Mt. Usu is an active volcano located within the Toya Caldera in Hokkaido. The volcano has erupted about every 20-30 years since the 20th century. The location and type of eruptive activity vary with each eruption. Understanding the eruptive activity requires an understanding of its subsurface structure, which remains a key challenge. We conducted a broadband and audio-frequency band magnetotelluric survey to model the 3-D resistivity structure of Mt. Usu. Additionally, drilling surveys around Mt Usu were conducted to a depth of approximately 1 km in the area affected by the 2000 eruption. In this study, we conducted a 3-D resistivity inversion using existing MT data to investigate the subsurface structure and compared the 3-D resistivity structure with the drilling data. We carried out a 3-D inversion based on the FEMTIC code (Usui 2015) to obtain a 3-D resistivity model. The input data were given an error floor of 5 % of the absolute value for the four impedance components and 0.03 for the two tipper components. We started from the initial model with a uniform resistivity at 100 Ωm (model calculation area: 600 km(NS)×600 km(EW)×780 km(vertical)). The atmosphere and sea water were fixed at 108 Ωm and 0.3 Ωm, respectively. We compared the resistivity structure with the existing drilling data. The geological structure estimated from the drilling survey does not correspond to the 3-D resistivity structure. However, the conductive layer corresponds to the drilling core, which is rich in smectite. The drilling core is also classified as a smectite alteration zone. Therefore, it is highly likely that the conductive layer extending throughout Mt. Usu reflects layers that have undergone diagenetic or hydrothermal alteration. Furthermore, the drilling survey revealed an andesite dyke, estimated to have intruded during the Pleistocene, at approximately 0.3 km BSL. On the other hand, the upper end of the dyke estimated from GPS data during the 2000 eruption, was located about 0.5 km (Okazaki et al. 2002). Both the dyke identified in the drilling survey and the dyke inferred from GPS data correspond to the depth of the conductive layer. The 3-D resistivity structure obtained in this study corresponds the alteration zone and volcanic activity sources, as inferred from the drilling survey and geophysical data. In the future, we expect to gain further insights into the mechanisms of volcanic activity by investigating the relationship between the resistivity structure and volcanic activities such as dyke intrusion and earthquake swarms.

Continuous observation of the total magnetic intensity (F) was started by Kakioka Magnetic Observatory at a station (MEA) on the southern slope of the 96-1 Pommachineshiri Crator of Meakandake Volcano in 2003. Since 2014, when an array was formed with the addition of two stations (ME2 and ME3), array observation with the three F stations, apart by some 100 m from one another, has been continuously made. I use the Extended Kalman Filter method to infer the time evolution of demagnetization source described by four parameters (horizontal position, altitude and intensity of the dipole moment) such as to explain monthly variations of the three F data. This has been enabled by taking a priori information on dipole demagnetization sources from the repeat measurement surveys as an initial model. Although the obtained models depend highly on prior setting of the observation and transition errors, I find that the resolution of the time variation of the moment intensity (including the notable increase of intermittent demagnetization) is generally high. On the other hand, the resolution of the elevation (about 500 m depth) is low, limiting the ability of clearly imaging upward migration of the heat source. In order to obtain a model more reliable for the volcanic activity assessment, it is desirable to add further information to constrain parameters, or to reconsider the station layout for enhancing the resolution of the depth variation of the heat source.

