基本情報

所属

• 理学研究院　地球惑星科学部門　地球惑星ダイナミクス分野

職名

• 教授

学位

• Ph.D.（オーストラリア国立大学）

ホームページURL

https://www.sci.hokudai.ac.jp/seismolab/

J-Global ID

• 200901097277030626

研究キーワード

• アセノスフェア   リソスフェア   異方性   トモグラフィー   表面波   地球内部構造   地震学   Seismic Tomography   Earth Structure   seismology   Physics of Earth's Interior   

研究分野

• 自然科学一般 / 固体地球科学 / 地震学

担当教育組織

•  学士課程　理学部
•  修士課程　理学院
•  博士課程　理学院

職歴

• 2023年02月 - 現在 北海道大学 大学院理学研究院 教授
• 2016年04月 - 現在 防災科学技術研究所 客員研究員
• 2007年05月 - 2023年01月 北海道大学 大学院理学研究院 准教授
• 2022年04月 - 2022年09月 広島大学 大学院先進理工系科学研究科 客員准教授
• 2018年10月 - 2018年10月 東北大学 大学院理学研究科 非常勤講師
• 2015年04月 - 2016年03月 東京大学 地震研究所 客員准教授
• 2015年08月 - 2015年08月 神戸大学 大学院理学研究科 非常勤講師
• 2012年04月 - 2013年03月 東京大学 地震研究所 客員准教授
• 2011年06月 - 2011年07月 オーストラリア国立大学 地球科学研究所 客員研究員
• 2008年02月 - 2010年02月 コロンビア大学 ラモントｰドハティ地球研究所 客員研究員
• 2007年04月 - 2007年04月 北海道大学 大学院理学研究院 助教（職名変更）
• 2006年04月 - 2007年03月 北海道大学 大学院理学研究院 助手（改組）
• 2002年04月 - 2006年03月 北海道大学 大学院理学研究科 助手
• 1998年04月 - 1998年12月 日本学術振興会特別研究員（DC1）

学歴

• 1999年 - 2002年   オーストラリア国立大学   地球科学研究所（博士課程）
• 1996年 - 1998年   広島大学   大学院理学研究院   地球惑星システム学専攻（修士課程）
• 1993年04月 - 1996年03月   広島大学   理学部   地球惑星システム学科（飛び級）
• 1992年04月 - 1993年03月   大阪府立大学   工学部

所属学協会

• 米国物理探査学会   日本地球惑星科学連合   米国地震学会   アメリカ地球物理学連合   日本地震学会   

研究活動情報

論文

• Layered evolution of the oceanic lithosphere beneath the Japan Basin, the Sea of Japan

  Sanxi Ai, Takeshi Akuhara, Manabu Morishige, Kazunori Yoshizawa, Masanao Shinohara, Kazuo Nakahigashi

  Journal of Geophysical Research: Solid Earth 2023年02月03日 [査読有り]
  
• Eruption sequence of the 2022 Hunga Tonga-Hunga Ha'apai explosion from back-projection of teleseismic P waves

  Kotaro Tarumi, Kazunori Yoshizawa

  Earth and Planetary Science Letters 602 117966 - 117966 2023年01月 [査読有り][通常論文]
  
• Anomalous radial anisotropy and its implications for upper mantle dynamics beneath South China from multimode surface wave tomography

  Qingya Tang, Weijia Sun, Kazunori Yoshizawa, Li‐Yun Fu

  Journal of Geophysical Research: Solid Earth 2022年08月06日 [査読有り][通常論文]
  
• Recovery of Love wave overtone waveforms and dispersion curves from single-station seismograms using time-warping

  M G Brown, G Lin, H Matsuzawa, K Yoshizawa

  Geophysical Journal International 230 70 - 83 2022年02月08日 [査読有り][通常論文]
  
• Continental Drift with Deep Cratonic Roots

  Masaki Yoshida, Kazunori Yoshizawa

  Annual Review of Earth and Planetary Sciences 49 1 2021年05月30日 [査読有り][招待有り]
   

  The influence of the continental lithosphere and its root (or keel) on the continental drift of Earth is a key element in the history of plate tectonics. Previous geodynamic studies of mantle flow suggested that the cratonic root is moderately mechanically coupled with the underlying mantle, and stable continental drift on Earth's timescales occurs when the effective viscosity contrast between the continental lithosphere and the underlying mantle is approximately 103. Both geodynamics and seismological studies indicate that mechanically weak mobile belts (i.e., orogenic or suture zones) that surround cratons may play a role in the longevity of the cratonic lithosphere over geologically long timescales (i.e., over 1,000 million years) because they act as a buffer region against the high-viscosity cratons. Low-viscosity asthenosphere, characterized by slow seismic velocities, reduces the basal drag force acting on the cratonic root, which may also contribute to the longevity of the cratonic lithosphere. ▪ The role of the continental lithosphere and its root on the continental drift is reviewed from recent geodynamic and seismological studies. ▪ The cratonic root is moderately mechanically coupled with the underlying mantle and deformed by mantle flow over geological timescales. ▪ Orogenic belts or suture zones that surround cratons act as a buffer to protect cratons and are essential for their longevity. ▪ Low-viscosity asthenosphere may reduce the basal drag acting on the cratonic root and also contribute to its stability and longevity. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
• Seismic evidence for a thermochemical mantle plume underplating the lithosphere of the Ontong Java Plateau

  Takehi Isse, Daisuke Suetsugu, Akira Ishikawa, Hajime Shiobara, Hiroko Sugioka, Aki Ito, Yuki Kawano, Kazunori Yoshizawa, Yasushi Ishihara, Satoru Tanaka, Masayuki Obayashi, Takashi Tonegawa, Junko Yoshimitsu

  Communications Earth & Environment 2 1 2021年05月24日 [査読有り][通常論文]
   

