the method works well at regional distances, allowing us to perform a high-resolution mapping of multimode phase speeds in North America. The results of the automated waveform analysis also indicate some intrinsic limitations in the higher mode phase speed measurements from a single seismogram particularly in the short period range, mainly due to the overlapping of higher mode arrivals as well as coupling between mode branches. Despite such an innate difficulty in the higher mode dispersion measurements, the automated method allows us to construct reliable multimode phase speed maps. The current data set of ray paths is significantly biased towards the western half of North America, resulting in non-uniform horizontal resolution across the continent. This issue will be resolved by the future migration of the USArray stations to cover the central and eastern United States. The new automated method can be a useful tool for high-resolution mapping of regional 3-D shear wave structure including possible anisotropy.
  • K. Yoshizawa, K. Miyake, K. Yomogida
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 (1-2) 4 - 19 0031-9201 2010/11 [Refereed][Not invited]
     
    Three-dimensional shear wave speed structure of the upper mantle beneath the Japanese Islands and the surrounding regions is constructed from the measurements of inter-station phase speeds of surface waves. We employ the dense broad-band seismic network (F-net), deployed throughout Japan, as well as permanent stations of global seismic network in East Asia. A temporary broad-band seismic network in Far-East Russia, which has been constructed as a part of the Stagnant Slab Project since 2004, is also used in combination with the permanent stations. Using seismic events from 2005 to 2007 with moment magnitude greater than 6.0 and depth shallower than 100 km, we measure the phase speeds of the fundamental-mode Rayleigh and Love waves, working with the conventional two-station technique. Phase speed maps of the Japanese Islands and the Sea of Japan are obtained from the inversions of inter-station phase speeds that are measured for a large number of station-pairs. We have collected over 5300 paths for Rayleigh waves and 3800 paths for Love waves. The phase speed maps of Rayleigh waves, in the period range from 25 to 140 s, are inverted to obtain an isotropic 3D shear wave speed model in the depth range from 40 to 200 km. The isotropic S wave model represents prominent fast wave speed anomalies in northeastern Japan, associated with the subducting Pacific Plate. Fast anomalies of the lithosphere beneath the Sea of Japan are also imaged, implying that the average thickness of lithosphere beneath the Yamato Rise and Japan Basin is about 60 +/- 10 km. Immediately below this oceanic lithosphere, slow anomalies are found in the asthenosphere beneath the entire region of the Sea of Japan in the depth down to about 200 km. In southwestern Japan, the low-angle subduction of the Philippine Sea Plate is mapped clearly, indicating that the northern end of the plate extends beyond the northern coast of the Chugoku region, reaching beneath the Oki Islands in the Sea of Japan. A strong low velocity anomaly off the west coast of Kyushu is found in the depth down to about 130 km, which supports the existence of an upwelling flow beneath this area. We also invert the phase speed maps of Rayleigh and Love waves in the period range from 30 to 80 s simultaneously, to construct the radially anisotropic shear wave speed model. A remarkable anisotropy with SH > SV is found in the mantle wedge beneath northeastern Japan, while only weak anisotropy is seen beneath the Sea of Japan. Such anisotropic properties, which cannot readily be obtained from body wave studies only, will be an important key to understanding the dynamic processes in this tectonically actively region. (C) 2010 Elsevier B.V. All rights reserved.
  • Takehi Isse, Hajime Shiobara, J. -P. Montagner, Hiroko Sugioka, Aki Ito, Azusa Shito, Toshihiko Kanazawa, Kazunori Yoshizawa
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 (1-2) 33 - 43 0031-9201 2010/11 [Refereed][Not invited]
     
    Seismic anisotropy can provide fundamental information on past and present-day deformation processes in the upper mantle. Using Rayleigh and Love waves recorded by land and seafloor broadband seismometers, we analyzed the isotropic and anisotropic shear-wave velocity structures in the northern Philippine Sea region. We found that the fast directions of azimuthal anisotropy are parallel to the directions of ancient seafloor spreading in the lithosphere of the Shikoku and West Philippine Basins and Pacific Ocean, whereas they are parallel to the direction of the present-day absolute plate motion (APM) in the asthenosphere of the Shikoku Basin, and oblique to the direction of the APM in the Pacific Ocean (by similar to 30 degrees) and in the northern part of the West Philippine Basin (by similar to 55 degrees). In the subduction zones around the Philippine Sea plate, the fast direction of azimuthal anisotropy is trench-parallel in the Ryukyu arc, and oriented NW-SE in the Izu-Ogasawara island arc. The Philippine Sea plate, which is a single plate, shows very large lateral variations in azimuthal and radial anisotropies compared with the Pacific plate. (C) 2010 Elsevier B.V. All rights reserved.
  • Kinue Osada, Kazunori Yoshizawa, Kiyoshi Yomogida
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 (1-2) 63 - 72 0031-9201 2010/11 [Refereed][Not invited]
     
