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

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Papers - WATANABE Toshiki