  The Ontong Java Plateau in the western Pacific Ocean is the world’s largest oceanic plateau. It was formed 122 million years ago by a massive volcanic event that significantly affected Earth’s environment. The cause of the magmatic event remains controversial because the upper mantle structure beneath the plateau is poorly known. Here we use passive seismic data obtained through seafloor observations, alongside existing seismic data, to determine the three-dimensional radially anisotropic shear wave velocity to depths of up to 300 km. We find that the lithosphere–asthenosphere boundary is approximately 40 km deeper beneath the centre of the Ontong Java Plateau than beneath the surrounding seafloor. Based on our results and petrological and rheological constraints, we propose that the lithosphere–asthenosphere boundary has deepened as a result of underplating of dehydrated residual material beneath the pre-existing lithosphere during formation of the Ontong Java Plateau by a thermochemical mantle plume.
• Upper-mantle discontinuities beneath Australia from transdimensional Bayesian inversions using multimode surface waves and receiver functions

  Toru Taira, Kazunori Yoshizawa

  Geophysical Journal International 223 3 2085 - 2100 2020年10月14日 [査読有り][通常論文]
   

  Radially anisotropic S-wave structures under the permanent seismic stations in Australia are reconstructed using multimode surface waves (SWs) and receiver functions (RFs) in a framework of the Bayesian inference. We have developed a fully nonlinear method of joint inversions incorporating P-RFs and multimode Rayleigh and Love waves, based on the transdimensional Hierarchical Bayesian formulation. The method allows us to estimate a probabilistic Earth model taking account of the complexity and uncertainty of Earth structure, by treating the model parameters and data errors as unknowns. The Parallel Tempering algorithm is employed for the effective parameter search based on the reversible-jump Markov Chain Monte Carlo method. The use of higher modes enables us to enhance the sensitivity to the depth below the continental asthenosphere. Synthetic experiments indicate the importance of higher mode SWs for the better recovery of radial anisotropy in the whole depth range of the upper mantle. The method is applied to five Global Seismographic Network stations in Australia. While the S-wave models in eastern Australia show shallow lithosphere–asthenosphere boundary (LAB) above 100 km depth, those in central and Western Australia exhibit both mid-lithosphere discontinuities (MLDs) and LAB. Also, seismic velocity jumps equivalent to the Lehmann discontinuity (L-D) are found in all seismic stations. The L-D under the Australian continents is found at around 200–300 km depth, depending on locations. Radial anisotropy in the depth range between LAB and L-D tends to show faster SH anomalies, which may indicate the effects of horizontal shear underneath the fast-moving Australian plate.
• Dynamics of continental lithosphere extension and passive continental rifting from numerical experiments of visco-elasto-plastic thermo-chemical convection in 2-D Cartesian geometry

  Masaki Yoshida, Saneatsu Saito, Kazunori Yoshizawa

  Tectonophysics 228659 - 228659 2020年10月 [査読有り][通常論文]
  
• Possible tectonic patterns along the eastern margin of Gondwanaland from numerical studies of mantle convection

  Masaki Yoshida, Saneatsu Saito, Kazunori Yoshizawa

  Tectonophysics 787 228476 - 228476 2020年07月 [査読有り][通常論文]
  
• Array-based analysis of multimode surface waves: application to phase speed measurements and modal waveform decomposition

  H Matsuzawa, K Yoshizawa

  Geophysical Journal International 218 1 295 - 312 2019年07月01日 [査読有り][通常論文]
  
• Surface wave tomography for the Pacific Ocean incorporating seafloor seismic observations and plate thermal evolution

  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年 [査読有り][通常論文]
  
• Erratum: Interstation phase speed and amplitude measurements of surface waves with nonlinear waveform fitting: application to USArray

  K Hamada, K Yoshizawa

  Geophysical Journal International 212 2 795 - 797 2018年02月01日 [査読有り][通常論文]
  
• Interactions of multi-scale heterogeneity in the lithosphere: Australia

  B. L. N. Kennett, K. Yoshizawa, T. Furumura

  TECTONOPHYSICS 717 193 - 213 2017年10月 [査読有り][通常論文]
   

  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.
• Atmospheric resonant oscillations by the 2014 eruption of the Kelud volcano, Indonesia, observed with the ionospheric total electron contents and seismic signals

  Yuki Nakashima, Kosuke Heki, Akiko Takeo, Mokhamad N. Cahyadi, Arif Aditiya, Kazunori Yoshizawa

  EARTH AND PLANETARY SCIENCE LETTERS 434 112 - 116 2016年01月 [査読有り][通常論文]
   

  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.
• Interstation phase speed and amplitude measurements of surface waves with nonlinear waveform fitting: application to USArray

  K. Hamada, K. Yoshizawa

  GEOPHYSICAL JOURNAL INTERNATIONAL 202 3 1463 - 1482 2015年09月 [査読有り][通常論文]
   

  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.
• The lithosphere-asthenosphere transition and radial anisotropy beneath the Australian continent

  K. Yoshizawa, B. L. N. Kennett

  GEOPHYSICAL RESEARCH LETTERS 42 10 3839 - 3846 2015年05月 [査読有り][通常論文]
   

  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.
• Radially anisotropic 3-D shear wave structure of the Australian lithosphere and asthenosphere from multi-mode surface waves

  K. Yoshizawa

  PHYSICS OF THE EARTH AND PLANETARY INTERIORS 235 33 - 48 2014年10月 [査読有り][通常論文]
   

  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.
• 2011年東北沖超巨大地震が明らかにした超巨大地震の多様性

  小山 順二, 都筑 基博, 蓬田 清, 吉澤 和範

  北海道大学地球物理学研究報告 76 76 129 - 146 北海道大学大学院理学研究院 2013年03月 [査読無し][通常論文]
   