　本研究では、2021年から2022年にかけて、東京大学地震研究所と海洋研究開発機構が共同で実施した伊豆大島周辺での陸上および海底での広帯域電磁場観測データを用いて比抵抗構造解析を実施し、伊豆大島下深さ10kmまでの地下構造を初めて明らかにした。 　陸上データは、伊豆大島の11地点で約40日間にわたり取得され、海底データは、伊豆大島周辺の海底7地点に設置された海底電位磁力計(OBEM）から約4.5カ月間にわたり取得された。 　観測されたデータを時系列解析し、陸上データについては、384 Hz～4.88×10⁻⁴ Hzの周波数帯域で、海底データについては3.75×10⁻² Hz～9.77×10⁻⁵ Hzの周波数帯域でのMT応答関数・地磁気変換関数を求めた。さらに、その応答関数を入力値とし、3次元比抵抗構造インバージョンを実施した。 　その結果は、海水準以浅は数100Ωm以上の高比抵抗領域が広がり、およそ海水準以深は、特に伊豆大島直下は低比抵抗体が10km以深まで続く特徴を持つ構造であった。浅部高比抵抗は空隙の多い不飽和層であり、その下の低比抵抗体は深さ数kmの空隙が地下流体で飽和していると考えられる。その下はさらに比抵抗が低くなっており、熱水のみならずメルトの存在も示唆される。 　先行研究の浅部比抵抗構造の結果や、伊豆大島の震源分布の結果と調和的な構造であり、伊豆大島の地下構造に対して、電磁気学的観点から新たな制約を与えるものとなった。

　火山の全磁力観測点においては顕著な地磁気の季節変化がしばしば観測され，火山活動監視ではノイズとなることが知られている．この季節変化は，表層の温度変化に伴う岩石の磁化強度変化による影響であることが指摘されており，表層温度を全磁力季節変化の補正に利用できる（Utada et al., 2000）．気象庁地磁気観測所は，2007年以来伊豆大島の火山防災に寄与する目的で三原山火口の北側に全磁力連続観測点（MIK1, MIK2）を約40mの近接した間隔で運用しているが，両者ではあまり似ていない全磁力季節変化が観測されてきた（Fig. 1）．また，伊豆大島における全磁力連続データから火山性のシグナルを誤認しないために適切な補正に関する議論が進展している（浅利・長町, 2023）．一方で，伊豆大島における全磁力季節変化には，観測点付近で測定した地中温度の変化では整合が取れない全磁力変動が含まれることが指摘されており，その全磁力変動の要因の一つに多孔質のスコリアに浸透する天水が挙げられたが，降水量と全磁力連続データを突合しても両者の因果関係は解明されず，全磁力変動における天水の役割は不明であった（三島ほか, 2010）．伊豆大島の全磁力季節変化の要因については，土壌雨量（田口ほか, 2014）や地磁気擾乱（笹岡, 2015）などをその候補とする議論があったが，特定には至っていない．本稿の目的は，地中深さ方向の熱収支の関係から1次元熱伝導モデルを作成し，地表面からの熱伝導のほか，表層透水による天水と地層との熱接触並びにスコリア層の間隙に生じる空気の熱対流に伴う温度変化を同時に熱平衡式（e.g., 赤坂, 2018）により計算し，地中温度と伊豆大島地磁気全磁力の相関について明らかにすることである．また，火山監視能力を高度化するために全磁力季節変化を補正する． 　伊豆大島の地質調査（川辺, 2012）を参考にして，①表層モデルは，厚さ10.0m（各層厚さ0.5m）とし，深さ1.0m～3.0mは玄武岩スコリア層とみなし，深さ0.0m～1.0m並びに深さ3.0m～10.0mは玄武岩と土壌から成る層として作成する．②地中温度の計算は，大島（アメダス）の1時間ごとの気温及び降水量に基づき，③スコリア層の4層分の鉛直温度プロファイルを用いて伊豆大島における全磁力変動を補正する．熱特性に係るパラメータについては，玄武岩や土壌などの熱伝導係数と熱容量の数値を導出するため，各パラメータ（熱伝導率および容積比熱）の資料を参考にした（北野ほか, 1988; 土木学会, 2006）．例えば富士山のスコリアの調査によると，スコリアの間隙率は70％程度あり，1時間に約10mの速さで透水する実験結果が示された（笹田, 2017）．ここでは，全磁力観測点付近のスコリア層について，間隙率は60％，かつ1時間に3mの深さまで透水し，天水はスコリア層には残留しないとした．ところで，混合物質の各パラメータの導出は一般的に構造を反映するため煩雑である（稲葉, 1989）．各パラメータについては，玄武岩，空気及び水が混合する場合は並列式で簡便的に算出した． 　玄武岩スコリア層（深さ1.0m～3.0m）の地中温度プロファイルには，夏季の天水による表層透水並びにスコリア内に発生する冬季の冷気沈降による熱対流の影響が見られる（Fig. 2）．全磁力季節変化と最小二乗法によりスコリア層の地中温度から求めた補正量とは良い一致を示す（Fig. 3）．補正量の要因別の内訳によると，MIK1は天水由来と熱対流由来の地中温度の影響が大きく，MIK2は季節変化由来と天水由来の地中温度の影響が大きいことが示された（Fig. 4）．両観測点の温度依存は，それぞれの観測点付近におけるスコリア中の磁気分布，そして岩石磁気に対する透水や熱対流による影響の大きさにより現れているものと考えられる．MIK1とMIK2の補正後の全磁力は，観測の不調等によるデータの乱れが散見されるものの，概ねノイズ軽減の効果が確認された（Fig. 5）．2021～2024年の補正後の全磁力連続観測はやや増加傾向を示すが，現在伊豆大島では噴火の予兆が現れていないため明瞭な火山活動による影響ではないと推察する（Fig. 6）．本調査では，地表面温度の日較差に対応する全磁力変動が観測されておらず，伊豆大島観測点の全磁力観測は地表面付近の岩石磁気の影響は受けていないのではないかと推察するが，表層スコリアの温度に対しては非常に顕著な依存がみられることが判明した．

