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This paper presents and interprets two new self-potential data measured over a hydrothermally active field associated with a Quaternary rhyolitic volcano, Oomuro-dashi, in the northern Izu-Ogasawara Arc, south of Japan. The measured data show a pair of positive and negative anomalies of the order of one millivolt at 5 m above the seafloor. The observation of a positive self-potential near a seafloor hydrothermal system is new, in spite that negative self-potential anomalies have been regularly reported in various studies for different locations. Determining the dominant mechanism(s) is therefore key to further understanding the subseafloor structure of seafloor hydrothermal systems. To this end, we also conducted long-term observations of subseafloor temperatures at two sites in the area of the self-potential anomaly to estimate the Darcy velocity. We found a downward fluid flow of the order of tens of metres per year at both sites. The flow in the area of the negative self-potential anomaly is stronger than in the area of the positive anomaly. Based on these observations, we propose two end-member models to explain the paired self-potential anomaly. The first model considers a horizontal geo-battery, in which part of a subhorizontal electrically conductive body is crossed by a subvertical redox front. In this model, the oxidised part of the geo-battery causes a negative self-potential anomaly, as in the previous observations, while the reduced counterpart of the geo-battery, which is normally buried, is exposed near the seafloor and causes a positive anomaly. In this case, a conductive body is expected to lie beneath both anomalies, and we could access the reduced part of the geo-battery. This model is consistent with the results of the Darcy velocity estimation if the strong hydrothermal circulation would cause the redox horizon to deepen. The second model is a combination of the thermal and streaming potentials causing both positive and negative self-potential anomalies. This model does not necessarily require a buried conductive body beneath the self-potential anomalies. These end-member models could be distinguished by resistivity imaging, which identifies the distribution of conductive bodies beneath self-potential anomalies, although they would overlap in natural systems. Graphical Abstract

We performed a simultaneous survey of self-potential and plume turbidity using an autonomous underwater vehicle (AUV) above the Sunrise deposit in the Myojin Knoll caldera of the Izu-Ogasawara arc. A 10-m-long electrode rod, on which five electrodes referenced with a common electrode were mounted, was connected at the tail of an AUV. The survey was conducted at a typical speed of 2 knots, covering the 1500 m × 1500 m area with a typical spacing of survey lines of 100 m. With AUV altitude of 100 m above the seafloor, a negative self-potential anomaly of a few millivolts was observed. The self-potential anomaly was found to spread 300 m × 300 m. The self-potential is probably attributable to the geo-battery mechanism: electric current is generated by redox reactions occurring around an ore body crossing a redox contrast. Assuming that the source of the self-potential is an electric current dipole, we can image a southward-dipping dipole with the moment of approximately 103 A m, approx. 30 m below the southern part of the ore deposit. Anomalies of turbidity, which are correlated to ambient temperature and which are signatures of discharged hydrothermal fluids, were distributed more broadly than the self-potential. Some turbidity anomalies were found without self-potential anomalies. They were probably transported by the ocean current. Spatial decoupling between the self-potential and turbidity anomalies suggests that the direct contribution of hydrothermal fluids to the self-potential anomalies is probably a secondary effect. The survey altitude of 100 m and the survey speed of 2 knots in the present study represent practical limitations for the self-potential survey when active hydrothermal fields are targeted. We have observed that the self-potential method responds exclusively to the presence of hydrothermal ore deposits. This behavior differs from other methods for exploring seafloor hydrothermal ore deposits: The geomagnetic method responds not only to ore deposits but also to volcanic bodies. The plume method can detect remote hydrothermal activities, but the source locations are not necessarily specified. The self-potential method is useful as an excellent exploration tool, particularly for initial surveys.

We conducted a self-potential survey at an active hydrothermal field, the Izena hole in the mid-Okinawa Trough, southern Japan. This field is known to contain Kuroko-type massive sulphide deposits. This survey measured the self-potential continuously in ambient seawater using a deep-tow array, which comprises an electrode array with a 30-m-long elastic rod and a stand-alone data acquisition unit. We observed negative self-potential signals not only above active hydrothermal vents and visible sulphide mounds but also above the flat seafloor without such structures. Some signals were detectable >50 m above the seafloor. Analysis of the acquired data revealed these signals’ source as below the seafloor, which suggests that the self-potential method can detect hydrothermal ore deposits effectively. The self-potential survey, an easily performed method for initial surveys, can identify individual sulphide deposits from a vast hydrothermal area.