    Three-dimensional geometry of the upper boundary of the Pacific plate subducting beneath Hokkaido, Japan, was obtained using the ScSp phase: the phase converted from ScS (S wave reflected at the core-mantle boundary) to P wave at the plate boundary. Taking the advantage of a dense seismic network, "Hi-net", recently deployed across the Japanese islands, we applied several seismic array analyses to the recorded waveform data for a large nearby deep earthquake, in order to enhance very weak ScSp signals in the original records. At first, we set up five blocks for the region in plate dip directions. After aligning the travel times of ScS and stacking seismograms among stations in a given sub-block perpendicular to each dip direction, we searched for the optimal plate model (i.e., two-dimensional geometry of the upper boundary) for each block. The model was parameterized by seven depth grids, and seismograms were stacked based on the travel time of ScSp as a time lag of each sub-block, so that the optimal model would yield the maximum spectral energy of ScSp after stacking. This model parameter search was conducted, using ray tracings of ScSp with a reference velocity model and a non-linear inversion scheme (Neighbourhood Algorithm). The optimal model of each block was combined each other by cubic spline interpolation, in order to construct an overall three-dimensional geometry of the upper boundary of the plate. Next, we performed the frequency-wavenumber (f-k) spectral analysis to refine the above result. Assuming each station as a reference point, we made beam output from records of its adjacent stations as a function of wavenumber vector (k(x),k(y)) and frequency. The peak of its power spectrum was considered to represent the wavenumber vector of ScSp, that is, azimuth of arrival and slowness, so that we can estimate the position and depth of the corresponding ScS-ScSp conversion. In the frequency range from 0.5 to 1.5 Hz, we could estimate the conversion points for 21 stations or hypothetical arrays, and revised the geometry of the upper boundary obtained by the non-linear stacking approach in the previous step. The final plate model was compared with the distribution of intraplate earthquakes in the Pacific plate. This comparison clearly reveals that the upper seismic zone merges with the lower from 150 to 200 km in depth, deviating systematically away from the upper boundary where the boundary is slightly bumped in a convex manner. (C) 2010 Elsevier B.V. All rights reserved.
  • E. Bourova, K. Yoshizawa, K. Yomogida
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 (1-2) 20 - 32 0031-9201 2010/11 [Refereed][Not invited]
     
    The upper mantle structure of marginal seas (the Seas of Japan and Okhotsk) and subduction zones in northeastern Eurasia is investigated, using the three-stage multimode surface wave tomography incorporating finite-frequency effects. Broadband waveform data from 305 events with magnitude greater than 5.5 from 1990 to 2005 recorded at 25 stations of the IRIS network in northeastern Eurasia and Japan and at 8 stations of the broadband seismic network in Far-Eastern Russia from 2005 to 2008 are employed in our analysis. The dispersion curves of the fundamental mode and first two higher modes of Rayleigh waves are simultaneously inverted for the shear-wave velocity structure of the region. The off-great circle propagation due to strong heterogeneities in the region is also taken into account in the construction of intermediary phase velocity models for each mode as a function of frequency. The obtained 3D S-wave velocity model is well resolved down to 200 km depth. Checkerboard tests show the average horizontal resolution of 5 in the study region. The subducting Pacific plate is clearly imaged as a high velocity anomaly up to 6%. The mantle wedge above the Pacific plate is associated with low velocity anomalies. The absolute minimum S-wave velocity in the mantle wedge is 4 km/s in the Sea of Okhotsk in the depth range from 80 to 160 km, probably indicating the presence of partial melt. The anomalous spot with conspicuous low velocity in the southern end of the Sea of Okhotsk may indicate the existence of hot upwelling flow in the mantle. A high velocity anomaly subparallel to the present subduction zone is found in the northwestern Sea of Okhotsk in the depth range from 100 to 200 km. The position of this anomaly correlates well with the high velocity anomaly found in the P-wave tomography of Gorbatov et al. (2000), which may be interpreted as a relict of the Okhotsk plate subducted in the past. We also attempted a mapping of azimuthal anisotropy in this region. The fast phase velocity directions near the Pacific plate are observed subparallel to the Kuril and Japan Trenches at all the periods, indicating a strong effect of the subducting Pacific plate on the mantle flow, while the anisotropy appears to be weak in tectonically inactive marginal seas. (C) 2010 Elsevier B.V. All rights reserved.
  • Kinue Osada, Kazunori Yoshizawa, Kiyoshi Yomogida
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 (1-2) 63 - 72 0031-9201 2010/11 [Refereed][Not invited]
     
    Three-dimensional geometry of the upper boundary of the Pacific plate subducting beneath Hokkaido, Japan, was obtained using the ScSp phase: the phase converted from ScS (S wave reflected at the core-mantle boundary) to P wave at the plate boundary. Taking the advantage of a dense seismic network, "Hi-net", recently deployed across the Japanese islands, we applied several seismic array analyses to the recorded waveform data for a large nearby deep earthquake, in order to enhance very weak ScSp signals in the original records. At first, we set up five blocks for the region in plate dip directions. After aligning the travel times of ScS and stacking seismograms among stations in a given sub-block perpendicular to each dip direction, we searched for the optimal plate model (i.e., two-dimensional geometry of the upper boundary) for each block. The model was parameterized by seven depth grids, and seismograms were stacked based on the travel time of ScSp as a time lag of each sub-block, so that the optimal model would yield the maximum spectral energy of ScSp after stacking. This model parameter search was conducted, using ray tracings of ScSp with a reference velocity model and a non-linear inversion scheme (Neighbourhood Algorithm). The optimal model of each block was combined each other by cubic spline interpolation, in order to construct an overall three-dimensional geometry of the upper boundary of the plate. Next, we performed the frequency-wavenumber (f-k) spectral analysis to refine the above result. Assuming each station as a reference point, we made beam output from records of its adjacent stations as a function of wavenumber vector (k(x),k(y)) and frequency. The peak of its power spectrum was considered to represent the wavenumber vector of ScSp, that is, azimuth of arrival and slowness, so that we can estimate the position and depth of the corresponding ScS-ScSp conversion. In the frequency range from 0.5 to 1.5 Hz, we could estimate the conversion points for 21 stations or hypothetical arrays, and revised the geometry of the upper boundary obtained by the non-linear stacking approach in the previous step. The final plate model was compared with the distribution of intraplate earthquakes in the Pacific plate. This comparison clearly reveals that the upper seismic zone merges with the lower from 150 to 200 km in depth, deviating systematically away from the upper boundary where the boundary is slightly bumped in a convex manner. (C) 2010 Elsevier B.V. All rights reserved.
  • Takehi Isse, Hajime Shiobara, J. -P. Montagner, Hiroko Sugioka, Aki Ito, Azusa Shito, Toshihiko Kanazawa, Kazunori Yoshizawa
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 (1-2) 33 - 43 0031-9201 2010/11 [Refereed][Not invited]
     
