Faculty Profiles - WATANABE Toshiki

Papers - WATANABE Toshiki

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Surface wave monitoring using ambient noise for detecting temporal variations in underground structures in landslide area Reviewed

Feng C., Yamaoka K., Ikuta R., Watanabe T., Tsuji S.

Engineering Geology Vol. 341 2024.9

Application of machine learning methods for earthquake detection from high-density temporary observation seismic records on a volcanic island Reviewed

Azuma H., Kunimasa H., Kusumo A. W., Oda Y., Watanabe T., Matsuoka T.

Journal of Applied Geophysics Vol. 230 2024.9

Electrical resistivity tomography combined with seismic data estimates heterogeneous distribution of near‑seafloor concentrated gas hydrates within gas chimneys Reviewed

Keiichi Ishizu, Ayako Oda, Tada-nori Goto, Takafumi Kasaya, Toshiki Watanabe Hideaki Machiyama

Scientific Reports Vol. 14:15045 2024.7

Shallow S-wave velocity estimation using microtremor observations for liquefaction prediction Reviewed

Sato K., Watanabe T.

6th Asia Pacific Meeting on Near Surface Geoscience and Engineering: Smart Technologies Kind to the Planet 2024.6

Three-dimensional structure of the Omiya and the Iriyamase faults and its formation process, the eastern margin of the Fujigawa-kako fault zone, the northwestern border of the Izu collision zone, central Japan Invited Reviewed

Ito, T., Kano, K., Sato, G., Fujiwara, A., Watanabe, T., Higahinaka, M., Abe, S., Abe, S., Tsumura. N., Yamamoto, G., Sato, H., Iwasaki, T., Takeda, T., Ishiyama, T., Odawara, K., Harada, M., and Komori, J.

Journal of Geography (Chigaku Zasshi) Vol. 133 ( 2 ) page： 101 - 128 2024.4

Crustal structure of the northwestern border of the Izu collision zone from the Fujikawa-kako fault zone to the Itoigawa‒Shizuoka tectonic line, central Japan: Outcomes of integrated seismic exploration FIST2012 Invited Reviewed

Fujiwara, A., Watanabe, T., Higahinaka, M., Abe, S., Ito, T., Kano, K., Sato, G., Abe, S., Tsumura. N., Iwasaki, T., Takeda, T., Yamamoto, G., Sato, H., Ishiyama, T., Odawara, K., Harada, M., and Komori, J.

Journal of Geography (Chigaku Zasshi) Vol. 133 ( 1 ) page： 23 - 48 2024.2

Seismic reflection imaging of deep crustal structures using local earthquakes in the Kanto region, Japan Reviewed

Kazuya Shiraishi, Toshiki Watanabe

Earth, Planets and Space Vol. 75 ( 1 ) 2023.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Nature

We applied a novel method of passive seismic reflection imaging to actual local earthquake data collected by a dense seismic network in the Kanto region, Japan. This method, which implements reverse time migration (RTM), is based on the cross-correlation of wavefields that are extrapolated forward and backward in time from receiver locations using passively observed seismic records. Using multiple reflections between the Earth’s surface and subsurface boundaries, internal structures are imaged using many earthquakes without well-defined source information. The objective of this case study is to evaluate the possibility of acquiring seismic reflection images of the deep crustal structure by applying the RTM-based method using P-wave reflections in the earthquake data collected by a dense seismic network. The P-wave reflection profile along a 191-km-long pseudo-survey line down to a 100 km depth is obtained using the seismic records of hundreds of local earthquakes observed at 72 receiver stations. A P-wave velocity model for RTM imaging is extracted from an existing 3D model obtained by seismic tomography in a previous study. The resulting image shows several continuous reflectors at depths of 15–70 km. These reflectors correspond to the spatially variable velocity and suggest deep structures related to dual plate subduction in this region. Two eastward-dipping reflectors imaged at depths of 15–50 km are likely the top and bottom surfaces of the crust of the Philippine Sea slab, and the westward-dipping reflector at depths of 50–70 km implies the top surface of the Pacific slab. The en-échelon reflectors at depths of 15–20 km may be reflective boundaries between the upper and lower crust in the overlying Okhotsk plate. Our case study results confirm the possibility of obtaining profiles at higher resolutions than are typically obtained by earthquake-based seismic tomography and of imaging at depths beyond the limits of artificially controlled-source seismic surveys. Further implementation of the RTM-based imaging method will improve its potential use for subsurface imaging and monitoring from dense passive seismic data. Graphical Abstract: [Figure not available: see fulltext.].

DOI： 10.1186/s40623-023-01772-0

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Seismic structure and its implication on the hydrothermal system beneath Mt. Ontake, central Japan Reviewed

Yuta Maeda, Toshiki Watanabe

Earth, Planets and Space Vol. 75 ( 1 ) 2023.12

Language：English Publishing type：Research paper (scientific journal)

Mt. Ontake is an active volcano in central Japan where phreatic eruption activity is prominent. A shallow subsurface structure in the summit region of this volcano has been scarcely studied despite its importance. Our study examines the structure from three seismic analyses: a typical P-wave velocity from the semblance of the vertical propagation of intermediate-depth earthquakes (IDEs), a layered velocity model from P- and S-wave arrival times of shallow volcanic earthquakes, and a pseudoreflection profile from the autocorrelation functions of the IDEs. Our results consistently indicate the presence of three layers, which are interpreted as younger (> 0.1 Ma) and older (0.39–0.78 Ma) eruptive deposits and the basement. A comparison of the structure with hypocentres and deformation sources suggests fluid migration controlled by the structure. Graphical Abstract: [Figure not available: see fulltext.].