The 2024 Noto Peninsula earthquake (M JMA 7.6) occurred, followed by a tsunami on January 1, 2024. The earthquake caused considerable damage by strong ground motion over a wide area centered on the Noto Peninsula. The tsunami also damaged coastal areas. Japan Agency for Marine–Earth Science and Technology (JAMSTEC), Earthquake Research Institute (ERI) and Atmosphere and Ocean Research Institute (AORI) of the University of Tokyo conducted emergency survey cruises with various observations such as ocean bottom seismometers (OBSs) deployment and recovery operations to identify aftershock activity promptly and acquisition of multibeam bathymetry data from the northeast area of the Noto Peninsula and Toyama Bay. Our survey mainly covered the northeast part of the aftershock area and the downstream part of the Toyama Deep–Sea Channel (TDSC) in the Toyama Bay and the southern Toyama Trough. Survey lines in the northern survey area generally trend NNE–SSW, roughly parallel to the channel; some lines in the NNW–SSE direction cross the active faults. We compiled a bathymetric map (with 25 m grid spacing), using all available multibeam echo sounder (MBES) data. We identified some important bathymetric features, such as gentle wavy topography developed over the levee and horseshoe-shaped landforms. In addition, we conducted a dense survey to detect depth differences before and after the earthquake in the area along TDSC in the middle of our survey area. Results revealed four small-scale landslide areas in the dense survey area. Obtaining detailed topographic data using modern multibeam sonar is extremely important to assess the risk of tsunami damage for various marine infrastructures as well as the potential occurrence of submarine landslides and slope failures. Graphical Abstract

The 2011 Tohoku-oki earthquake (Mw 9.0) was characterized by a huge fault slip on the shallowest part of the plate interface, where fault behavior had been believed to be aseismic. In this study, we modeled the two-dimensional resistivity distribution across the slip area based on ocean-bottom electromagnetic measurements to understand the physical properties around the plate interface controlling fault rupture processes. The optimal 2D resistivity model showed a conductive area around the shallowest plate interface where the huge coseismic slip was observed, whereas the deeper plate interface where the fault rupture was nucleated was relatively more resistive. The shallowest plate interface was interpreted to have a high pore seawater fraction, whereas the deeper interface was interpreted as a dry area. These findings are consistent with the hypothesis that aseismic frictional conditions changed to conditions enhancing fault rupture when the rupture propagated to the wet, clay-rich shallowest plate area. The optimal resistivity model also revealed a conductive area under the outer-rise area of the Pacific Plate. This finding supports the existence of a hydrated oceanic crust that supplied aqueous water to the subduction zone, including to the huge fault slip area.

Detecting resistivity and self-potential (SP) anomalies is useful for the exploration of hydrothermal deposits. Using autonomous underwater vehicles (AUVs) can increase survey effectiveness because it allows stable posture control without a towing wire cable from a ship. We propose a new style for geophysical surveys using multiple AUVs without a towing electrode cable for marine direct current resistivity (MDCR) and SP survey. We used two AUVs for electrical signal transmission and their receiver. We successfully conducted MDCR and SP surveys in hydrothermal deposit areas using two AUVs with 20 m tow-rods. One AUV was assisted by an autonomous surface vehicle (ASV) for monitoring and controlling via satellite and the public broadband mobile communications radiowave. The survey covered an area of about 1 square kilometer, spending only 4 hours near the seafloor with the vehicle’s speed maintained at 2 - 2.5 knots at distance between the AUVs of 200 - 300 m during most of the survey. The SP and apparent resistivity were calculated along the main survey line crossing known hydrothermal mounds of sulfide ore. The distribution of the negative SP anomalies obtained in the dive is similar to that obtained from our earlier survey using a deep-tow. The apparent resistivity is generally low (0.2 ohm-m or less) above the mounds. The averaged distance between the vehicles and the averaged altitude are respectively about 250 and 70 m. Therefore, the estimated apparent resistivities are the averaged value to several tens of meters below the seafloor. These positions show good agreement with the locations of known hydrothermal deposits.