    Seismic anisotropy can provide fundamental information on past and present-day deformation processes in the upper mantle. Using Rayleigh and Love waves recorded by land and seafloor broadband seismometers, we analyzed the isotropic and anisotropic shear-wave velocity structures in the northern Philippine Sea region. We found that the fast directions of azimuthal anisotropy are parallel to the directions of ancient seafloor spreading in the lithosphere of the Shikoku and West Philippine Basins and Pacific Ocean, whereas they are parallel to the direction of the present-day absolute plate motion (APM) in the asthenosphere of the Shikoku Basin, and oblique to the direction of the APM in the Pacific Ocean (by similar to 30 degrees) and in the northern part of the West Philippine Basin (by similar to 55 degrees). In the subduction zones around the Philippine Sea plate, the fast direction of azimuthal anisotropy is trench-parallel in the Ryukyu arc, and oriented NW-SE in the Izu-Ogasawara island arc. The Philippine Sea plate, which is a single plate, shows very large lateral variations in azimuthal and radial anisotropies compared with the Pacific plate. (C) 2010 Elsevier B.V. All rights reserved.
  • Takehi Isse, Hajime Shiobara, Yoshihiko Tamura, Daisuke Suetsugu, Kazunori Yoshizawa, Hiroko Sugioka, Aki Ito, Toshihiko Kanazawa, Masanao Shinohara, Kimihiro Mochizuki, Eichiro Araki, Kazuo Nakahigashi, Hitoshi Kawakatsu, Azusa Shito, Yoshio Fukao, Osamu Ishizuka, James B. Gill
    EARTH AND PLANETARY SCIENCE LETTERS 278 (1-2) 107 - 119 0012-821X 2009/02 [Refereed][Not invited]
     
    We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of a finite frequency and ray bending. The new ocean bottom data provided us with improved spatial resolution (similar to 300 km) in the Philippine Sea region. In the upper 120 km, the shear wave speed structure is well correlated with seafloor age. At depths greater than 160 km, fast anomalies of the subducting Pacific Plate are clearly defined. We also found slow speed anomalies beneath the Kyushu-Palau ridge at depths greater than 120 km. Along the Izu-Bonin(Ogasawara)-Mariana arc, we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc. Each anomaly has a width of similar to 500 km. Moreover, these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios. We suggest that each of the anomalies is a site of large-scale flow of upper mantle into the mantle wedge, and that each already contains a component from the adjacent subducting slab. (c) 2008 Elsevier B.V. All rights reserved.
  • S. Fishwick, M. Heintz, B. L. N. Kennett, A. M. Reading, K. Yoshizawa
    TECTONICS 27 (4) TC4009  0278-7407 2008/08 [Refereed][Not invited]
     
    A series of steps in the lithospheric thickness of eastern Australia are revealed by the latest seismic surface wave tomographic model and calculations of the horizontal gradient of shear wave speed. The new images incorporate data from the recent Tasmal experiment, improving resolution in continental Australia. Through comparisons with surface geology and geochemical studies, it is possible to infer that the steps in lithospheric thickness are related to boundaries between blocks of different age. The westernmost boundary marks the edge of the Archaean to Early-Proterozoic core of the continent. A second lithospheric boundary is observed in the central part of east Australia. To the west of this line, geochemical evidence suggests that there is Proterozoic lithospheric mantle, and this boundary may therefore represent the change from Proterozoic to Phanerozoic basement. The structure on the eastern margin of the continent is dominated by slow velocities, suggesting that in this area the continental lithosphere is very thin. There is a strong correlation between the slow wave speeds and the location of both the highest topography and recent volcanic activity. Inland of the continental margin, a zone of strong gradients in the seismic wave speed is observed, indicating a distinct step in lithospheric structure. If the step in lithospheric thickness was in place prior to volcanism, it may have acted as a boundary, with volcanism mainly occurring beneath the thinner lithosphere to the east.
  • 吉澤和範
    北海道大学地球物理学研究報告 (71) 39-48  0439-3503 2008/03 [Not refereed][Not invited]
  • Takehi Isse, Daisuke Suetsugu, Hajime Shiobara, Hiroko Sugioka, Kazunori Yoshizawa, Toshihiko Kanazawa, Yoshio Fukao
    GEOPHYSICAL RESEARCH LETTERS 33 (16) L16303 - L16303 0094-8276 2006/08 [Refereed][Not invited]
     
    We determined three-dimensional shear wave speed structure beneath the South Pacific superswell down to a depth of 200 km by analyzing Rayleigh wave records from broadband ocean bottom seismograph stations and island stations in the Pacific Ocean. The ocean bottom stations were deployed from 2003 to 2005 on the seafloor in the French Polynesian region, which enabled us to study the upper mantle structure beneath the Superswell with unprecedentedly high resolution. We measured the dispersions of fundamental mode Rayleigh waves at periods between 40 and 140 seconds by the two-station method. We found pronounced slow anomalies near the hot spots and in the Lau Basin. The slow anomalies beneath the hot spots have deeper-rooted than those associated with the Lau basin. The slow anomalies near the Society, Macdonald, Marquesas, and Pitcairn hot spots continue down to at least 200 km depth.
  • ISSE Takehi, YOSHIZAWA Kazunori, SHIOBARA Hajime, SHINOHARA Masanao, NAKAHIGASHI Kazuo, MOCHIZUKI Kimihiro, SUGIOKA Hiroko, SUETSUGU Daisuke, OKI Satoko, KANAZAWA Toshihiko, SUYEHIRO Kiyoshi, FUKAO Yoshio
    J Geophys Res 111 (B6) B06310 - B06310 0148-0227 2006/06 [Refereed][Not invited]
  • 吉澤和範
    北海道大学地球物理学研究報告 (69) 15-21  0439-3503 2006/03 [Not refereed][Not invited]
  • K Yoshizawa, BLN Kennett
    GEOPHYSICAL JOURNAL INTERNATIONAL 162 (3) 910 - 926 0956-540X 2005/09 [Refereed][Not invited]
     