DOI： 10.1186/s40623-023-01870-z

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Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan Reviewed

Hiroki Miyamachi, Hiroshi Yakiwara, Reiji Kobayashi, Shuichiro Hirano, Takeshi Kubo, Masakazu Souda, Kenyu Sakao, Naohiro Unno, Takeshi Matsushima, Kazunari Uchida, Rintaro Miyamachi, Kenshin Isoda, Yoshiko Teguri, Yoshinosuke Kamiya, Agnis Triahadini, Hiroshi Shimizu, Hiroshi Katao, Takuo Shibutani, Takeshi Tameguri, Yusuke Yamashita, Tsutomu Miura, Jun Nakagawa, Itaru Yoneda, Shinya Kato, Kosei Takishita, Kazuho Nakai, Yuta Maeda, Toshiki Watanabe, Shinichiro Horikawa, Kenjiro Matsushiro, Takashi Okuda, Shuhei Tsuji, Naoki Sogawa, Daima Hasegawa, Kazuo Nakahigashi, Eiji Kurashimo, Tomoaki Yamada, Hideji Abe, Miwako Ando, Shinichi Tanaka, Satoshi Ikezawa, Takaya Iwasaki, Masanao Shinohara, Toshinori Sato, Mare Yamamoto, Ryosuke Azuma, Satoshi Hirahara, Takashi Nakayama, Syuichi Suzuki, Shuhei Otomo, Ryota Hino, Tomoki Tsutsui, Yusuke Inoue, Ryuichi Takei, Yuya Tada, Hiroaki Takahashi, Yoshio Murai, Hiroshi Aoyama, Mako Ohzono, Takahiro Shiina, Masamitsu Takada, Masayoshi Ichiyanagi, Teruhiro Yamaguchi, Natsuki Ono, Kazuma Saito, Chihiro Ito, Yuuki Susukida, Tatsuya Nakagaki, Yasuhisa Tanaka, Yasuhiko Akinaga

Earth, Planets and Space Vol. 75 ( 1 ) 2023.12

Publishing type：Research paper (scientific journal)

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-023-01919-z

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Waveform inversion of the ultra-long-period seismic event associated with ground tilt motion during an eruption of Mount Kusatsu–Shirane, Japan, on January 23, 2018 Reviewed

Ryohei Takahashi, Yuta Maeda, Toshiki Watanabe

Earth, Planets and Space Vol. 74 ( 1 ) page： 85 2022.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Nature

We conducted waveform inversions of an ultra-long-period (~ 240-s) event associated with the phreatic eruption of Mount Kusatsu–Shirane on January 23, 2018. We used broadband seismic and tilt records from three stations surrounding the eruption site. The horizontal components of the broadband seismic records were severely contaminated by tilt motions. We applied a waveform inversion algorithm to account for both the translational and tilt motions. To reduce the number of free parameters, we assumed a tensile crack source and conducted grid searches for the centroid location and orientation of the crack. The results showed a rapid inflation of 105 m3 of the crack, followed by a slow deflation starting 8–11 s prior to the onset of the eruption. The source location and crack orientation were not uniquely determined. The most likely source is a north–south-opening sub-vertical crack near the eruptive craters. This ultra-long-period event may represent volcanic fluid migration from depth to the surface through a vertical crack during the eruption. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01644-z

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Estimating errors in autocorrelation functions for reliable investigations of reflection profiles Reviewed

Yuta Maeda, Toshiki Watanabe

Earth, Planets and Space Vol. 74 ( 1 ) page： 48 2022.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Nature

Autocorrelation functions (ACFs) of vertically incident seismic waves are used to image subsurface reflectors. However, the reflection responses derived from ACFs usually contain many false signals. We present a method to quantify the errors in ACFs and extract true reflectors with high reliability. We estimated the errors for each earthquake at each station as follows. We calculated the amplitude of the observed waveform within the noise window and generated 1000 random noise traces that have this amplitude. By subtracting the random noise traces from the observed waveform, we created 1000 candidate earthquake waveforms. We computed the ACF for each of the 1000 waveforms and calculated the ensemble average and standard deviation of the 1000 different ACF amplitudes at each lag time. Then, we applied weighted stacking to the ACFs of many earthquakes to obtain the reflection response at the station. We calculated the standard deviation of the weighted stack to estimate errors in the reflection response. We evaluated the method by applying it to seismic data from the metropolitan area of Japan. The subsurface structure of the study area has been studied extensively and consists of a strong velocity discontinuity between sedimentary and basement layers. Following our method, the discontinuity was imaged as a clear reflector with an amplitude that was substantially greater than three times the standard deviation, which corresponds to statistical significance at the 99% confidence level. At other depths where reflectors are not expected to be present, the amplitudes of the peaks were less than or close to three times the standard deviation. The signal of the discontinuity was clearly visible at frequencies below 10 Hz and was less prominent at higher frequencies. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01606-5

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Passive seismic reflection imaging based on acoustic and elastic reverse time migration without source information: theory and numerical simulations Reviewed

Kazuya Shiraishi, Toshiki Watanabe

Exploration Geophysics Vol. 53 ( 2 ) page： 198 - 210 2022

Language：English Publishing type：Research paper (scientific journal) Publisher：Taylor and Francis

To explore crustal structures using seismic records of earthquakes, we proposed a new seismic reflection imaging method based on reverse time migration (RTM) without source information. In the proposed RTM method using multiple reflections between surface and subsurface boundaries, we correlate forward- and backward-extrapolated wavefields of the same observation records from all receivers at once. We implemented acoustic RTM based on a scalar wave equation and elastic RTM based on a velocity-stress formulation for wavefield modelling. Furthermore, we adopted scalar and vector potential formulations in elastic RTM to obtain separate P- and S-wave reflection images. In our study, numerical simulations with a regional crustal structure model were conducted to compare combinations among the synthetic data by acoustic or elastic wave modelling, observation conditions, and RTM formulations. We demonstrated the validity of the proposed method with idealised dense observations and confirmed a limitation due to quasi-realistic sparse observations. Artefacts due to the mixture of P- and S-waves in the acoustic RTM images from elastic synthetic data were attenuated by elastic RTM, which addresses the proper illumination of P- and S-wave reflections.