  2011 年3 月11 日マグニチュード9.0 の超巨大地震が東北地方太平洋沿岸をおそった．この地震は過去千年以上にわたる日本付近で発生したどの地震よりも大きな津波を励起し，地震動災害ばかりではなく歴史に残る甚大な津波災害を発生させた．従来，このような超巨大地震が日本付近で発生することは，地震学的に想定されてこなかった．我々は，この超巨大地震の発生を考えるうえで，今まで見過ごされてきた超巨大地震の発生場には二つの異なった特徴があることに気が付いた．それはAlong-dip Double Segmentation（ADDS）とAlong-strike Single Segmentation（ASSS）という異なった地震活動である．我々はこの考えに基づき，世界中で発生した超巨大地震を調べなおし，超巨大地震の発生場を，地震活動の特徴（ADDS/ASSS），地震メカニズム，破壊様式，沈み込み帯の形状，上盤プレートの性質や背弧海盆の活動といった性質から，明らかにする．
• Australian Seismological Reference Model (AuSREM): mantle component

  B. L. N. Kennett, A. Fichtner, S. Fishwick, K. Yoshizawa

  GEOPHYSICAL JOURNAL INTERNATIONAL 192 2 871 - 887 2013年02月 [査読有り][通常論文]
   

  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.
• Three-dimensional S-wave structure of the upper mantle beneath Turkey from surface wave tomography

  Taciser Bakirci, Kazunori Yoshizawa, Mithat Firat Ozer

  GEOPHYSICAL JOURNAL INTERNATIONAL 190 2 1058 - 1076 2012年08月 [査読有り][通常論文]
   

  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.
• Variability of megathrust earthquakes in the world revealed by the 2011 Tohoku-oki Earthquake

  Junji Koyama, Kazunori Yoshizawa, Kiyoshi Yomogida, Motohiro Tsuzuki

  EARTH PLANETS AND SPACE 64 12 1189 - 1198 2012年 [査読有り][通常論文]
   

  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.
• Optimal spherical spline filters for the analysis and comparison of regional-scale tomographic models

  Andreas Fichtner, Stewart Fishwick, Kazunori Yoshizawa, Brian L. N. Kennett

  PHYSICS OF THE EARTH AND PLANETARY INTERIORS 190 44 - 50 2012年01月 [査読有り][通常論文]
   

  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.
• Along-dip segmentation of the 2011 off the Pacific coast of Tohoku Earthquake and comparison with other megathrust earthquakes

  Kiyoshi Yomogida, Kazunori Yoshizawa, Junji Koyama, Motohiro Tsuzuki

  EARTH PLANETS AND SPACE 63 7 697 - 701 2011年 [査読有り][通常論文]
   

  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.
• Automated multimode phase speed measurements for high-resolution regional-scale tomography: application to North America

  Yoshizawa K., Ekström Göran

  Geophysical Journal International 183 3 1538 - 1558 Oxford University Press 2010年12月 [査読有り][通常論文]
   

  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.
• 3D upper mantle structure beneath Japan and its surrounding region from inter-station dispersion measurements of surface waves

  K. Yoshizawa, K. Miyake, K. Yomogida

  Physics of the Earth and Planetary Interiors 183 1-2 4 - 19 2010年11月 [査読有り]
  
• Upper boundary of the Pacific plate subducting beneath Hokkaido, Japan, estimated from ScSp phase

  Kinue Osada, Kazunori Yoshizawa, Kiyoshi Yomogida

  PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 1-2 63 - 72 2010年11月 [査読有り][通常論文]
   

  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.
• Upper mantle structure of marginal seas and subduction zones in northeastern Eurasia from Rayleigh wave tomography

  E. Bourova, K. Yoshizawa, K. Yomogida

  PHYSICS OF THE EARTH AND PLANETARY INTERIORS 183 1-2 20 - 32 2010年11月 [査読有り][通常論文]
   

  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.
• Anisotropic structures of the upper mantle beneath the northern Philippine Sea region from Rayleigh and Love wave tomography

  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 2010年11月 [査読有り][通常論文]
   

  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.
• Seismic structure of the upper mantle beneath the Philippine Sea from seafloor and land observation: Implications for mantle convection and magma genesis in the Izu-Bonin-Mariana subduction zone

  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 2009年02月 [査読有り][通常論文]
   

  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.
• Steps in lithospheric thickness within eastern Australia, evidence from surface wave tomography

  S. Fishwick, M. Heintz, B. L. N. Kennett, A. M. Reading, K. Yoshizawa

  TECTONICS 27 4 TC4009  2008年08月 [査読有り][通常論文]
   

  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.
• 北朝鮮核実験による地震波形記録

  吉澤和範

  北海道大学地球物理学研究報告 71 39-48 - 48 北海道大学大学院理学研究院自然史科学部門（地球物理学） 2008年03月 [査読無し][通常論文]
   

  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.
• Shear wave speed structure beneath the South Pacific superswell using broadband data from ocean floor and islands

  Takehi Isse, Daisuke Suetsugu, Hajime Shiobara, Hiroko Sugioka, Kazunori Yoshizawa, Toshihiko Kanazawa, Yoshio Fukao

  GEOPHYSICAL RESEARCH LETTERS 33 16 L16303 - doi:10.1029/2006GL026872 2006年08月 [査読有り][通常論文]
   

  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.
• Three-dimensional shear wave structure beneath the Philippine Sea from land and ocean bottom broadband seismograms

  Isse Takehi, Yoshizawa Kazunori, Shiobara Hajime, Shinohara Masanao, Nakahigashi Kazuo, Mochizuki Kimihiro, Sugioka Hiroko, Suetsugu Daisuke, Oki Satoko, Kanazawa Toshihiko, Suyehiro Kiyoshi, Fukao Yoshio

  Journal of Geophysical Research 111 B6 B06310 - doi:10.1029/2005JB003750 2006年06月 [査読有り]
   

