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Abstract In 2018, a phreatic eruption occurred at Mt. Motoshirane, a pyroclastic cone group of the Kusatsu-Shirane Volcano in central Japan. The eruption was abrupt, and no signs of volcanic activity have been observed in recent years, in contrast to the other major pyroclastic cone group, Mt. Shirane, which hosts the active crater lake and has endured repeated phreatic eruptions. To understand the mechanism of the eruption at Mt. Motoshirane, information on the shallow hydrothermal system, which is thought to be the source region of phreatic eruptions, is required; however, few studies have been conducted on this particular cone group. In this study, we conducted an audio-frequency magnetotelluric survey in 2020 to reveal the shallow resistivity structure around the 2018 craters. A three-dimensional resistivity structure model showed generally two layers, with high resistivities at shallow depths overlain by low resistivities underneath. The boundary between the layers corresponded to the top boundary of the Neogene basement rocks. These low resistivities were not found beneath the 2018 craters; therefore, part of the Neogene basement rocks could have been lost by the eruption beneath the 2018 craters. This is consistent with the geochemical study on the mineral assemblage of the erupted deposits, which suggested that the explosions reached the depth of the basement. Graphical Abstract

Located in the central part of the Tibetan Plateau, the Qiangtang terrane preserves important record of the uplift and deformation history of the Plateau, and therefore remains an attractive area of research. However, deep geophysical investigations of its western part are still limited. To further understand the deep structure of the western Qiangtang terrane and its surroundings, we use magnetotelluric array data to generate a 3D electrical structure. It reveals high resistivity anomalies in the upper crust and scattered high conductivity anomalies in the mid-lower crust. The electrical structure also suggests that the Longmu Co-Gozha Co fault once believed to be a major regional deformation boundary, may not have cut through the crust. The melt content and rheological parameters derived from the electrical structures show dominant ductile-type deformation in most of the study area, which contributes to block extrusion along the slip faults. Viscous deformation regions formed by mantle melt upwelling in the mid-lower crust may contribute to the formation of the N-S directed normal faults on the surface.

In 2018, a phreatic eruption occurred at Mt. Motoshirane, a pyroclastic cone group of the Kusatsu-Shirane Volcano in central Japan. The eruption was abrupt, and no signs of volcanic activity have been observed in recent years, in contrast to the other major pyroclastic cone group, Mt. Shirane, which hosts the active crater lake and has endured repeated phreatic eruptions. To understand the mechanism of the eruption at Mt. Motoshirane, information on the shallow hydrothermal system, which is thought to be the source region of phreatic eruptions, is required; however, few studies have been conducted on this particular cone group. In this study, we conducted an audio-frequency magnetotelluric survey in 2020 to reveal the shallow resistivity structure around the 2018 craters. A three-dimensional resistivity structure model showed generally two layers, with high resistivities at shallow depths overlain by low resistivities underneath. The boundary between the layers corresponded to the top boundary of the Neogene basement rocks. These low resistivities were not found beneath the 2018 craters; therefore, part of the Neogene basement rocks could have been lost by the eruption beneath the 2018 craters. This is consistent with the geochemical study on the mineral assemblage of the erupted deposits, which suggested that the explosions reached the depth of the basement. [ABSTRACT FROM AUTHOR]

The Yunkai Magmatic Arc (YKMA) is located southwest of the South China Block. It has experienced the amalgamation, splitting, and intracontinental orogeny caused by multistage tectonic thermal events. It is also a concentrated area of strong earthquakes in South China. On 12 October 2019, the Beiliu M5.2 earthquake occurred in the hinterland of the YKMA. To reveal the deep electrical structure of the YKMA and the seismogenic environment of the Beiliu earthquake, 101 high-quality data from the magnetotelluric (MT) survey points were acquired. The deep electrical structure images were obtained by three-dimensional electromagnetic inversion imaging. The results indicated that the deep part of the hinterland of the YKMA is characterized by a mushroom-shaped electrical structure composed of ultra-high resistance (R1, with a resistivity value exceeding 10,000 Ωm) and sub-high resistance (R2, with a resistivity value of about 1,000–10,000 Ωm) bodies. The epicenter of the Beiliu M5.2 earthquake was located in R1, close to the contact region between R1 and R2. There are broad low resistivity zones on the southeast and northwest sides of the YKMA. The low resistivity zones is considered to be correspond to the deep extension of the Wuchuan-Sihui and Hepu-Beiliu brittle-ductile shear zones, respectively. The brittle-ductile shearing of the boundary zones and the oblique upwelling of deep mantle-derived magma from the Leiqiong region are the main reasons for the activation of faults and the activity of moderate and strong earthquakes in the YKMA. In this geodynamic environment, local stress and strain accumulation easily occur in the brittle high resistivity body (R1). When the strain energy accumulation exceeded the threshold value that the rock could withstand, new fracture dislocations occurred in the weak region where R1 and R2 contact, which finally resulted in the 2019 Beiliu M5.2 earthquake.