    Sensitivity kernels for fundamental mode surface waves at finite frequency for 2-D phase speed and 3-D shear wave speed are constructed based on the Born and Rytov approximations working with a potential representation for surface waves. The use of asymptotic Green's functions for scalar wave equations provides an efficient way to calculate the Born or Rytov kernels. The 2-D sensitivity kernels enable us to incorporate the finite-frequency effects of surface waves, as well as off-great-circle propagation, in tomographic inversions for phase-speed structures. We derive examples of the 2-D sensitivity kernels both for a homogeneous background model (or a spherically symmetric model), and for a laterally heterogeneous model. The resulting distortions of the shape of the sensitivity kernels for a heterogeneous background model indicate the importance of the use of proper kernels to account of the heterogeneity in the real Earth. By combining a set of 2-D sensitivity kernels with 1-D vertical sensitivity kernels for a particular frequency range and taking the inverse Fourier transform, we can derive 3-D sensitivity kernels for surface waves in the time domain. Such 3-D kernels are useful for efficient forward modelling of surface waveforms incorporating finite-frequency effects, and will also enable us to perform direct inversion of surface waveforms into 3-D structure taking account of finite-frequency effects.
  • S Goes, FJ Simons, K Yoshizawa
    EARTH AND PLANETARY SCIENCE LETTERS 236 (1-2) 227 - 237 0012-821X 2005/07 [Refereed][Not invited]
     
    We derive estimates of temperature of the Australian continental mantle between 80 and 350 km depth from two published S-velocity models. Lithospheric temperatures range over about 1000 degrees C, with a large-scale correlation between temperature and tectonic age. In detail however, variations ranging from 200 to 700 degrees C occur within each tectonic province. At the current seismic resolution, strictly Proterozoic and Archean blocks do not have substantially different temperatures, nor does the Phanerozoic lithosphere east and west of the Tasman line. Temperatures close to an average (moist) MORB source mantle solidus characterize the eastern seaboard and its offshore. Differences between the temperatures derived from the two velocity models illustrate the importance of well-constrained absolute velocities and gradients for physical interpretation. The large range of lithospheric temperatures cannot be explained solely with documented variability in crustal heat production, but requires significant variations in mantle heat flow as well. (c) 2005 Elsevier B.V. All rights reserved.
  • YOSHIZAWA KAZUNORI
    地震 57 (3) 393-408  0037-1114 2005/02 [Refereed][Invited]
  • YOSHIZAWA K, KENNETT B L N
    J Geophys Res 109 (B2) B02310.1-B02310.19  0148-0227 2004/02 [Refereed][Not invited]
  • New report charts course for future of geosciences
    Adlen, S, Brodsky, E.E, Oki, T, Ridley, A.J, Sanchez, L, Simionato, C, Yoshizawa, K, Shamir, U
    EOS Trans AGU 85 25, 31  2004 [Refereed][Not invited]
  • BLN Kennett, K Yoshizawa
    GEOPHYSICAL JOURNAL INTERNATIONAL 150 (1) 37 - 44 0956-540X 2002/07 [Refereed][Not invited]
     
    A three-stage inversion scheme for surface wave tomography working with multimode phase dispersion as a function of frequency provides a means of combining a wide range of data in a common framework. The phase average approximation is applied directly to phase slowness and there is no need to invoke perturbation arguments for the interpretation of path-averaged velocity models derived from waveform inversion of surface waves. By treating such wave speed profiles as summaries of path specific dispersion behaviour it is possible not only to combine results from different style of inversion but also to provide maximum exploitation of Love and Rayleigh wave information. Inversions of all suitable waveforms can be undertaken in terms of isotropic models. Dispersion information from all paths is combined to form multimode phase speed distributions as a function of frequency in linearized inversion which takes account of path bending and finite frequency effects. The final inversion for 3-D wave speed structure is based on a cellular inversion of the multimode frequency dispersion including angular effects in terms of a local stratified model including anisotropy. The smoothing from inclusion of finite frequency effects and damping of the linearized inversion for the phase speed distributions will control the smoothness of the 3-D shear wave speed model.
  • K Yoshizawa, BLN Kennett
    GEOPHYSICAL JOURNAL INTERNATIONAL 149 (2) 440 - 453 0956-540X 2002/05 [Refereed][Not invited]
     