DOI： 10.1080/08123985.2021.1917293

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Active-source Seismic Exploration of Fuji Volcano in 2003 Reviewed

Oikawa, J., Tanaka, S., Kagiyama, T., Miyamachi, H. Tsutsui, T., Ikeda, Y., Katayama, H., Matsuo, N., Oshima, H., Nishimura, Y., Keigo Yamamoto, K., Watanabe, T. and Yamazaki, F.

Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 96 page： 1 - 10 2021.12

Ground water-induced changes in velocities of P and S waves (Vp and Vs) measured using an accurately controlled seismic source Reviewed

Rina Suzuki, Koshun Yamaoka, Shuhei Tsuji, Toshiki Watanabe

Earth, Planets and Space Vol. 73 ( 1 ) 2021.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Nature

We analyze temporal variations in the travel times of both P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could efficiently produce both P and S waves. Seismic waves produced by the source, which is called the accurately controlled routinely operated signal system (ACROSS), are recorded by three nearby seismic stations, and the travel time variation at each station is estimated using the transfer function calculated from the recorded data. Long-term variations in Vp and Vs are observed and can be interpreted by the change in fluid saturation and crack density of the subsurface rocks. The variation in fluid saturation and crack density are consistent with that in the groundwater level, which is measured at the station nearest to the ACROSS. Short-term responses to rainfalls are observed at the station nearest to the ACROSS system; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation, partly owing to the initial response to rainfall. This can be interpreted as an air–water mixture within pores or cracks on a fine scale.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-021-01484-3

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Seismic imaging of deep crustal structures via reverse time migration using local earthquakes Reviewed

Shiraishi, K. and Watanabe, T.

Proceedings of the 14th SEGJ International symposium, Road ahead for geophysics in digital society: Acquisition, processing, and interpretation 2021.10

Urgent Joint Seismic Observation Data of the 2016 Kumamoto Earthquakes

SHITO Azusa, MITSUOKA Ayaho, MATSUMOTO Satoshi, MATSUSHIMA Takeshi, AIZAWA Koki, SHIMIZU Hiroshi, UCHIDA Kazunari, KAMIZONO Megumi, TEGURI Yoshiko, NAKAMOTO Manami, MIYAMACHI Rintaro, ICHIYANAGI Masayoshi, OHZONO Mako, OKADA Kazumi, KATSUMATA Kei, TAKADA Masamitsu, TAKAHASHI Hiroaki, TANIOKA Yuichiro, YAMAGUCHI Teruhiro, KOSUGA Masahiro, AZUMA Ryosuke, UCHIDA Naoki, EMOTO Kentaro, OHTA Yusaku, OKADA Tomomi, KAIDA Toshiki, KOZONO Tomofumi, SUZUKI Syuichi, TAKAGI Ryota, DEMACHI Tomotsugu, NAKAHARA Hisashi, NAKAYAMA Takashi, HIRAHARA Satoshi, MATSUZAWA Toru, MIURA Satoshi, YAMAMOTO Mare, IMANISHI Kazutoshi, UCHIDE Takahiko, YOSHIMI Masayuki, AOI Shin, ASANO Youichi, UENO Tomotake, FUJITA Eisuke, ABE Eiji, IIDAKA Takashi, IWASAKI Takaya, KATO Aitaro, KURASHIMO Eiji, SAKAI Shin’ichi, SHIINA Takahiro, SERIZAWA Masato, TANAKA Shinichi, NAKAGAWA Shigeki, HIRATA Naoshi, MASUDA Masataka, MIYAKAWA Koji, YAGI Takeo, WATANABE Atsushi, GOTO Kazuhiko, ITO Takeo, OKUDA Takashi, TERAKAWA Toshiko, HORIKAWA Shinichiro, MAEDA Yuta, MATSUHIRO Kenjiro, YAMANAKA Yoshiko, WATANABE Toshiki, IIO Yoshihisa, KATAO Hiroshi, KANO Yasuyuki, TSUDA Hiroo, MIURA Tsutomu, MURAMOTO Tomoya, YAMASHITA Yusuke, OKUBO Makoto, YAMASHINA Tadashi, OHKURA Takahiro, NAKAO Shigeru, HIRANO Shuichiro, MIYAMACHI Hiroki, YAKIWARA Hiroshi

Zisin (Journal of the Seismological Society of Japan. 2nd ser.) Vol. 73 ( 0 ) page： 149 - 157 2020.10

3‐D Intrinsic and Scattering Seismic Attenuation Structures Beneath Kyushu, Japan Reviewed

A. Shito, S. Matsumoto, T. Ohkura, H. Shimizu, S. Sakai, Y. Iio, H. Takahashi, H. Yakiwara, T. Watanabe, M. Kosuga, T. Okada, M. Yoshimi, Y. Asano

Journal of Geophysical Research: Solid Earth Vol. 125 ( 8 ) 2020.8

Language：English Publishing type：Research paper (scientific journal) Publisher：American Geophysical Union (AGU)