This study aims to understand the current magma supply system leading to giant caldera eruptions. The Kikai submarine caldera volcano is located on the Southwest Japan Volcanic Front and lies within southern Kagoshima Prefecture. This volcano is known for its 7.3 ka giant caldera-forming eruption, the most recent giant caldera eruption in Japan. Topographic and petrological studies suggest that a new magma supply led to the formation of the central lava dome even after the giant caldera eruption, which has been constrained to have occurred after 3.9 ka. While geological studies provide insights into past magmatic activity, the present state of magma supply can be constrained by investigating the current structure beneath the caldera volcano using geophysical methods. A seismic structure provided by seismic tomography has identified a low-velocity zone within the mantle wedge beneath this region. In this study, we present the resistivity structure obtained from an MT survey, which provides an additional constraint on the current structure beneath the caldera volcano independent of seismic velocity data. We estimated the MT (magnetotelluric) response function using the BIRRP from the data acquired by Ocean Bottom Electro-Magnetometers at 32 sites. The magnetic data from two land-based stations in Kagoshima Prefecture (Kanoya and Haraigawa) was used as remote reference data. The coordinate system was defined such that the x-axis is oriented parallel to the trench axis. We examined the power spectral density (PSD) of the electric field and found that the PSD in the x-direction was smaller than the y-direction. As a result, the MT response function exhibited the following features: the apparent resistivities in the xx- and xy-components tended to be generally low, and the associated uncertainties tended to be large. These characteristics were observed consistently across all survey sites, rather than being limited to specific areas. We estimated the three-dimensional resistivity structure beneath the seafloor in the Kikai Caldera area using the 3-D MT inversion code FEMTIC. The model domain covered approximately 2700 x 2700 x 2000 km, and a non-conforming deforming hexahedral mesh was utilized to easily and appropriately handle the effect of the seafloor topography. The initial model was based on a 1-D structure estimated from the average MT response function obtained at the sites outside the caldera rim and incorporates the Ryukyu slab, referring to previous research on the regional tectonic setting. Data selection was performed using statistical quality criteria and visual inspection. To assess the robustness of the inferred resistivity structure, we conducted inversions with several different values of the trade-off parameters and compared the resulting resistivity models. As a result, all models corresponding to trade-off parameters near the corner of the L-curve consistently showed two distinct structures: a conductive anomaly (C1) within the mantle wedge and an overlying resistive layer (R1). These resistivity features showed minimal lateral variation along the x-axis (trench-parallel direction). Further interpretations will involve comparisons with seismicity distributions and velocity structures in this region.

We have carried out several campaigns of geophysical explorations in the Goshogake geothermal area, Akita prefecture, northeast of Japan. GPR surveys was conducted five times along the part of walk-trail since 2017. We can find the signals of scattering of the electromagnetic wave around the alteration, although we cannot recognize the outstanding time change of the structure of reflection or scattering. SP measurements was conducted nine times since 2019 in the season of no snow. A calculated source point of virtual electric source is located around the active fumarole zone.