  [1] We obtained three-dimensional (3-D) shear wave speed structure beneath the Philippine Sea and the surrounding region from seismograms recorded by land-based and long-term broadband ocean bottom seismographic stations. The ocean bottom data gave us a better station coverage to obtain a higher spatial resolution (about 300–400 km) in the Philippine Sea than in previous studies. We employed a new technique of surface wave tomography, in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of finite frequency and ray bending. There is a sharp speed contrast along the Izu-Bonin-Mariana trench, across which the Philippine Sea side has a significantly slower upper mantle than the Pacific Ocean side. In the upper 120 km, the shear wave speed structure is well correlated with the age of the provinces. At depths greater than 160 km, the pattern is dominated by fast anomalies of the subducted slabs of the Pacific plate and two slow anomalies to the south of the Daito ridge and in the southernmost part of the Philippine Sea.
• 2004年スマトラ島沖大地震で発生した地球周回表面波

  吉澤和範

  北海道大学地球物理学研究報告 69 15-21 - 21 北海道大学大学院理学研究科地球惑星科学専攻（地球物理学） 2006年03月 [査読無し][通常論文]
   

  The great Sumatra earchquake 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 analyse 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 smaal signals up to R18.
• Sensitivity kernels for finite-frequency surface waves

  K Yoshizawa, BLN Kennett

  GEOPHYSICAL JOURNAL INTERNATIONAL 162 3 910 - 926 2005年09月 [査読有り][通常論文]
   

  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.
• Seismic constraints on temperature of the Australian uppermost mantle

  S Goes, FJ Simons, K Yoshizawa

  EARTH AND PLANETARY SCIENCE LETTERS 236 1-2 227 - 237 2005年07月 [査読有り][通常論文]
   

  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.
• 表面波インバージョンによる上部マントル内部の推定—幾何学的波線から有限幅波線への発展—

  吉澤和範

  地震 57 3 393 - 408 2005年02月 [査読有り][招待有り]
  
• Multimode surface wave tomography for the Australian region using a three-stage approach incorporating finite frequency effects.

  YOSHIZAWA K, KENNETT B L N

  J Geophys Res 109 B2 B02310.1-B02310.19  2004年02月 [査読有り][通常論文]
  
• New report charts course for future of geosciences

  By Sam Adlen, Emily E. Brodsky, Taikan Oki, Aaron J. Ridley, Laura Sanchez, Claudia Simionato, Kazunori Yoshizawa, Uri Shamir

  Eos 85 3 25 - 31 2004年01月20日 [査読有り][通常論文]
  
• 有限波長効果を考慮した新しい表面波トモグラフィー法の探究

  吉澤和範

  月刊地球 25 7 524-529 - 529 海洋出版 2003年07月 [査読無し][招待有り]
  
• A reappraisal of regional surface wave tomography

  BLN Kennett, K Yoshizawa

  GEOPHYSICAL JOURNAL INTERNATIONAL 150 1 37 - 44 2002年07月 [査読有り][通常論文]
   

  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.
• Determination of the influence zone for surface wave paths

  K Yoshizawa, BLN Kennett

  GEOPHYSICAL JOURNAL INTERNATIONAL 149 2 440 - 453 2002年05月 [査読有り][通常論文]
   

  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.
• Non-linear waveform inversion for surface waves with a neighbourhood algorithm - application to multimode dispersion measurements

  K Yoshizawa, BLN Kennett

  GEOPHYSICAL JOURNAL INTERNATIONAL 149 1 118 - 133 2002年04月 [査読有り][通常論文]
   

  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.
• Development and application of new techniques for surface wave tomography

  Kazunori Yoshizawa

  PhD thesis, Australian National University 1 - 204 2002年03月 [査読有り]
  
• Resolving power of surface wave polarization data for higher-order heterogeneities

  K Yoshizawa, K Yomogida, S Tsuboi

  GEOPHYSICAL JOURNAL INTERNATIONAL 138 1 205 - 220 1999年07月 [査読有り][通常論文]
   

  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.

書籍

• 図説地球科学の事典

  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 (担当:分担執筆範囲:7.8章 大陸マントル [吉澤和範])
  朝倉書店 2018年 (ISBN: 9784254160727)
• 地球の物理学事典

  本多了, 岩森光, 歌田久司, 大久保修平, 栗田敬, 土屋卓久, 中井俊一, 平賀岳彦, 宮武隆, 吉澤和範 (担当:共訳範囲:16-17章)
  朝倉書店 2013年07月
• 自然科学実験

  北海道大学自然科学実験編集委 (担当:分担執筆範囲:地学第5章：地震計で測る大地の震動)
  学術図書出版社 2012年

講演・口頭発表等

• Eikonal tomography of North American upper mantle using multi-mode surface waves  [通常講演]

  Kazunori Yoshizawa, Hitoshi Matsuzawa, Fan-Chi Lin

  IAGA-IASPEI Joint Scientific Assembly 2021 2021年08月 ポスター発表
• The influence of multi-scale stochastic heterogeneity on surface-wave phase speeds and apparent radial anisotropy  [通常講演]

  Yunao Xu, Kazunori Yoshizawa, Takashi Furumura

  IAGA-IASPEI Joint Scientific Assembly 2021 2021年08月 口頭発表（一般）
• Radially anisotropic 3-D shear wave structure of the upper mantle beneath North America by multi-mode eikonal tomography  [通常講演]

  Hitoshi Matsuzawa, Kazunori Yoshizawa, Fan-Chi Lin

  JpGU Meeting 2021 2021年06月 口頭発表（一般）
• Three dimensional shear wave structure in the upper mantle beneath the oldest Pacific plate  [通常講演]

  Takehi Isse, Hitoshi Kawakatsu, Shiobara Hajime, Nozomu Takeuchi, Hiroko Sugioka, YoungHee Kim, Hisashi Utada, Sang-Mook Lee, Kazunori Yoshizawa

  AGU Fall Meeting 2020 2020年12月
• Radially anisotropic 3-D S-wave model using multi-mode surface waves: Comparisons of linearized and nonlinear Bayesian approaches  [通常講演]

  Kazunori Yoshizawa, Toru Taira

  AGU Fall Meeting 2020 2020年12月 ポスター発表
• Multi-mode phase speed distribution in North America using eikonal tomography  [通常講演]