The attenuation of seismic wave energy is caused by intrinsic absorption and scattering. The former involves the conversion of seismic wave energy to heat energy via internal friction due to the anelasticity of the medium, whereas the latter involves the scattering of seismic wave energy due to random elastic heterogeneities in the medium. Quantifying both intrinsic ((Formula presented.)) and scattering ((Formula presented.)) attenuation is therefore important for understanding the physical properties of the Earth's interior and predicting seismic wave propagation. Here we develop a new procedure to separately map three-dimensional (3-D) (Formula presented.) and (Formula presented.) structures. The path-averaged (Formula presented.) and (Formula presented.) values are obtained via an envelope-fitting approach that employs a multiple-scattering model. The path-averaged (Formula presented.) and (Formula presented.) structures are then mapped into 3-D space using a tomographic inversion technique based on sensitivity kernels, which are calculated from a Monte Carlo simulation of the radiative transfer equations. We apply this method to map the crustal structure beneath Kyushu, Japan, and obtain 3-D (Formula presented.) and (Formula presented.) structures for the 1–2, 2–4, and 4–8 Hz frequency bands. The spatial attenuation patterns are similar for each of the analyzed frequency bands, with the spatial variability in (Formula presented.) being more pronounced than that in (Formula presented.). The high- (Formula presented.) and (Formula presented.) regions correspond to the locations of active volcanoes. Conversely, the northern Kyushu area possesses low- (Formula presented.) and (Formula presented.) zones that correspond to low heat flow and Cretaceous granite, respectively. The overall (Formula presented.) and (Formula presented.) distributions correlate with thermal and geological structures, respectively.

DOI： 10.1029/2019jb018742

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2019JB018742

Time-lapse approach to detect possible preslip associated with the nankai trough mega-earthquake by monitoring the temporal change of the strong seismic reflector at the subducting philippine sea plate Reviewed

Junzo Kasahara, Kayoko Tsuruga, Yoko Hasada, Takahiro Kunitomo, Ryoya Ikuta, Toshiki Watanabe, Koshun Yamaoka, Naoyuki Fujii

Active Geophysical Monitoring page： 425 - 439 2019.1

Publishing type：Part of collection (book)

To examine the usefulness of the accurately controlled routinely operated signal system (ACROSS) for the detection of the temporal change of scattering or reflection signatures, we conducted a 9-month experiment using a seismic ACROSS source in Toki City, Japan, and a temporal linear seismic array extending from Toki City toward the south. The seismic experiment was carried out from December 2005 to September 2006. The observations show that the 8-day stacking at ~60 km of offset distances is characterized by prominent arrivals similar to that of the 30-day stacking. This suggests that 1-week stacking is sufficient to detect prominent arrivals up to ~60 km offset distances. We calculated theoretical travel times and synthetic seismograms using the reanalyzed crustal structural model and compared the observed seismograms with theoretical travel times and synthetic seismograms. The synthetic and observed records match well with each other. However, the phases reflected from the subducting plate boundary could not be identified for the offset range of receivers used in this study. Nevertheless, the 8-day stacking of the data received at station “OSD” at an offset distance of 61.2 km provides a sufficient signal-to-noise ratio to identify Pg, which is a refracted wave in the crust, and shallow reflected arrivals.

DOI： 10.1016/B978-0-08-102684-7.00020-0

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Seismic interferometry imaging of subsurface structure in the southernmost area of South Japanese Alps Reviewed

Hasegawa, D, Watanabe, T, Ito, T, Kano, K, Abe, S, Fujiwara, A, Kouchi, Y

Proceedings of the 13th SEGJ International Symposium -What's Next?: Technology for Discovery, Safety and Sustainability page： S02-P02 2018.11

Secular and coseismic changes in S-wave velocity detected using ACROSS in the Tokai region Reviewed

Tsuji Shuhei, Yamaoka Koshun, Ikuta Ryoya, Kunitomo Takahiro, Watanabe Toshiki, Yoshida Yasuhiro, Katsumata Akio

Earth Planets and Space Vol. 70 2018.9

Time-lapse changes in tube and guided waves in cross-well Mallik experiment Reviewed International coauthorship International journal

Andrey Bakulin, Valeri Korneev, Toshiki Watanabe, Serge Ziatdinov

Society of Exploration Geophysicists - SEG International Exposition and 76tth Annual Meeting 2006, SEG 2006 page： 379 - 383 2018

Language：Japanese Publishing type：Research paper (international conference proceedings)

We analyze cross-well seismic data from the Mallik experiment and demonstrate time-lapse changes in tube and guided waves. Although such changes are challenging to interpret, they are generally of a larger magnitude compared to any time-lapse signatures of the first P-wave arrivals reported elsewhere. This suggests better sensitivity of tube and guided waves to small production-related changes and their feasibility for reservoir monitoring.

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2016年熊本地震と関連する活動に関する総合調査 Invited

清水洋, 松本聡, 酒井慎一, 岡田知己, 渡辺俊樹, 飯尾能久, 相澤広記, 松島健, 高橋浩晃, 中尾茂, 鈴木康弘, 後藤秀昭, 大倉敬宏, 山本希, 中道治久, 山中浩明, 神野達夫, 三宅弘恵, 纐纈一起, 浅野公之, 松島信一, 福岡浩, 若井明彦, 大井昌弘, 田村圭子, 木村玲欧, 井ノ口宗成, 前原喜彦, 赤星朋比古

自然災害科学総合シンポジウム講演論文集 Vol. 53 page： 73 - 90 2016.9

Development of surface-wave monitoring system for leaked CO2 using a continuous and controlled seismic source Reviewed

Tatsunori Ikeda, Takeshi Tsuji, Toshiki Watanabe, Koshun Yamaoka

INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL Vol. 45 page： 94 - 105 2016.2