Phreatic eruptions are primarily driven by vapor layers, making the detection of changes in these layers essential for volcanic disaster prevention. Inferno Crater Lake in New Zealand, characterized by its periodic 38-day fluctuations in water level and temperature, is hypothesized to experience vapor layer variations that contribute to these phenomena. To investigate this, a six-month observation campaign was conducted in 2023 using the EM ACROSS method, a geophysical technique sensitive to high-resistivity layers. This method involved continuous transmission of artificial electromagnetic signals, allowing precise monitoring of subsurface resistivity structures. By focusing on frequencies near the transmission frequency, errors in electric field and current measurements were evaluated, enabling observations with a time resolution of one hour. Variations in the amplitude and phase of the apparent resistivity tensor were found to correlate strongly with fluctuations in the lake's water level. However, significant phase variations were not observed below 46.95 Hz, and the phase tensor was undetectable in this frequency range. Resistivity fluctuations at these lower frequencies were attributed to changes at depths of approximately 300 m, suggesting that the sensitivity of the method decreases with depth. To further interpret the observed resistivity changes, a 3-D finite element method was employed to model the subsurface resistivity structure. The results indicate that a vapor layer expanding to a thickness of 180–240 m and rising to 60 m below the surface during high water levels provided the best explanation for the observed phase tensor variations. This finding aligns with previous resistivity surveys that identified a resistivity-altered zone near the lake, although the EM-ACROSS method demonstrated greater sensitivity to deeper regions. These results highlight the potential of the EM-ACROSS method as a highly sensitive tool for monitoring vapor layer dynamics, which are critical to understanding and forecasting phreatic eruption processes. The method’s ability i to provide high-resolution temporal and spatial data makes it particularly valuable for observing phreatomagmatic systems, offering new insights into subsurface resistivity changes and their relationship with surface-level phenomena. Future applications of this method could significantly enhance volcanic monitoring efforts and improve predictive capabilities for eruption-related hazards.

地球物理データの継続的な取得とタイムラプス・イメージングの適用は，地下環境の監視に不可欠である．一般的には，parallel inversionおよびsequential inversionという手法が，火山地帯や地熱地域における流体の時空間分布の推定に広く用いられている．しかし，これらの手法は観測配置の変動に非常に敏感であり，実際の地下構造変化とは関係のない見かけの時間変化を生じ得ることが知られている．この課題を克服するために，本研究では，Group Lassoというスパース正則化技術を組み込んだ新たなタイムラプス・イメージング手法を提案する．本手法は，空間領域に加えて時間領域にも制約を課すことで，観測レイアウトの変化に対するモデルのロバスト性を向上させ，見かけの時間変化を抑制することが可能である．モデル計算により，提案手法が従来手法よりも優れた性能を発揮し，時間変化の推定精度を向上させることが確認された．さらに，繰り返し観測と連続観測を併用することにより，現状のモニタリング体制で，より時間分解能の高いタイムラプス・イメージングを実施し得ることが示唆された．今後，提案手法を実データに適用することで，火山地域や地熱地帯のダイナミクスに関するより正確な描像が得られることが期待される．

The project “Seismology TowArd Research innovation with data of Earthquake” (STAR-E Project), led by the Ministry of Education, Culture, Sports, Science and Technology Japan (MEXT), has accelerated developments and applications of artificial intelligence (AI) techniques in Japanese seismological community. Integration of seismology and AI, such as detection of earthquakes/slow earthquakes from seismic waveforms, simulation mitigation, and modeling, is going on rapidly and competitively in the world. Among various topics carried out in the SYNTHA-Seis, which is one of the five research subjects in the STAR-E Project, this paper introduces a neural operator to represent seismic wavefield propagation and a data-driven method to acquire a stochastic differential equation to represent low-frequency tremor waveforms based on deep learning.