    An approximate description of the zone of influence around the propagation path for a surface wave is provided by investigating the Fresnel zones for the frequency range of interest. The influence zone about surface wave paths, over which surface waves are coherent in phase, is identified as approximately one-third of the width of the first Fresnel zone. A technique called Fresnel-area ray tracing (FRT) for surface waves has been used to estimate this region around the ray path for each frequency. The FRT technique is developed by combining two standard ray tracing methods, i.e. kinematic ray tracing (KRT) and dynamic ray tracing (DRT). To obtain the exact Fresnel area in a laterally heterogeneous structure would require the solution of a large number of KRT equations. In contrast, the FRT approach requires just a few ray tracing calculations. In the first step, the trajectory of the surface wave is computed by solving the KRT system for the phase-velocity distribution at the required frequency. In the next step, the behaviour of rays in the zone surrounding the KRT path is calculated by solving the DRT system twice; once from the source to the receiver and once more from the receiver to the source along the same trajectory. Finally, combining the solutions of these ray tracing systems using paraxial ray theory, the Fresnel area around a central ray can be estimated. Using FRT, stationary-phase fields can be constructed around a central ray path in a laterally heterogeneous structure. The influence zone around the ray path is then estimated from the stationary-phase function with simple assumptions concerning the perturbed wavefield. The estimate of the influence zone can be efficiently calculated in laterally heterogeneous structure by using the FRT technique, and allows an extension of current methods of surface wave analysis, which have commonly been based on geometrical ray theory and on the approximation of great-circle propagation. This approach allows the treatment of finite-width rays as well as deviations in propagation from the great circle induced by moderate lateral heterogeneity as revealed by recent tomography models. Such finite-width rays should be of major benefit in enhancing ray-based surface wave tomography.
  • K Yoshizawa, BLN Kennett
    GEOPHYSICAL JOURNAL INTERNATIONAL 149 (1) 118 - 133 0956-540X 2002/04 [Refereed][Not invited]
     
    A ne technique for multimode dispersion Measurement has been developed by employing fully non-linear waveform inversion for a path-specific 1-D profile using a neighbourhood algorithm (NA). One-dimensional models derived from a waveform inversion are quite sensitive to the model parametrization and the reference model used to start the inversion. Willi different approaches to the parametrization of the shear wave speed profile, we can find models with significant differences in velocity variation with depth, which provide similar levels of Ill to the observed waveforms. Although the models differ. the Calculated phase dispersion for the first few modes of the surface waves are very close indeed. We therefore regard the 1-D models derived from the multimode waveform inversion as an implicit description of the path-specific dispersion for each of the modes. The inversion procedure using NA samples a substantial number of models and we select the 1-D wave speed model that achieves minimum misfit. Phase speeds for each mode branch are then Calculated From the model parameters with the minimum misfit. Because we use the 1-D models as a summary of the average multimode dispersion along the path, we do not need to make assumptions concerning the nature of anisotropy. We can employ simple isotropic models and the method can be applied to both Rayleigh and Love waves independently The new method is applied to sets of paths in the Australian region and provides stable measurements of multimode dispersion from a single seismogram. The multimode phase speeds Measured using the NA inversion call be used to retrieve phase velocity maps as a function of frequency for higher-mode branches as well as the fundamental mode, which will be crucial constraints oil 3-D Earth models.
  • K Yoshizawa, K Yomogida, S Tsuboi
    GEOPHYSICAL JOURNAL INTERNATIONAL 138 (1) 205 - 220 0956-540X 1999/07 [Refereed][Not invited]
     
    The resolving power of polarization data compared with that of phase data is investigated by employing both synthetic and observed data sets, using the linear relationship between the phase velocity perturbation and the phase or polarization anomaly. In order to investigate the intrinsic differences between phase and polarization data, a synthetic test is first undertaken using a white noise model with sufficiently uniform coverage of ray paths. This test shows that polarization data can retrieve higher-order heterogeneities of degrees up to 20 almost completely, despite damping and smoothing effects, whilst phase data can only retrieve those of degrees lower than 8 with reasonable damping. Next, about 4000 phase and 2500 polarization records are collected for minor- and major-are Rayleigh waves (R1 and R2) in the frequency range 4-12 mHz, To correct the Rayleigh wave polarization data, the misorientation of each station is estimated from the polarization of long-period P waves propagating mainly in the lower mantle. The phase and polarization data are then inverted for the phase velocity distribution in spherical harmonics with degrees up to 15. The phase velocity maps derived from the phase data are quite consistent with previous studies, whilst those from the polarization data show some discrepancies. For example, the correlation between the phase and polarization models is quite good for low even degrees such as 2, 4 and 6, but not for low odd degrees or degrees higher than 8. The gradients of amplitude spectra from the polarization data are smaller than those from the phase data, especially at degrees higher than 6, which suggests a slightly higher sensitivity of the polarization data to higher-order heterogeneities. Nevertheless, the overall spectral characteristics of both models are similar; that is, low-order heterogeneities are dominant whilst higher orders are clearly reduced. Further investigation using a synthetic test with the same uneven paths as the observed data shows the suppression of higher-order heterogeneities. Since the synthetic test with even paths retrieves higher-order heterogeneities sufficiently, this result strongly suggests the path averaging effect of uneven ray paths that is intrinsic in the ray theoretical approach used in this study as well as almost all the global inversions. Although inversions based on geometrical ray theory have some limited resolving power with the current status of global records, polarization data are indeed helpful in resolving higher-order lateral heterogeneities with the dense and uniform path coverage that is becoming available.

Books etc

  • 鳥海 光弘, 入船 徹男, 岩森 光, Wallis Simon, 小平 秀一, 小宮 剛, 阪口 秀, 鷺谷 威, 末次 大輔, 中川 貴司, 宮本 英昭 (Contributor7.8章 大陸マントル [吉澤和範])
    朝倉書店 2018 (ISBN: 9784254160727)
  • Physics of the Earth
    Honda S, Iwamori H, Utada H, Okubo S, Kurita K, Tsuchiya T, Nakai S, Hiraga T, Miyatake T, Yoshizawa K (Joint translationChap 16-17)
    Asakura Publishing Co. Ltd. 2013/07
  • 自然科学実験
    北海道大学自然科学実験編集委 (Contributor地学第5章:地震計で測る大地の震動)
    学術図書出版社 2012