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

To detect CO2 leakage from CO2 geological storage, we describe a seismic monitoring method using a continuous and controlled seismic source system, the Accurately Controlled Routinely Operated Signal System (ACROSS). The method applies surface-wave analysis to monitor the shallow subsurface from the temporal-variation (time-variation) of surface-wave phase velocity. Our numerical simulation study for CO2 leakage through fault zones indicated that the spatial distribution of leaked CO2 can be estimated from small temporal-variation of local phase velocities (similar to 1-3%). To demonstrate the method in a field case, we analyzed continuous seismic records acquired with ACROSS. We clearly extracted a dispersion curve of surface waves in the frequency range excited by the ACROSS (5.015-15.015 Hz). In particular, we obtained reliable estimates of phase velocities in 10-15 Hz frequency range, in which the time-variation of phase velocities was better than 1% accuracy. This temporal stability was sufficient to allow us to detect changes in phase velocities associated with CO2 leakage before leaked CO2 reached the surface. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijggc.2015.11.030

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A subsurface structure change associated with the eruptive activity at Sakurajima Volcano, Japan, inferred from an accurately controlled source Reviewed

Yuta Maeda, Koshun Yamaoka, Hiroki Miyamachi, Toshiki Watanabe, Takahiro Kunitomo, Ryoya Ikuta, Hiroshi Yakiwara, Masato Iguchi

GEOPHYSICAL RESEARCH LETTERS Vol. 42 ( 13 ) page： 5179 - 5186 2015.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Temporal variations of Green functions associated with the eruptive activity at Sakurajima Volcano, Japan, were estimated using an accurately controlled routinely operated signal system (ACROSS). We deconvolved 400s waveforms of the ACROSS signal at nearby stations by a known source time function and stacked the results based on the time relative to individual eruptions and the eruption intervals; the quantities obtained by this procedure are Green functions corresponding to various stages of the eruptive activity. We found an energy decrease in the later phase of the Green functions in active eruptive periods. This energy decrease, localized in the 2-6s window of the Green functions, is difficult to explain by contamination from volcanic earthquakes and tremors. The decrease could be more reasonably attributed to a subsurface structure change caused by the volcanic activity.

DOI： 10.1002/2015GL064351

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弾性波アクロスによる東海地域地殻のP波およびS波速度構造の推定 Reviewed

國友孝洋, 山岡耕春, 渡辺俊樹, 吉田康宏, 勝間田明男, 生田領野, 加藤愛太郎, 飯高隆, 津村紀子, 大久保慎人

地震II Vol. 67 ( 1 ) page： 1 - 24 2014

Active monitoring at an active volcano: amplitude-distance dependence of ACROSS at Sakurajima Volcano, Japan Reviewed

Yamaoka, K., Miyamachi, H., Watanabe, T., Kunitomo, T., Michishita, T., Ikuta, R. and Iguchi, M.

Earth, Planets and Space 2014

Effects of pore fluid pressure and tectonic stress on diverse seismic activities around the Mt. Ontake volcano, central Japan Reviewed

Terakawa, T., Yamanaka, Y., Nakamichi, H., Watanabe, T., Yamazaki, F., Horikawa, S. and Okuda, T

Tectonophysics Vol. 608 page： 138 - 148 2013

東海地域下の三次元地震波減衰構造 Reviewed

高岡宏之, 津村紀子, 高橋福助, 野崎謙治, 加藤愛太郎, 飯高隆, 岩崎貴哉, 酒井慎一, 平田直, 生田領野, 國友孝洋, 吉田康弘, 勝俣啓, 山岡耕春, 渡辺俊樹, 山崎文人, 大久保慎人, 鈴木貞臣

地震第2輯 Vol. 65 ( 2 ) page： 175 - 187 2012.12

Imaging basement structure beneath the Noubi Plain, central Japan using strong motion records of natural earthquakes Reviewed

Watanabe, T., Shimizu, H., Tobita, J., Abe, S. and Shiraishi, K.

Proceedings of the 10th SEGJ International Symposium -Imaging and Interpretation- page： 96 - 99 2011.11

Monitoring the plate boundary in Tokai area using ACROSS -Role of Toyohashi ACROSS source-

Watanabe, T., Yamaoka, K., Yamazaki, K. and Suzuki, Y.

Proceedings of 14th International Symposium on Recent Advances in Exploration Geophysics page： 65 - 68 2010.11

Variations of fluid pressure within the subducting oceanic crust and slow earthquakes Reviewed

Kato, A., Iidaka, T., Ikuta, R., Yoshida, Y., Katsumata, K., Iwasaki, T., Sakai, S., Thurber, C., Tsumura, N., Yamaoka, K., Watanabe, T., Kunitomo, T., Yamazaki, F., Okubo, M., Suzuki, S. and Hirata, N.

Geophysical Research Letter Vol. 37 page： L14310 2010.7

Detection and continuous monitoring of strong seismic reflectors at the subducting Philippine Sea Plate using ACROSS methodlogy Reviewed

Kasahara, J, Tsuruga, K., Hasada, Y., Kunitomo, T., Ikuta, R., Watanabe, T., Yamaoka, K., Fujii, N. and Kumazawa, M.

Active Geophysical Monitoring, Vol. 40, Handbook of Geophysical Exploration page： 435 - 447 2010.5

構造境界面を組込んだMT法の2次元インバージョン Reviewed

木村俊則, 後藤忠徳, 笠谷貴史, 岡本拓, 三ケ田均, 真田佳典, 渡辺俊樹, 芦田讓

物理探査 Vol. 63 ( 2 ) page： 185 - 196 2010.4

Technical Trends on Seismic Reflection Method for Understanding Structure and Physical Properties in the Crust Invited Reviewed

Watanabe, T.

Zishin, Second Series Vol. 61 ( 特集号 ) page： S217 - S223 2009.7

Seismic interferometry of teleseicmic S-wave coda for retrieval of body waves: an application to the Philippine Sea slab underneath the Japanese Islands Reviewed

Tonegawa, T., Nishida, K., Watanabe, T. and Shiomi, K.