Some of the catastrophic earthquakes are accompanied by rich Slow Slip Events (SSEs) activities (Kato et al., 2012; Obara and Kato, 2016). This suggests that SSEs are likely closely related to the nucleation process of large earthquakes. However, the triggering mechanism for SSEs remains largely unclear. Many SSEs sources are in a deep area greater than 30 km (Todd et al., 2018), which significantly increases the difficulty of accurately detecting the structures and their temporal changes in the SSEs source regions. While SSEs were frequently triggered in the Northern Hikurangi Subduction Zone at a shallower depth of 2-15 km (Wallace, 2020), it is an ideal area on the earth to investigate the triggering mechanism for SSEs. As increasing evidence suggests that fluid movement at depth may play a critical role in triggering the SSEs, magnetotelluric (MT) data may be one of the best choices for monitoring fluid migration before, during, and after the SSEs. By collaborating with many scientists from GNS Science, the authors initiated a seafloor MT monitoring experiment on the upper plate of the Northern Hikurangi Subduction Zone in 2023 after modifying ocean bottom electromagnetometer (OBEM) to enable continuous recording on the seafloor for at least one year. This instrument improvement has made long-term seafloor MT monitoring happen for the first time. One-year seafloor MT monitoring data was recovered in October 2024 from three sites above previous SSEs sources in the Northern Hikurangi Subduction Zone.

Minute-means geomagnetic observatories capture short-period geomagnetic variations. The tippers' response, by analyzing the relationship between vertical and horizontal magnetic-field variations, provides insights into the heterogeneous electrical conductivity structure of the crust and upper mantle. A long-time series of minute-means data from 61 geomagnetic observatories (approximately 75–135°E, 15–55°N) in China was collected and analyzed. The estimated tippers were derived for sixteen periods, ranging from 240.48s to 9078.12s. We investigate a cost-effective 3-D resistivity modeling that explicitly incorporates topography, bathymetry, and shoreline data from the ETOPO Global Relief Model to enhance the accuracy of electrical structure recovery beneath China by future inversion analysis. While modeling using a finer mesh is expected to yield more accurate model responses, it also increases model complexity and computational cost. To balance the grid resolution and computational resources, we apply the FEMTIC package (Usui, 2015), which utilizes a non-conforming deformed hexahedral mesh, to construct the model with a nested mesh consisting of a regional mesh and local mesh (cuboids). By comparing the geomagnetic observatory tippers’ responses for different meshes from ETOPO1 data, we found that the local mesh around geomagnetic observatories plays a crucial role. Provided that the local mesh resolution is sufficiently high, the precision of the regional mesh has minimal impact on the results. Based on these findings, we designed a practical mesh configuration for accurately modeling topography and coastlines. The study area, covering 5000 × 5600 km², is represented using a regional mesh with 50 × 56 equal-sized grids (each 100 × 100 km²). Around each geomagnetic observatory, a local mesh of 200 × 200 km² is applied, refined to 3.125 × 3.125 km² at the center using unequal-sized grids. This forward modeling results for geomagnetic observatory tippers’ responses are validated using two test models—one with and without topography. Coastal geomagnetic observatory tippers are primarily influenced by the bathymetry effect, while some inland geomagnetic observatory tippers are affected beyond the typical observational errors by undulating surface topography. The verification shows the necessity of incorporating topography into the modeling process. To quantitatively evaluate the accuracy of tippers calculated from resistivity models that include topography, we employed a simple method based on the arbitrary selection of horizontal coordinate systems in 3D topography over 100 Ohm-m half-space. This method calculates the mean and standard deviation of tipper responses at each geomagnetic observatory, for each frequency and component, by randomly rotating the forward model across ten different azimuthal coordinate systems, including the conventional (XN, YE) coordinate system. Our results indicate that the accuracy of forward modeling is negatively correlated with the roughness of local topography. To account for this, we used high-resolution ETOPO data and locally refined meshes to better capture topographic features near each geomagnetic observatory. For observatories located in regions with complex topography, we employed a mesh based on ETOPO 2022 (30 Arc-Second) data, refined to 0.7812 × 0.7812 km² at the center. For observatories in less complex terrain, a coarser central mesh of 3.125 × 3.125 km² was used. Ultimately, the finalized mesh design for the entire study area consists of a regional grid of 50 × 56 cells (each 100 × 100 km²), combined with local meshes of 200 × 200 km² refined to either 3.125 × 3.125 km² or 0.7812 × 0.7812 km², depending on the surrounding topography. In the future inversion analysis, we will use this local mesh design. Also, we are planning to incorporate the forward modeling uncertainty into the inversion analysis in order not to overfit the data beyond the forward modeling accuracy.