  Hitoshi Matsuzawa, Kazunori Yoshizawa, Fan-Chi Lin

  AGU Fall Meeting 2020 2020年12月 ポスター発表
• Multi-mode surface wave tomography with trans-dimensional hierarchical Bayesian inversion: application to Australia  [通常講演]

  吉澤 和範, 平 亨

  JpGU-AGU Joint Meeting 2020 2020年07月 ポスター発表
• Mapping azimuthal anisotropy in the Australasian upper mantle with multi-mode surface waves  [通常講演]

  吉澤 和範, 西村 祐香

  JpGU-AGU Joint Meeting 2020 2020年07月 ポスター発表
• Multi-mode surface wave phase speed distribution in North America  [通常講演]

  松澤 仁志, 吉澤 和範, Lin Fan-Chi

  JpGU-AGU Joint Meeting 2020 2020年07月 ポスター発表
• Radially Anisotropic S-wave Structure of Australian Upper Mantle: Trans-dimensional Hierarchical Bayesian Inversions of Receiver Functions and Multi-mode Surface Waves  [通常講演]

  Toru Taira, Kazunori Yoshizawa

  American Geophysical Union 2019 Fall Meeting 2019年12月 ポスター発表
• モード分離波形を用いたマルチモード表面波の位相速度分布：eikonal tomographyへの適用  [通常講演]

  松澤 仁志, 吉澤 和範, Fan-Chi LIn

  日本地震学会秋季大会 2019年09月 ポスター発表
• マルチモード表面波による上部マントル異方的構造の推定：オーストラリア周辺域への応用  [通常講演]

  西村 祐香, 吉澤 和範

  日本地震学会秋季大会 2019年09月 口頭発表（一般）
• ラミナ状ランダム不均質性の表面波伝播と見かけの異方性への影響  [通常講演]

  徐 云遨, 吉澤 和範, 古村 孝志

  日本地震学会秋季大会 2019年09月 口頭発表（一般）
• 線形および非線形インバージョンによる大陸マントルの3次元異方的S波速度構造モデル比較  [通常講演]

  吉澤 和範, 平 亨

  日本地震学会秋季大会 2019年09月 ポスター発表
• Multi-mode Phase Speed Measurements of Surface Waves with a Hybrid Array-based Method: Application to USArray  [通常講演]

  H. Matsuzawa, K. Yoshizawa

  IUGG General Assembly 2019年07月 ポスター発表
• Trans-dimensional Bayesian Inversions for Australian Lithosphere and Asthenosphere Using Multi-mode Surface Waves and Receiver Functions  [通常講演]

  T. Toru, K. Yoshizawa

  IUGG General Assembly 2019年07月 ポスター発表
• Radial and Azimuthal Anisotropy in the Australian Upper Mantle: Tectonic Implications for Tasman Line and Suture Zone  [通常講演]

  Y. Nishimura, K. Yoshizawa, B.L.N. Kennett

  IUGG General Assembly 2019年07月 ポスター発表
• Quantitative Estimation of Apparent Radial Anisotropy in the Lithosphere and Asthenosphere: Effects of Multi-scale Heterogeneity  [通常講演]

  K. Yoshizawa, Y. Xu, T. Fukumura

  IUGG General Assembly 2019年07月 ポスター発表
• リソスフェア・アセノスフェアのランダム微細不均質性とみかけの鉛直異方性  [通常講演]

  吉澤 和範

  日本地震学会2018年秋季大会 2018年10月 口頭発表（一般）
• Mapping the upper mantle beneath North American continent with joint inversion of surface-wave phase and amplitude  [通常講演]

  K. Yoshizawa, K. Hamada

  American Geophysical Union 2017 Fall Meeting 2017年12月 口頭発表（一般）
• 地震波トモグラフィーとテクトニクス  [招待講演]

  吉澤 和範

  地震学夏の学校 2017年09月 公開講演，セミナー，チュートリアル，講習，講義等
• A seismological constraint on the asthenosphere: mapping radial anisotropy with multi-mode surface waves  [招待講演]

  YOSHIZAWA Kazunori

  JpGU-AGU Joint Meeting 2017 2017年05月 口頭発表（招待・特別）
• Lithosphere-asthenosphere transition, mid-lithosphere discontinuity and radial anisotropy from multi-mode surface wave tomography  [招待講演]

  YOSHIZAWA Kazunori

  American Geophysical Union 2015 Fall Meeting 2015年12月 口頭発表（招待・特別）
• Mapping the upper mantle with multi-mode surface waves: Lithosphere-ashtenosphere transition and radial anisotropy  [招待講演]

  YOSHIZAWA Kazunori

  The 14th International Symposium on Study of the Earth's Deep Interior 2014年08月 口頭発表（招待・特別） the Shonan Village Center, Kanagawa, Japan
• The Lithosphere-Asthenosphere Transition and Radial Anisotropy Beneath Australian Continent  [招待講演]

  YOSHIZAWA Kazunori

  Asia and Oceania Geoscieces Society 11th Annual Meeting 2014年07月 口頭発表（招待・特別） Royton Sapporo Hotel, Japan
• Radially anisotropic shear wave model of North America from multi- mode surface tomography using USArray  [招待講演]

  YOSHIZAWA Kazunori

  JpGU Meeting 2010 2010年05月 口頭発表（招待・特別）

その他活動・業績

• 遠地P波の逆投影解析によるフンガトンガ・フンガハアパイ火山の噴火シーケンス:大気-固体振動モードと噴火サイクルの関係

  吉澤和範, 吉澤和範, 垂水洸太郎 日本地震学会秋季大会講演予稿集(CD-ROM) 2022 2022年
• マルチモード表面波とレシーバ関数を用いた豪州大陸下の上部マントル不連続面

  垂水洸太郎, 吉澤和範, 吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2022 2022年
• 線形および非線形インバージョンによる大陸マントルの3次元異方的S波速度構造モデルの比較

  吉澤和範, 吉澤和範, 平亨 日本地震学会秋季大会講演予稿集(CD-ROM) 2019 2019年
• マルチモード表面波による上部マントル異方的構造の推定:オーストラリア周辺域への応用