Conference Activities & Talks

  • Phase speed mapping of multi-mode surface waves with eikonal tomography using mode-branch waveforms  [Not invited]
    H. Matsuzawa, K. Yoshizawa, F.-C. Lin
    Fall Meeting, Seismological Society of Japan  2019/09
  • Azimuthal anisotropy in the upper mantle using multi-mode surface waves: Application to the Australian regio  [Not invited]
    Y. Nishimura, K. Yoshizawa
    Fall Meeting, Seismological Society of Japan  2019/09
  • Effects of laminated random heterogeneity on surface wave propagation and apparent radial anisotropy  [Not invited]
    Y. Xu, K. Yoshizawa, T. Furumura
    Fall Meeting, Seismological Society of Japan  2019/09
  • Comparisons of anisotropic 3D S wave speed models derived from linearized and non-linear inversions of multi-mode surface waves  [Not invited]
    K. Yoshizawa, T. Taira
    Seismological Society of Japan  2019/09
  • Multi-mode Phase Speed Measurements of Surface Waves with a Hybrid Array-based Method: Application to USArray  [Not invited]
    H. Matsuzawa, K. Yoshizawa
    IUGG General Assembly  2019/07
  • Trans-dimensional Bayesian Inversions for Australian Lithosphere and Asthenosphere Using Multi-mode Surface Waves and Receiver Functions  [Not invited]
    T. Toru, K. Yoshizawa
    IUGG General Assembly  2019/07
  • Radial and Azimuthal Anisotropy in the Australian Upper Mantle: Tectonic Implications for Tasman Line and Suture Zone  [Not invited]
    Y. Nishimura, K. Yoshizawa, B.L.N. Kennett
    IUGG General Assembly  2019/07
  • Quantitative Estimation of Apparent Radial Anisotropy in the Lithosphere and Asthenosphere: Effects of Multi-scale Heterogeneity  [Not invited]
    K. Yoshizawa, Y. Xu, T. Fukumura
    IUGG General Assembly  2019/07
  • Fine-scale random heterogeneity and apparent radial anisotropy of S-waves in the lithosphere and asthenosphere  [Not invited]
    YOSHIZAWA Kazunori
    Seismological Society of Japan 2018 Fall Meeting  2018/10
  • Mapping the upper mantle beneath North American continent with joint inversion of surface-wave phase and amplitude  [Not invited]
    K. Yoshizawa, K. Hamada
    American Geophysical Union 2017 Fall Meeting  2017/12
  • Seismic Tomography and Tectonics  [Invited]
    YOSHIZAWA Kazunori
    Summer School of Seismology  2017/09
  • A seismological constraint on the asthenosphere: mapping radial anisotropy with multi-mode surface waves  [Invited]
    YOSHIZAWA Kazunori
    JpGU-AGU Joint Meeting 2017  2017/05
  • Lithosphere-asthenosphere transition, mid-lithosphere discontinuity and radial anisotropy from multi-mode surface wave tomography  [Invited]
    YOSHIZAWA Kazunori
    American Geophysical Union 2015 Fall Meeting  2015/12
  • Mapping the upper mantle with multi-mode surface waves: Lithosphere-ashtenosphere transition and radial anisotropy  [Invited]
    YOSHIZAWA Kazunori
    The 14th International Symposium on Study of the Earth's Deep Interior  2014/08  the Shonan Village Center, Kanagawa, Japan
  • YOSHIZAWA Kazunori
    Asia and Oceania Geoscieces Society 11th Annual Meeting  2014/07  Royton Sapporo Hotel, Japan
  • Radially anisotropic shear wave model of North America from multi- mode surface tomography using USArray  [Invited]
    YOSHIZAWA Kazunori
    JpGU Meeting 2010  2010/05

MISC

  • 北米大陸の内部をのぞく − アース・スコープ計画の10年 (T. Feder, "Scoping out the North American continent, 10 years on"の翻訳)
    吉澤 和範  パリティ  29-  (9)  46  -49  2014/08  [Refereed][Invited]
  • Junji Koyama, Motohiro Tsuzuki, Kiyoshi Yomogida, Kazunori Yoshizawa  Journal of the Hokkaido University, Faculty of Science, Series VII: Geophysics  129  -146  2013/12/01  [Not refereed][Not invited]
  • Kazunori Yoshizawa  Journal of the Hokkaido University, Faculty of Science, Series VII: Geophysics  39  -48  2008/12/01  [Not refereed][Not invited]
     
    Seismic waves generated by a nuclear test carried out by North Korea on October 9, 2006 were observed throughout the Japanese islands. Clear arrivals of Pn waves, which traveled below the Moho discontinuity underneath the Japan Sea, were recorded by the Japanese broad-band seismic network, F-net, deployed by National Research Institute for Earth Science and Disaster Prevention (NIED). We analyzed the waveforms of the nuclear event observed at F-net stations as well as some global seismic stations in East Asia. Apparent path-average velocity of Pn waves and their maximum amplitudes are estimated from vertical component seismograms of the F-net stations. We found conspicuous regional variations of apparent path-average velocity and maximum amplitude. For example, fast apparent velocity and larger amplitude are observed in the paths across the Japan basin in the northern Japan Sea, whereas slow velocity as well as relatively smaller amplitudes are found in the path to stations in Kyushu. Such regional variability of Pn waves is likely to make it difficult to estimate source parameters for this small-scale explosive event in the Korean Peninsula, only using the Japanese seismic network. These results suggest the necessity of a precise three-dimensional seismic model of the Japan Sea to utilize Japanese seismic network data for analysis of regional seismic waves that propagate along a variety of paths in the Japan Sea.
  • Kazunori Yoshizawa  Journal of the Hokkaido University, Faculty of Science, Series VII: Geophysics  15  -21  2006/12/01  [Not refereed][Not invited]
     