函館平野西縁断層帯は，函館平野の西縁から函館湾の西岸にかけて発達する全長約26kmの活断層帯である．本断層帯の陸域部では，これまでに重力探査や電気探査，反射法地震探査など複数の物理探査を実施することによって活断層の位置や形状等を推定してきたが，海域部では音波探査の反射記録断面のみで推定しており，陸域部に比べて活断層の評価に資する物理探査データが十分に得られていなかった．そこで本研究は，函館平野西縁断層帯の海域延長部を対象に海上磁気探査と音響測深を実施し，活断層に起因した磁気異常の取得を試みた．観測の結果，音波探査によって推定された活断層付近で磁気異常が大きく変動し，活断層に起因するとみられる磁気異常を捉えることができた．また，二次元のダイク・モデルとステップ・モデルによって計算される磁気異常と観測された磁気異常を比較・検討することによって，調査海域に伏在する活断層の形態を推定することができた．これらの結果から，海上磁気探査は沿岸海域の活断層調査に有効であり，音波探査と併用することによって，より精度の高い活断層評価が行える手法としての活用が期待できる．

Geophysical methods have been used in the characterization and mapping of metallic ore. This study employed magnetic and Electrical resistivity tomography (ERT) survey to characterize nickel ore in Gwandi Prospect, Tanzania. The high-resolution airborne data was used to generate a target for ground survey. The interpreted 2D images of the results provide detailed information on the magnetic and resistivity properties of subsurface geology. The ground magnetic data reveal the high magnetic body at the center, interpreted to be associated with mafic or ultramafic rocks. The mineralized zone was marked based on magnetic intensity range from negative to positive anomaly and low to high resistivity. The integrated results suggest that the nickel ore zone is characterized by low magnetic and resistivity properties. The depth was estimated from the ERT section, and the mineralized zone was found to be located at a shallow depth from the surface. The results were tested and compared with the results from geological and geochemical analysis to validate the presence of Nickel mineralization in the study area. The geological, geochemical, and geophysical results were integrated to delineate the mineralized zone, structure trend, and depth of occurrence at Gwandi prospect.

　本研究では岩手県奥州市胆沢地区に位置する角塚古墳(つのづかこふん)で電気探査、地中レーダを用いて調査を行った。角塚古墳は本州最北端の前方後円墳で、岩手県内最古の古墳かつ墳丘が現存する古墳では最大の規模を所有し、胆沢扇状地水沢段丘面の中央に位置している。昭和49、50年に範囲確認調査が行われ、多量の埴輪、人頭大の玉石が確認された。平成10年、11年、12年に後円部を中心とした調査が行われ、後円部に隙間なく立て並べられている埴輪列と思われるものが確認された。令和4年8月20、22日、11月10、30日に後円部を中心とした地中レーダ探査が行われ、埋葬施設や葺石と思われるものが確認された。本研究では、令和6年7月11日、9月20日、12月9日の計3日間にわたり調査を行った。以前の地中レーダ探査では解像度が不十分で浅いところを詳しく確認することができなかった。 　ゆえに、本研究の調査では後円部のより浅い墳丘部分を対象とし遺構が検出できるのではないかと考えた。地中レーダの周波数は、400 MHz、900 MHzのものを用いた。後円部全体を見るために電気探査も行った。結果として、埋葬施設と思われる反応が過去に行われたトレンチ調査の跡による反応であり、遺構のような反応が他に見られなかったが、多数の葺石を確認し、正確な位置まで示すことができ、遺構調査における電気探査とGPRの有効性を示すことができた。