  西村祐香, 吉澤和範, 吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2019 2019年
• オントンジャワ海台の上部マントルS波速度構造

  一瀬建日, 末次大輔, 塩原肇, 杉岡裕子, 伊藤亜妃, 石川晃, 川野由貴, 吉澤和範, 石原靖, 田中聡, 大林政行, 利根川貴志, 吉光淳子, 小林拓史 日本地震学会秋季大会講演予稿集(CD-ROM) 2019 2019年
• モード分離波形を用いたマルチモード表面波の位相速度分布:eikonal tomographyへの適用

  松澤仁志, 吉澤和範, LIN Fan-Chi 日本地震学会秋季大会講演予稿集(CD-ROM) 2019 2019年
• アレイ解析による北米大陸のマルチモード表面波位相速度分布

  松澤仁志, 吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2018 2018年
• η_κを用いた太平洋及びオーストラリア周辺域の上部マントル3次元鉛直異方性構造

  奥山秀弥, 吉澤和範, 川勝均, 一瀬健日 日本地震学会秋季大会講演予稿集(CD-ROM) 2018 2018年
• マルチモード表面波を用いたオーストラリア周辺域の上部マントル方位異方性

  西村祐香, 吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2018 2018年
• リソスフェア・アセノスフェアのランダム微細不均質性とみかけのS波鉛直異方性

  吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2018 2018年
• Trans-dimensional Bayesian inversionによる大陸域の地殻-上部マントル速度構造推定

  平亨, 吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2018 2018年
• 新しい異方性パラメータη_κの表面波位相速度インバージョンへの影響

  奥山秀弥, 吉澤和範, 川勝均 日本地震学会秋季大会講演予稿集(CD-ROM) 2017 2017年
• アレイ解析による表面波のモード波形分離法の開発

  松澤仁志, 吉澤和範 日本地震学会秋季大会講演予稿集(CD-ROM) 2017 2017年
• 震源間の位相速度解析によるローカルスケールの表面波位相速度分布の推定

  吉澤和範, GINBOI Kuria 日本地震学会秋季大会講演予稿集(CD-ROM) 2017 2017年
• アレイ解析によるマルチモード表面波の位相速度計測

  松澤仁志, 吉澤和範 日本地震学会秋季大会講演予稿集 2016 2016年
• 表面波二点法解析による位相・振幅比データを用いた北米大陸下の3次元S波速度構造

  浜田広太, 吉澤和範 日本地震学会秋季大会講演予稿集 2016 2016年
• 広帯域アレイを用いた日本海下の上部マントル3次元S波速度構造

  成田涼, 浜田広太, 吉澤和範, 川勝均 日本地震学会秋季大会講演予稿集 2016 2016年
• リソスフェア内のランダム不均質性とみかけの鉛直異方性:表面波位相速度への影響

  吉澤和範 日本地震学会秋季大会講演予稿集 2016 2016年
• 大陸域におけるリソスフェア内不連続面と鉛直異方性

  吉澤和範 日本地震学会秋季大会講演予稿集 2015 2015年
• 2観測点間の表面波の位相及び振幅比のジョイントインバージョンによる北米大陸下の上部マントル構造

  浜田広太, 吉澤和範 日本地震学会秋季大会講演予稿集 2015 2015年
• 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年08月 [査読無し][招待有り]
  
• 上部マントルS波鉛直異方性に対する地震表面波の分解能

  吉澤和範 日本地震学会秋季大会講演予稿集 2014 2014年
• 2観測点間の表面波振幅比を用いた北米大陸の位相速度分布

  浜田広太, 吉澤和範 日本地震学会秋季大会講演予稿集 2014 2014年
• 非線形波形フィッティングを用いた表面波振幅解析による北米大陸の減衰構造

  浜田広太, 吉澤和範 日本地震学会秋季大会講演予稿集 2013 2013年
• 豪州大陸下のリソスフェア-アセノスフェア境界の地震学的検出

  吉澤和範, KENNETT B. L. N. 日本地震学会秋季大会講演予稿集 2013 2013年
• USArrayを用いた北米大陸の減衰構造:2点法による表面波振幅解析

  浜田広太, 吉澤和範 日本地震学会秋季大会講演予稿集 2012 2012年
• マルチモード表面波を用いたリソスフェア-アセノスフェア境界の推定

  吉澤和範 日本地震学会秋季大会講演予稿集 2012 2012年
• マルチモード表面波による豪州及び北米大陸の3次元上部マントル構造の復元

  吉澤和範, KENNETT B.L.N., EKSTROEM G. 日本地震学会秋季大会講演予稿集 2011 2011年
• 2011年東北沖巨大地震が明らかにした巨大地震の多様性

  小山順二, 吉澤和範, 蓬田清, 都築基博 日本地震学会秋季大会講演予稿集 2011 2011年
• 2011年東北地方太平洋沖地震における二重セグメント:その特異性と他の超巨大地震との比較

  蓬田清, 小山順二, 吉澤和範, 都筑基博 日本地震学会秋季大会講演予稿集 2011 2011年
• マルチモード表面波の自動位相速度計測:大陸地域への応用

  吉澤和範 日本地震学会秋季大会講演予稿集 2010 2010年
• 「地震学夏の学校2007」総合報告

  吉澤和範, 青山裕, 高橋浩晃 日本地震学会ニュースレター 19 (4) 9 -13 2007年 [査読無し][通常論文]
  
• 日本列島周辺域の表面波方位異方性分析

  吉澤和範, LITVINA Ekaterina 日本地震学会秋季大会講演予稿集 2006 2006年
• 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年 [査読無し][招待有り]
  
• Societal Impact

  Yoshizawa K IUGG report "Geosciences: The Future" 95 -100 2003年07月 [査読有り][招待有り]
  
• Seismology and Physics of the Earth's Interior

  Yoshizawa K IUGG report "Geosciences: The Future" 11 -21 2003年07月 [査読有り][招待有り]
  
• 長周期P波の偏向異常 : 高速度スラブの効果?