Geophysical Journal International Vol. 178 ( 3 ) page： 1183 - 1189 2009

Language：English Publishing type：Research paper (scientific journal)

In this study, we present the propagations of the body waves, such as direct P and S waves, and the reflected wave from the Philippine Sea slab, with the spatial correlation of thewavefields induced by teleseismic S wave observed at the Hi-net tiltmeters. The used frequency band is 0.07-0.5 Hz. To enhance the contribution of the S coda, we depressed the source-time function and the later deterministic phases, such as sS, ScS, SS, and surface wave. The cross correlation functions (CCFs) with the processed S coda are stacked by taking incident angle-dependence of direct S wave to stations into account for performing an isotropic incidence. Through these processes, the propagations of direct P, S, and reflected waves can be seen whenthe stacked CCFs are aligned as a function of separation distance between the two stations. The gradients of the travel time curves for the direct P and S waves are approximately 7 km/s and 3 km/s, corresponding to the seismic velocities in the crust (or uppermost mantle for the case of the P wave). In order to enhance the reflected waves, we search the reflected points by assuming that the later phases in the CCFs are SS reflections and mapping the amplitudes onto depth sections. As a result, the northerly-dipping negative phases can be traced right below the hypocenter distribution, probably corresponding to the oceanic Moho within the Philippine Sea slab. Moreover, we show that the oceanic Moho can be also traced by assuming the case of PP reflections. These results indicate that the CCFs plausibly contain the propagation information for both P and S waves between the two receivers, and are utilizable to the detections of reflected phases in addition to the direct waves.

DOI： 10.1111/j.1365-246X.2009.04249.x

2008年桜島人工地震探査の目的と実施

井口正人, 為栗健, 山本圭吾, 大島弘光, 前川徳光, 森済, 鈴木敦生, 筒井智樹, 今井幹浩, 封馬和希, 八木直史, 植木貞人, 中山貴史, 山本芳裕, 高木涼太, 猪井志織, 古賀祥子, 西村太志, Titi Anggono, 山本希, 及川純, 長出昇構, 市原美恵, 辻浩, 青木陽介, 森田裕一, 渡邊篤志, 野上健治, 山脇輝夫, 渡辺俊樹, 中道治久, 奥田隆, 立花健二, Enrique Hernandez, 橋田悠, 平井敬, 吉本昌弘, 山崎賢志, 毛利拓治, 清水洋, 中元真美, 山下裕亮, 三ケ田均, 尾西恭亮, 田中暁, 岡野豊, 川林徹也, 藤谷淳司, 坂口弘訓, 今泉光智哲, 大倉敬宏, 吉川慎, 安部祐希, 安藤隆志, 横尾亮彦, 相澤広記, 高山鐵朗, 山崎友也, 多田光弘, 市川信夫, 加茂正人, 富阪和秀, 宮町宏樹, 小林励司, 八木原寛, 平野舟一郎, 泊知里, 西山信吾, 吉田沙由美, 畠山謙吾, 西田誠, 加藤幸司, 宮村淳一, 小枝智幸, 増田与志郎, 平松秀行, 河野太亮, 松末伸一, 大薄富士男, 五藤大仁, 宮下誠, 伊藤弘志, 音成陽二郎

京都大学防災研究所年報 Vol. 52 ( B ) page： 293 - 307 2009

A marine deep-towed DC resistivity survey in a methane hydrate area, Japan Sea Reviewed

Goto, T., Kasaya, T. Machiyama, H., Takagi, R., Matsumoto, R., Okuda, Y., Satoh, M., Watanabe, T., Seama, N., Mikada, H., Sanada, Y. and Kinoshita, M.

Exploration Geophysics Vol. 39 ( 1 ) page： 52 - 59 2008.2

Time-variation of late arrivals in the transfer functions obtained by a permanent seismic source and receiver array (ACROSS)

Watanabe, T., Furukawa, T., Hasada, Y., Ikura, R., Yamaoka

Proceedings of the 12th International Symposium on Recent Advances in Exploration Geophysics page： 65 - 68 2008.1

Temporal change in reflected waves from deep crust possibly synchronized with the activity of DLFTs detected using ACROSS

Watanabe, T., Soma, T., Ikuta, R., Yamaoka, K.

Proceedings of the 11th International Symposium on Recent Advances in Exploration Geophysics page： 59 - 62 2007.4

東海監視に向けた地震波アクロスと小規模地震計アレイによる試験観測

渡辺俊樹, 生田領野, 相馬知征, 山岡耕春, 藤井直之,

月刊地球 Vol. 29 ( 8 ) page： 475 - 480 2007

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

プレート間カップリングの時間変化の監視を目的とする能動的な連続地震波観測を実現するために，弾性波アクロス震源と小規模地震計アレイを用いて10ヶ月にわたる試験観測を実施した．連続地震記録からアクロス伝達関数を求めた．その結果，地殻深部を伝播したと考えられる波群を検出し，それらの微小な時間変化を検知した．伝達関数の変化は低周波微動の活動と関連しているように見え，興味深い．地表に設置した地震計では気温，降雨などの地表付近の影響が避けられない．現在，ボアホール内に設置され環境変動の影響が小さいと考えられるHi-net記録を用いて，アレイ解析で見いだされた後続波群の時間変動について詳細に検討している．

Scattering analysis and estimation of structure heterogeneity in seismic exploration Invited

Vol. 29 ( 4 ) page： 227 - 231 2007

Continuous observation of seismic wave velocity and apparent velocity using a precise seismic array and ACROSS seismic source Reviewed

Saiga, A., Yamaoka, K., Kunitomo, T. and Watanabe, T.