Eighty-six new acoustic survey lines along and across the Japan Trench revealed active sediment creep deformation on a deep-sea terrace at water depths of 400–1200 m in an area of arcuate-shaped depressions that are probably associated with tectonic erosion. The most active region of creep is located on the top at the surface of the depression south of 38° N. The area of creep deformation is characterized by arcuate-shaped topographic lineaments with active folds and active normal faults stepping down trenchward. In contrast to the southern region, normal faults at the top of the depression north of 38° N cut a sedimentary sequence (Unit 1) that is acoustically transparent with continuous weak reflectors, and this is covered by the undeformed layered sediment sequence of Unit 2. Unit 2 corresponds to the period of rising sea level that extended from the latest Pleistocene to the early Holocene (14–6 ka). Thus, creep is ongoing at the top of the depression south of 38° N in the surface layer, whereas it stopped north of the depression between 14 and 6 ka. These observations might indicate that the active region jumped from north to south due to probably retrogressive sliding.

Megathrust and slow earthquakes are known to occur in the area off Kumano, along the eastern part of the Nankai Trough, Japan. Pore fluids along the fault surface play an important role in earthquake occurrence, but the detailed fluid distribution remains unknown. In this study, based on seafloor electromagnetic field observations, we estimated the three-dimensional resistivity structure, which reflects the fluid distribution off Kumano. The optimal three-dimensional resistivity model showed a low-resistivity layer at shallow depths below the seafloor. The low-resistivity layer matches geological features observed in seismic reflection surveys, including forearc basin fill, the frontal prism, and underthrust sediments. Along the plate boundary (décollement), the resistivity increased markedly with increasing depth. Comparison with slow earthquakes along the plate boundary reveals that shallow very low-frequency earthquakes, shallow slow slip events, and tremor only occur at depths < 10 km and on the conductive zone of the plate boundary. Furthermore, the area of coseismic slip during the 1944 Tonankai earthquake was characterized by higher resistivity than the slow earthquake epicentral area. These findings indicate that the pore-fluid distribution along the plate boundary may influence earthquake occurrence and type. Graphical Abstract

The physical properties of seafloor massive sulfides are crucial for interpreting sub-seafloor images from geophysical surveys, shedding light on the evolution of seafloor mineral deposits. While some studies have explored the relationship between electrical properties and the volume of conductive minerals in rocks from seafloor massive sulfide deposits, they primarily focused on artificial samples, leaving the characteristics of natural samples less understood. Moreover, there has been no comprehensive study detailing the general characteristics of electrical properties, particularly chargeability and relaxation time, in relation to the volumetric fraction of sulfides in rocks from massive sulfide mounds in typical hydrothermal areas. In this study, we employed complex conductivity measurements, elemental concentration analysis, and mineral content identification on to rock samples from the active hydrothermal zones of the Okinawa Trough in Japan. The complex conductivity observed was remarkably high, with a pronounced imaginary component and a broad frequency range. This is attributed to induced polarization extending beyond our measurement range. The rock samples were rich in conductive sulfide minerals such as pyrite, chalcopyrite, and galena. Using the Cole–Cole rock physics model, we established a correlation between rock chargeability and relaxation time coefficient with the volume fraction of conductive sulfide minerals, which deviated from previous findings. The intensity of induced polarization was notably higher than anticipated in earlier studies using artificial samples. Furthermore, we observed a distinct positive correlation between the coefficient of relaxation time and the increase in sulfide volume, likely due to the geometric characteristics of the sulfide minerals. Our findings suggest that rocks in massive sulfide mounds may generally construct sulfide clusters that lengthen the conductive path of the electrical carrier. Graphical Abstract

Near-seafloor concentrated gas hydrates (GHs) containing large amounts of methane have been identified at various gas chimney sites. Although understanding the spatial distribution of GHs is fundamental for assessing their dissociation impact on aggravating global warming and resource potential, the spatial distribution of GHs within gas chimneys remains unclear. Here, we estimate the subseafloor distribution of GHs at a gas chimney site in the Japan Sea using marine electrical resistivity tomography data. The resulting two-dimensional subseafloor resistivity structure shows high anomalies (10–100 Ωm) within seismically inferred gas chimneys. As the resistivity anomalies are aligned with high amplitude seismic reflections and core positions recovering GHs, we interpret the resistivity anomalies are near-seafloor concentrated GH deposits. We also detect various distribution patterns of the high resistivity anomalies including 100-m wide and 40-m thick anomaly near the seafloor and 500-m wide anomaly buried 50 m below the seafloor, suggesting that GHs are heterogeneously distributed. Therefore, considering such heterogeneous GH distribution within gas chimneys is critical for in-depth assessments of GH environmental impacts and energy resources.