  吉澤 和範, 蓬田 清 日本地震学会講演予稿集 = Programme and abstracts, the Seismological Society of Japan 1998 (2) 1998年10月09日
• Rayleigh波偏向面異常による位相速度分布 : 観測点方位補正の効果

  吉澤 和範, 蓬田 清, 坪井 誠司 日本地震学会講演予稿集 = Programme and abstracts, the Seismological Society of Japan 1997 (2) 1997年09月15日

受賞

• 2007年05月 日本地震学会 若手学術奨励賞
   「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: 吉澤 和範
• 2001年12月 アメリカ地球物理学連合 Outstanding Student Paper Award
   「Surface Wave Tomography: A Three-Stage Approach」

共同研究・競争的資金等の研究課題

• 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)

  研究期間 : 2020年04月 -2023年03月 

  代表者 : 吉澤 和範

   

  本研究では，マルチモード表面波や実体波レシーバ関数等の複数の観測情報を用いて，ベイズ推定に基づく非線形インバージョン法に適用することで，上部マントル内部に存在するMLD (Mid-Lithosphere Discontinuity）やLAB (Lithosphere-Asthenosphere Boundary)，Lehmann面等の主要な不連続面の空間分布を含む，高精度なリソスフェア－アセノスフェア構造の推定法の確立を目指している．本年度はその基礎となるP波レシーバ関数とマルチモード表面波の同時インバージョンを行う手法を豪州大陸の複数観測点に適用した結果を国際学術誌に発表した．この手法により，豪州の安定大陸下に複数のMLDが観測されると共に，LABやその下のレーマン面（アセノスフェアの底に相当すると考えられる）も検出された．今後，より高密度な観測網に適用するすることで，高精度な空間マッピングへの実用化が期待される．また，ベイズ推定法による表面波トモグラフィーの研究も行い，既存の線形インバージョンモデルとの比較も行った．ベイズ推定モデルでは，データ誤差の空間変化を考慮することができ，より精度の高い3次元構造解析が可能となることが示された．さらに，S波速度モデルの高精度復元に不可欠なマルチモード表面波の新しい位相速度マッピング法（稠密アレイを用いたeikonal tomography法）の北米大陸への応用も行った．また，上部マントル内のマルチスケールのランダム不均質性が，特に中～長周期表面波の波動場や内部境界面・鉛直異方性の推定に与える影響の定量的検証も進めた．一方，内部不連続面の更なる高精度推定に向け，S波レシーバ関数の活用に向けた波形解析法の基礎研究も行い，次年度以降の応用に向けて準備を進めている．これら一連の研究成果の一部は，国内外の主要学会にて成果発表を行った．
• 数値シミュレーションによるゴンドワナ大陸東縁での大陸分裂のメカニズムの解明

  日本学術振興会：科学研究費助成事業 基盤研究(C)

  研究期間 : 2018年04月 -2021年03月 

  代表者 : 吉田 晶樹, 斎藤 実篤, 吉澤 和範

   

  約1億年前の大陸分布を初期状態とした三次元全球マントル対流数値シミュレーションを実施した。その結果、対流が安定化するにつれてゴンドワナ大陸東縁に沿って広範な沈み込み帯が発達することが分かった。さらに、二次元粘弾塑性・熱組成対流モデルを用いた数値シミュレーションによって、海溝の後退により大陸縁辺域に伸張応力が働く条件下でのリフティングを再現した。大陸の伸張速度等を系統的に変化させて計算した結果、約1600万年以内で大陸が分裂することが分かった。当時のゴンドワナ大陸東縁下に大規模なマントルプルームが存在しなかった可能性も考慮すると、ジーランディア形成の主原因が海溝の後退であったことが支持される。
• 微細不均質性を考慮したリソスフェア－アセノスフェアの高精度イメージングの研究

  日本学術振興会：科学研究費助成事業 基盤研究(C)

  研究期間 : 2017年04月 -2020年03月 

  代表者 : 吉澤 和範

   

  本研究ではリソスフェア－アセノスフェアの詳細な構造推定に向け，複数のアプローチでの研究を行った．まず，ラミナ状微細不均質性が中～長周期の表面波に与えるみかけの異方性の影響を，地震波動シミュレーション等を通じて検証し，微細不均質性の水平スケールや強さに応じて，レイリー波速度が低下し，鉛直異方性の推定に影響をすることを示した．また，ベイズ推定に基づく可変次元インバージョン法を開発し，マルチモード表面波と実体波レシーバ関数を同時に用いた上部マントル構造の推定を行った．特に豪州地域の観測点に応用し，リソスフェア内不連続面やリソスフェア－アセノスフェア境界，レーマン面等，複数の不連続面検出にも成功した．
• 太平洋アレイ (Pacific Array)

  日本学術振興会：科学研究費助成事業 挑戦的萌芽研究

  研究期間 : 2015年04月 -2018年03月 

  代表者 : 川勝 均, 塩原 肇, 一瀬 建日, 西田 究, 歌田 久司, 末次 大輔, 杉岡 裕子, 吉澤 和範

   

  「太平洋アレイ」とは，十数台の海底広帯域地震計をアレイ一単位とした，海底地震計アレイによるアレイ観測計画である．技術の革新により，1－2年程度の観測により，一単位アレイ直下の一次元地震波速度構造（異方性も含む）が，海水面から150－200kmの深さ（アセノスフェアの深度まで）まで推定できるようになった．この技術革新により，アレイによるアレイ観測を行うことで，広大な太平洋を効果的にカバーする観測計画の可能性が浮び上ってきた．本研究の最大の成果は，国際協力体制の構築により，観測が開始されるようなったことである．
• 地球物理学的観測によるアジア北東地域のテクトニクスモデルの刷新

  文部科学省：科学研究費補助金(基盤研究(A))

  研究期間 : 2013年 -2017年 

  代表者 : 高橋 浩晃

   