Earth Planets and Space Vol. 58 ( 8 ) page： 993 - 1005 2006

Language：English Publishing type：Research paper (scientific journal)

We report the results of continuous monitoring.using a seismometer array.of the travel time of seismic waves generated by an ACROSS artificial seismic source. The seismometer array, which was deployed in a surface vault located 2.4 km from the source, recorded both direct P- and S-waves and refracted P- and Swaves that traveled along a velocity boundary between the granite basement and overlying sedimentary rocks. We analyzed temporal variation in differential travel time and apparent velocity for these phases for a period of 1 month and found significant temporal variation in the differential travel time. Most of the variation can be attributed to changes in environmental conditions, such as atmospheric temperature and rainfall. Variation is even observed in the seismogram that is located 50 m from the vibration source, although much smaller variation is observed in the vibration of the foundation to which the source is attached. The spectral study revealed that the effects of temperature and rainfall depend strongly on the frequency range used by ACROSS and that a large variation occurs in the 15- to 20-Hz range, especially between 17 and 20 Hz. The environmental effect on the temporal variation is comparable to the record of refracted S waves and that of a distance of 50 m, whereas a larger variation was observed in the direct S wave. This result shows that the signal is affected by the environmental change near the vibration source. The environmental effect can be drastically reduced when the signal from the 15- to 20-Hz range is eliminated in the analysis.

DOI： 10.1186/BF03352604

Seismic exploration at Fuji volcano with active sources : The outline of the experiment and the arrival time data Reviewed

Oikawa, J., Kagiyama, T., Tanaka, S., Miyamachi, H., Tsutsui, T., Ikeda, Y., Katayama, H., Matsuo, N., Oshima, H., Nishimura, Y., Yamamoto, K., Watanabe, T., Yamazaki, F., Watanabe, H., Fujii, T., Nakada, S., Takeo, M., Ohminato, T., Kaneko, T., Yoshimoto, M., Takeda, T., Koyama, E., Osada, N., Saka, M., Haneda, T., Hashimoto, S., Tsuji, H., Imoto, Y., Shimano, T., Furukawa, A., Sagiya, T., Fujii, I., Hayashi, Y., Miyajima, R., Yamada, M., Okuda, T., Itoh, T., Hashimoto, T., Maekawa, T., Suzuki, A., Itoh, T., Miura, Y., Ueki, S., Nishimura, T., Nita, K., Satoh, M., Simomura, Y., Nogami, K., Onizawa, S., Oyamada, H., Funasaki, J., Chikazawa, S., Fujiwara, K., Hamada, N., Aoki, G., Takagi, A., Yamamoto, T., Hayashi, Y., Kanao, M., Yamashita, M., Shimizu, H., Watanabe, A., Korenaga, M., Ohkura, T., Yoshikawa, S., Ikeda, S., Iguchi, M., Tameguri, T., Yakiwara, H., Hirano, S.

Bulletin of the Earthquake Research Institute Vol. 81 page： 71 - 94 2006

Continuous Active Seismic Observation using ACROSS Source and Seismometer Array Reviewed

Watanabe, T., Ikuta, R., Yamaoka, K., Soma, T. and Fujii, N.

Proceedings of the 8th SEGJ International Symposium -Imaging and interpretation- page： 369 - 372 2006

Authorship：Lead author Language：English Publishing type：Research paper (international conference proceedings)

A team in Nagoya University has been extensively working on developing Accurately-Controlled Routinely-Operated Signal System (ACROSS) in order to monitor temporal variations in the subsurface properties. Continuous transmission of a controlled seismic signal from the seismic ACROSS source and the long-term observation of the waves propagating through the deeper crust and the plate boundary enable us to understand the time-variable phenomena such as slow-slip and deep tremor and to monitor the state of the plate boundary. We deployed a short-spam seismometer array in the "Aichi Prefectural Forest Park" in defunct Hourai Town (presently Shinshiro City) in eastern Aichi. The array, composed of 12 stations, is about 2 km wide in NS and 1 km wide in EW. The observation started from November, 2004 and terminated at the end of September, 2005. The seismic ACROSS source located at Toki has been operated to generate FM signal from 10 to 20 Hz in 50 seconds of period during the continous observation. After one or two weeks of data stacking, signals with a good S/N ratio were obtained using the surface seismometers, which is comparable to the data of the Hi-Net station deployed in a borehole. The transfer function clearly shows the P-wave, S-wave and their after-phases, having good correlation with the Hi-net data. A semblance analysis of array data shows the estimation of the azimuth and dip of the wave propagation and the apparent slowness. The waveform of after-phases, supposed to the reflected phase from the plate boundary, shows temporal changes during observation. Detailed analysis is underway, however, a possible explanation may be a correlation to the activity of deep low-frequency tremors in Tokai region.

DOI： 10.1190/segj082006-001.74

Time-lapse changes in tube and guided waves in cross-well Mallik experiment Reviewed

Bakulin, A., Korneev, V, Watanabe, T. and Ziatdinov, S.

Expanded Abstract of the 76th Annual Meeting, Society of Exploration Geophysicists page： 379 - 383 2006

南海トラフ沈みこみ帯の地殻比抵抗構造 Reviewed

木村俊則, 芦田讓, 後藤忠徳，笠谷貴史, 三ケ田均, 真田佳典, 渡辺俊樹, 山根一修

物理探査 Vol. 58 ( 3 ) page： 251 - 262 2005.8

Seismic Exploration by Using Active Sources at Kuchierabujima Volcano, Southwest Japan

Iguchi, M., Yamamoto, K., Hashimoto, T., Tsutsui, T., Tanaka, S., Onizawa, S., Aoki, Y., Watanabe, T., Ohkura, T., Shimizu, H., Yakiwara, H., Miyamachi, H., Hiramatsu, H., Tameguri, T., Takayama, T., Fukushima, M., Triastuty, H., Ishihara, K., Oshima, H., Suzuki, A., Maekawa, T., Kajii, T., Watanabe, Y., Ogiwara, Y., Nogami, K., Oikawa, M., Hirabayashi, J., Tsuji, H., Hiratao, Y., Okuda, T., Itoh, T., Yoshikawa, S., Inoue, H., Ikeda, S., Hori, M., Saito, M., Hirano, S., Uekama, Y., Iguchi, M., Takeshita, T., Mori, T.