    The great Sumatra earthquake on December 26, 2004, which devastated the coastal areas of the Indian Ocean by the subsequent tsunami, generated tremendous numbers of surface-wave trains which traveled around the globe for many times. We analyze vertical-component seismograms of the event recorded by the F-net broadband seismic network in Japan, which has been deployed and maintained by NIED (National Research Institute for Earth Science and Disaster Prevention, Japan). By applying a band-pass filter between 3 and 5 mHz, we extract the long-period Airy phase of the fundamental-mode Rayleigh wave. The long-period records display a magnificent series of multi-orbit Rayleigh wave trains that have circumnavigated the Earth for more than 6 times; i.e., we are able to observe clear signals up to R13-R14 phases. Although much later arrival phases than R14 become less visible mainly due to lateral heterogeneities in the Earth as well as dispersions of surface waves, we are still able to detect some small signals up to R18.
  • 2004年スマトラ沖地震で発生した地球を周回する表面波
    吉澤 和範  なゐふる(日本地震学会広報誌)  49-  6  2005  [Not refereed][Invited]
  • Societal Impact
    Yoshizawa K  IUGG report "Geosciences: The Future"  95  -100  2003/07  [Refereed][Invited]
  • Seismology and Physics of the Earth's Interior
    Yoshizawa K, Yomogida K, Tsuboi S  IUGG report "Geosciences: The Future"  11  -21  2003/07  [Refereed][Invited]
  • K. Yoshizawa, B. L N Kennet  Geophysical Journal International  149-  118  -133  2002/05/13  [Not refereed][Not invited]
     
    A new technique for multimode dispersion measurement has been developed by employing fully non-linear waveform inversion for a path-specific 1-D profile using a neighbourhood algorithm (NA). One-dimensional models derived from waveform inversion are quite sensitive to the model parametrization and the reference model used to start the inversion. With different approaches to the parametrization of the shear wave speed profile, we can find models with significant differences in velocity variation with depth, which provide similar levels of fit to the observed waveforms. Although the models differ, the calculated phase dispersion for the first few modes of the surface waves are very close indeed. We therefore regard the 1-D models derived from the multimode waveform inversion as an implicit description of the path-specific dispersion for each of the modes. The inversion procedure using NA samples a substantial number of models and we select the 1-D wave speed model that achieves minimum misfit. Phase speeds for each for each mode branch are then calculated from the model parameters with the minimum misfit. Because we use the 1-D models as a summary of the average multimode dispersion along the path, we do not need to make assumptions concerning the nature of anisotropy. We can employ simple isotropic models and the method can be applied to both Rayleigh and Love waves independently. The new method is applied to sets of paths in the Australian region and provides stable measurements of multimode dispersion from a single seismogram. The multimode phase speeds measured using the NA inversion can be used to retrieve phase velocity maps as a function of frequency for higher-mode branches as well as the fundamental mode, which will be crucial constraints on 3-D Earth models.

Awards & Honors

  • 2007/05 Seismological Society of Japan Young Investigator Award
     「表面波の波形解析と有限波長トモグラフィー法に関する理論的・実践的研究」 
    受賞者: Kazunori Yoshizawa
  • 2001/12 American Geophysical Union Outstanding Student Paper Award
     「Surface Wave Tomography: A Three-Stage Approach」

Research Grants & Projects

  • ベイズ推定による上部マントル不連続面の高精度マッピングの研究
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    Date (from‐to) : 2020/04 -2023/03 
    Author : 吉澤 和範
  • 文部科学省:科学研究費補助金(基盤研究(C))
    Date (from‐to) : 2018/04 -2021/03 
    Author : 吉田 晶樹
  • 文部科学省:科学研究費補助金(基盤研究(C))
    Date (from‐to) : 2017/04 -2020/03 
    Author : 吉澤 和範
  • 文部科学省:科学研究費補助金(基盤研究(A))
    Date (from‐to) : 2013 -2017 
    Author : 高橋 浩晃
  • 文部科学省:科学研究費補助金(基盤研究(C))
    Date (from‐to) : 2014 -2016 
    Author : 吉澤 和範
  • 文部科学省:科学研究費補助金(若手研究(B))
    Date (from‐to) : 2012 -2013 
    Author : 吉澤 和範
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2009 -2012 
    Author : TAKAHASHI Hiroaki
  • 文部科学省:科学研究費補助金(挑戦的萌芽研究)
    Date (from‐to) : 2010 -2011 
    Author : 吉澤 和範
     
    本研究では,マルチモード表面波の非線形波形解析法に基づいて表面波の振幅異常に関する情報を抽出し,さらにこの情報と高分解能な3次元S波速度構造モデルを用いて,地球内部の3次元非弾性減衰構造を復元するための新手法の確立を目的としている.今年度は,地震波線のfocusing/defocusingによる振幅異常への影響を定量的に評価する上で必須となる3次元S波速度構造モデルの高精度化と,モード毎の振幅異常値を抽出するための新しい表面波波形インバージョン法の開発に着手した.まず,北米・豪州・日本列島及びその周辺域など,高密度広帯域地震観測網のデータが利用可能な地域における高分解能な3次元S波速度モデルの構築を進め,それらの結果の一部を,国際学術誌及び学会において発表した.これと同時に,振幅情報を効率的に抽出するための新しい非線形波形インバージョン法の開発も進めている.この新手法では,周波数に依存して変化するマルチモードの振幅異常を,極力少ない独立変数で表現できるよう,波線平均の1次元Qモデル(及び1次元S波速度モデル)をパラメータとし,摂動理論を応用して非線形波形フィッティングを行う.これまでのマルチモード位相速度の自動計測による結果から,複数の高次モードの波群が重なる周波数帯域において,位相速度の計測精度が落ちることが明らかとなってきた.これは基本モードの計測では問題にはならないが,...
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(特定領域研究)
    Date (from‐to) : 2004 -2008 
    Author : Minoru KASAHARA, Kiyoshi YOMOGIDA, Hiroki MIYAMACHI, Yuuichiro TANIOKA, Hiroaki TAKAHASHI, Kazunori YOSHIZAWA, Rak Se SEN, Vladimir BORMOTOV
     