  ロシア極東地域から中国東北部を含むアジア北東地域のテクトニクスの解明を目指し，地震とGNSS観測を実施した．2011年東北地方太平洋沖地震による広域的な余効地殻変動が観測され，ロシア沿海州地方は地震時変動を上回る変位が得られた．ロシア極東地域に展開した広帯域地震観測網のデータから，当該地域の上部マントル地震波速度構造を明らかにし日本海下に低速度異常を確認した．上部マントルの粘弾性構造の推定から，日本列島周辺で繰り返し発生する巨大地震がアジア北東地域に長期的な余効変動を引き起こしてきた事実を明らかにした．また当該地域の特徴的な地震活動を明らかにした．
• 地震波解析によるリソスフェア－アセノスフェア境界の高精度マッピングの研究

  文部科学省：科学研究費補助金(基盤研究(C))

  研究期間 : 2014年 -2016年 

  代表者 : 吉澤 和範

   

  マルチモード表面波から得られる高精度な3次元S波速度モデルを用いて，リソスフェア－アセノスフェア境界の空間分布を推定すると共に，S波鉛直異方性の高精度な復元法について検証した．最新の豪州大陸や太平洋地域の3次元モデルには，リソスフェア直下のアセノスフェア内に顕著なSH>SVとなる鉛直異方性がみられ，高速移動するプレート直下に強いせん断力が影響していることが示された．さらに，レシーバ関数から推定される大陸リソスフェア内部の不連続面（深さ70-90km）の深さが，S波鉛直異方性プロファイルの変曲点と一致することや，内部不連続面と微細不均質性・鉛直異方性との関連性など，新たな知見が得られた．
• マルチモード表面波を用いたリソスフェア－アセノスフェア境界の研究

  文部科学省：科学研究費補助金(若手研究(B))

  研究期間 : 2012年 -2013年 

  代表者 : 吉澤 和範

   

  リソスフェア－アセノスフェア境界（LAB）の3次元的な空間分布を，マルチモード表面波による3次元S波速度分布とその鉛直勾配から推定する手法を開発した．特に高精度な3次元モデルが得られた豪州大陸に応用し，大陸全域のLABの空間分布復元に成功した．さらに，推定されたLAB分布と，3次元異方性分布や実体波レシーバ関数の結果との関係についても詳細な検討を行った．また表面波による3次元速度構造の更なる高精度化に向けた新しい2点間波形解析法の開発も行った．これを特に高密度な観測網が展開されている北米大陸に応用し，従来の1点法に基づくトモグラフィーモデルに比べて，水平分解を格段に向上できることが示された．
• 地球物理学的観測による北東アジア地域の新たなテクトニック・フレームの構築

  文部科学省：科学研究費補助金(基盤研究(A))

  研究期間 : 2009年 -2012年 

  代表者 : 高橋 浩晃
• マルチモード表面波の非線形波形解析による3次元非弾性減衰構造への挑戦

  文部科学省：科学研究費補助金(挑戦的萌芽研究)

  研究期間 : 2010年 -2011年 

  代表者 : 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,...
• 広帯域地震観測によるスタグナントスラブの微細構造の解明

  文部科学省：科学研究費補助金(特定領域研究)

  研究期間 : 2004年 -2008年 

  代表者 : 笠原 稔, 蓬田 清, 宮町 宏樹, 谷岡 勇市郎, 高橋 浩晃, 吉澤 和範, セン ラクス, ボルモトフ ウラジミール

   

  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。
• 表面波の有限波長トモグラフィー法を用いた3次元上部マントル異方性構造の研究

  文部科学省：科学研究費補助金(若手研究(B))

  研究期間 : 2005年 -2006年 

  代表者 : 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...
• 短周期マルチモード表面波の散乱・回折効果を考慮した次世代トモグラフィー法の研究

  文部科学省：科学研究費補助金(若手研究(B))

  研究期間 : 2003年 -2004年 

  代表者 : 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...
• 表面波偏向面異常を用いた波形インバージョンによる上部マントル3次元構造

  日本学術振興会：科学研究費助成事業 特別研究員奨励費

  研究期間 : 1998年 -1998年 

  代表者 : 吉澤 和範

教育活動情報

主要な担当授業

大学運営

委員歴

• 2021年04月 - 現在   日本地震学会   IASPEI委員会
• 2021年01月 - 現在   日本学術会議   地球惑星科学委員会IUGG分科会IASPEI小委員会
• 2010年04月 - 現在   日本地震学会   代議員
• 2021年04月 - 2023年03月   日本地震学会   大会・企画委員会
• 2018年01月 - 2022年08月   Earth, Planets and Space   編集委員
• 2018年04月 - 2020年03月   東京大学地震研究所   共同利用委員会委員
• 2016年 - 2020年   日本地球惑星科学連合   広報普及委員会委員
• 2013年 - 2020年   日本地球惑星科学連合   地球内部科学小委員会委員
• 2014年 - 2017年   日本地震学会   表彰委員会委員
• 2005年 - 2009年   日本地震学会   代議員
• 2002年 - 2008年   日本地震学会   地方連絡委員   日本地震学会
• 2002年 - 2003年   国際測地学・地球物理学連合 (IUGG)   将来検討ワーキンググループメンバー(IASPEI代表)   国際測地学・地球物理学連合

学術貢献活動

• 日本地震学会2022秋季大会

  期間 : 2022年10月23日 - 2022年10月26日

  役割 : 企画立案・運営等

  種別 : 大会・シンポジウム等

  主催者・責任者 : 日本地震学会
• 地震学夏の学校2007世話人（代表）

  期間 : 2007年09月

  役割 : 企画立案・運営等

  種別 : 学会・研究会等
• IUGG Working Group (Geosciences: The Future) Member (IASPEI representative)

  期間 : 2002年02月 - 2003年07月

  役割 : 企画立案・運営等

  種別 : 審査・学術的助言

  主催者・責任者 : International Union of Geodesy and Geophysics (IUGG)