Disaster Prevention Research Institute annuals Vol. 48 ( B ) page： 297 - 322 2005.4

Preliminary assessment of the waveform inversion method for interpretation of cross-well seismic data from the thermal production test, Mallik Gas Hydrate Production Research Well Reviewed

Watanabe, T., Shimizu, S., Asakawa, E., Kamei, R. and Matsuoka, T.

Scientific Results from Mallik 2000 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, Geological Survey of Canada, Bulletin, Vol. 585 2005

Seismic Exploration Using Active Sources at Kuchierabujima Volcano, Southwest Japan Reviewed

Bulletin of the Earthquake Research Institute Vol. 80 ( 1 ) page： 11 - 40 2005

Differential waveform tomography for time-lapse crosswell seismic data with application to gas hydrate production monitoring Reviewed

Watanabe, T., Shimizu, S., Asakawa, E., Matsuoka, T.

Expanded Abstract of the 74th Annual Meeting, Society of Exploration Geophysicists page： TOM1.1 2004.10

Authorship：Lead author Language：English Publishing type：Research paper (international conference proceedings)

To detect the change of physical properties in small areas, a series of high-resolution waveform inversions is applied to time-lapse seismic data. A procedure by use of ifferentiation between the time-lapse data and normalization using reference data is proposed in this study. The procedure is derived as a straightforward extension of waveform inversion as the scatterer imaging. Through numerical tests, the proposed approach was found to be more accurate than the conventional approach in obtaining the velocity change in small areas. The method was applied to the time-lapse crosswell seismic data obtained during the Mallik 2002 gas production test. A small area showing a velocity decrease near the production zone were found using the proposed method, indicating the existence of dissociated methane gas in the sand layers.

DOI： 10.1190/1.1845221

フルウェーブ・インバージョン Reviewed

渡辺俊樹

非破壊検査 Vol. 53 ( 5 ) page： 274 - 279 2004

人工地震を用いた富士山における構造探査

及川純, 鍵山恒臣, 田中聡, 宮町宏樹, 筒井智樹, 池田靖, 潟山弘明, 松尾のり道, 西村裕一, 山本圭吾, 渡辺俊樹, 大島弘光, 山崎文人

月刊地球 Vol. 号外No. 48 page： 23 - 26 2004

Viscoacoustic wave form inversion of transmission data for velocity and attenuation Reviewed

Watanabe, T., Nihei, K. T., Nakagawa, S., Myer, L. R.

Journal of Acoustical Society of America Vol. 115 ( 6 ) page： 3059-3067 2004

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

This study investigates the performance of a frequency domain viscoacoustic full waveform nonlinear inversion to obtain high resolution images of velocity and attenuation. An efficient frequency domain implementation is applied that consists of performing a series of single frequency inversions sweeping from low to high frequency. A cascaded inversion was adopted in which the real part of the velocity is first imaged using the phase information, then the quality factor (Q) is imaged using the amplitude information. Tests with synthetic data indicate that our approach yielded better images than the simultaneous determination of the real and imaginary part of the complex velocity. The method is applied to laboratory data obtained in a water tank with suspended acrylic bars. Broadband 200 kHz data is obtained for a crosshole configuration with a computer-controlled scanning system and piezofilm source and detector. The velocity image produced by the full waveform inversion is compared to a curved ray traveltime tomography velocity image, and was observed to possess higher resolution and more precise locations of the acrylic bars. The Q image shows a lower resolution than the velocity image, but recovers the correct Q for acrylic. This method can be applied for geophysical applications targeted to soil, unconsolidated rocks and marine sediments and also nondestructive evaluation and medical applications.

DOI： 10.1121/1.1710878

地震計アレイによる2.4km離れたACROSS震源からの弾性波の走時変動連続観測

雑賀敦, 山岡耕春, 國友孝洋, 渡辺俊樹

月刊地球 Vol. 号外No. 46 page： 105 - 112 2004

物理探査におけるアクティブ・モニタリング

渡辺俊樹

月刊地球 Vol. 号外No. 46 page： 166 - 172 2004

P-S converted seismic waves by a fracture and imaging Reviewed

Sanada, Y., Watanabe, T., Kiriyama, A., Ashida, Y.

ISRM 3rd Asian Rock Mechanics Symposium (ARMS) Vol. 1 page： 261 - 264 2004

周波数領域波形インバージョンによる孔間室内実験データの速度と減衰のイメージング Reviewed

渡辺俊樹, Nihei, K. T., Nakagawa, S. and Myer, L. R.

物理探査 Vol. 56 ( 2 ) page： 97 - 105 2003.6

Seismic imaging ahead of tunnel face with three-component geophones Reviewed

Ashida, Y., Matsuoka, T., Watanabe, T. and Kusumi, H.

Environmental Rock Engineering page： 371 - 375 2003

High resolution imaging of crosshole data using waveform inversion and reverse-time imaging Reviewed

Watanabe, T., Nihei, K. T., Nakagawa, S. and Myer, L. R.

Environmental Rock Engineering page： 317 - 322 2003

Simulation of Hopkinson effect by discrete element method Reviewed

Matsuoka, T., Kusumi, H., Ashida, Y. and Watanabe, T.

ISRM 2003 - Technology roadmap for rock mechanics - page： 813 - 816 2003

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