    潜り込んだ太平洋プレートのスタグナント(滞留)しているロシア極東地域に、8箇所の広帯域地震観測点の新設を目標に進めてきたが、3年目に完成し、さらに1箇所追加で、カムチャッカに設置できた。観測は、ロシア科学アカデミー・地球物理局・サハリン支所の定常観測点を利用できており、それぞれの観測点での維持管理は、順調にすすみ、全地球で発生するM5.5以上の地震データと、M4.5以上の千島海溝付近の地震データは、日本側に提供されて、日本側のデータセンターに保存され、研究に供されている。これらのデータは、世界の地震コミュニティに公開する予定である。
  • 文部科学省:科学研究費補助金(若手研究(B))
    Date (from‐to) : 2005 -2006 
    Author : 吉澤 和範
     
    本研究では,研究代表者がこれまで開発を進めてきた表面波の有限波長トモグラフィー法を,異方性媒体の場合にまで拡張し,高分解能な上部マントル内部の3次元異方的不均質構造を復元することを目的としている.今年度は,高密度な広帯域地震観測網が展開されている北西太平洋地域の地震波形データに対して独自のトモグラフィー手法を適用し,この地域直下の上部マントルの方位異方性分布の復元を行った.同時に,比較的観測点密度が低く,これまでトモグラフィーモデルの分解能に限界のあった北海道北部(苫前町)に,新たな広帯域地震観測点を設置し連続観測も行った.これまで進めてきた地震波解析により,北西太平洋地域を通過するレイリー波の位相速度データを十分収集することができたので,これらを用いて,日本列島及びその周辺域を含む北西太平洋の位相速度分布及び方位異方性分布の復元を行った.この結果,周期約100秒以下の基本モードレイリー波位相速度(深さ約150km以浅のS波速度構造に相当)の方位異方性分布が,GPS観測から判明しているユーラシアプレート東縁部の運動方向と整合的であることや,北海道北西部のユーラシアプレート及び北米プレートの境界を境にして方位異方性の方向が急激に変化すること等が分かった.これらの観測事実は,ユーラシアプレート東縁部の移動方向に沿った流れ場がその直下の上部マントル内部に存在することや,二つのプレー...
  • 文部科学省:科学研究費補助金(若手研究(B))
    Date (from‐to) : 2003 -2004 
    Author : 吉澤 和範
     
    本研究では,より詳細かつ正確な上部マントル内部の3次元不均質構造を探るために,不均質媒体を伝播する表面波の散乱・回折といった複雑な効果を考慮した新しいインバージョン法の開発と応用を主な目的としている.昨年度に引き続き,新手法の新しい理論とその数値計算手法の開発を行い,その成果は既に主要国際誌に投稿している.この方法では,波線周辺領域からの散乱波の影響も考慮するため,従来の波線理論に基づく手法に比べて膨大な計算量が必要となる.今年度は,計算効率の向上を図るために,昨年度購入した高速PCクラスタサーバーに,さらにメモリとCPUの増設を行い,より高速な大規模計算を可能とした.これを活用しオホーツク海からオーストラリア大陸までにいたる西太平洋地域の地震波形データの解析を行い,初期的な3次元不均質モデルの復元を行った.また,新しい有限波長トモグラフィー法による分解能に関するテストも行い,従来の方法に比べて分解能と信頼性の高いモデルが得られることが明らかとなった.さらに海洋研究開発機構の研究者と共同で,本研究に関連した有限波長トモグラフィー法を応用して,フィリピン海プレートの3次元S波速度構造の復元を行った.これら本研究に関連する研究成果は,AGU秋季大会(米国・サンフランシスコ)や,日本地球惑星科学関連合同大会(幕張),日本地震学会秋季大会(福岡)において発表を行った.さらに,本研究に...
  • Effects of Finite Frequency on Seismic Wave Propagation
    The Other Research Programs
    Date (from‐to) : 1999 -2004
  • Structure and Dynamics of Earth's Interior using Seismic Tomography
    The Other Research Programs
    Date (from‐to) : 1999 -2004
  • 文部科学省:科学研究費補助金(特別研究員奨励費)
    Date (from‐to) : 1998 
    Author : 吉澤 和範
  • Development of a simultaneous inversion technique of body and surface waves
    The Other Research Programs
  • Global tomography Incorporating Surface Wave Finite Frequency Effects
    The Other Research Programs

Educational Activities

Teaching Experience

  • Advanced Seismology
    開講年度 : 2018
    課程区分 : 修士課程
    開講学部 : 理学院
    キーワード : 地震波トモグラフィー,逆問題,地球内部構造,地震波動伝播,地震データ解析
  • Laboratory Work in Earth and Planetary Sciences II
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 堆積物,堆積岩,堆積環境,水圏環境,古海洋,古気候,気象,天気図,統計解析,地震,地震波,震源
  • Seismology
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 地震,弾性論,地震波動伝播,地球内部構造,震源メカニズム
  • Introduction to Modern Earth and Planetary Sciences II
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 地震,火山,地殻変動,地球深部構造,プレートテクトニクス,地震学,測地学

Committee Membership

  • 2006 - Today   Seismological Society of Japan   Board of Representatives
  • 2002   Seismological Society of Japan   A Local Liaison Committee   Seismological Society of Japan


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