To understand a field of volcanic activity, a survey on the Earth's subsurface structure is helpful. In particular, a magnetotelluric survey can image the subsurface structure necessary for volcanic disaster prevention because it is sensitive to high-temperature areas and areas where fluid and clay minerals exist. In 3-D magnetotelluric inversion using the data observed near volcanoes and the coastline, the topography and sea around the survey area need to be considered. Therefore, constructing a mesh of the analysis area by automatically using topography and bathymetry data is necessary. Additionally, the estimated subsurface structure should be compared to the sources of variation estimated from other physical observations, such as earthquakes, ground deformation, and changes in the geomagnetic total field, for interpretation. This study introduces a simple and easy-to-operate graphical pre- and post-processor developed to support the resistivity structure analysis of a volcano and the nearby coastline. In the developed software, the pre-process indicates the mesh construction, and the post-process indicates result derivation, file construction for the sensitivity tests, and file exportation to compare the inversion results with other measured quantities. This pre- and post-process can be performed easily on the same window of the software. [ABSTRACT FROM AUTHOR]

Abstract Subvolcanic hydrothermal systems can lead to hydrothermal eruptions as well as unrest phenomena without an eruptive event. Historical eruptions and recent unrest events, including ground inflation, demagnetization, and a gradual decrease in the plume height, at Mt. Tokachidake, central Hokkaido, Japan, are related to such a subvolcanic hydrothermal system. This study investigates the three-dimensional (3-D) resistivity structure of Mt. Tokachidake to image its subvolcanic hydrothermal system. A 3-D inversion of the magnetotelluric data, acquired at 22 sites around the crater area, was performed while accounting for the topography. Our resistivity model was characterized by a high-resistivity layer at a shallow depth (50–100 m) and two conductors near the active crater and dormant crater. The high-resistivity layer was interpreted to be composed of dense lava, which acts as a caprock surrounding the conductor. The high conductivity beneath the active crater can be explained by the presence of hydrothermal fluid in fractured or leached zones within the low-permeability lava layer, as the sources of ground inflation and demagnetization were identified within the conductive zone immediately beneath the resistive layer. The resistivity structure was used to estimate the volume of hydrothermal fluid within the pore space. The minimum volume of hydrothermal fluid beneath the active crater that can explain the resistivity structure was estimated to be 3 × 106 m3. This estimate is comparable to the water volume that was associated with the long runout and highly fluidized lahar in 1926. The resistivity structure and volume of hydrothermal fluid presented in this study can be used as a reference for further numerical simulations, which aim to reveal the mechanisms of recent unrest events and assess the risk of hazards, such as lahar.

The central-western region of the Indian subcontinent hosts the vast geological records of its evolution from the Archean to the Recent, including the youngest (∼65 Ma) episode of the Réunion mantle plume activity that produced a large igneous province, the Deccan Volcanic Province (DVP). A three-dimensional lithospheric resistivity image of central-western India is obtained to understand the lithospheric architecture and map any major eruption channels of the Deccan volcanism as no explicit geophysical revelation of such pathways of magma ascend has yet been made. Two high conductivity (< 30 Ωm) pipe-like geometric features originating from a common deep mantle conductive zone under the Malwa plateau (northernmost lobe of the DVP) and its proximity are detected in the resistivity model. These are interpreted to be remnants of the hitherto unknown primary lithospheric pathways of magma ascent from the deep mantle melt-rich zone related to the Reunion mantle plume upwelling under central-western India. This study gives first compelling geophysical evidence of key eruptive centers of the massive Deccan volcanism in central-western India at a locale not anticipated earlier. High to moderate conductivity crustal zones and weak to moderate lithospheric mantle resistivity in most parts of the study region represent an intense and multiphase tectono-magmatic evolution of the region spanning from the Neoproterozoic to the Cretaceous-Paleogene boundary. [Display omitted] • 3D lithospheric resistivity model of the central-western India. • Conductive pipes originating from a common deep mantle conductive zone. • Remnant mantle pathways of Reunion mantle plume upwelling. • Geophysical evidence of key eruptive centers of Deccan volcanism in India. • Support for the mantle plume theory for the Deccan volcanism. [ABSTRACT FROM AUTHOR]

Abstract Crustal earthquake ruptures tend to initiate near fluid-rich zones. However, it is relatively unknown whether fluid-rich zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal area of the 2016 Kumamoto earthquake sequence by using 200 sites broadband magnetotelluric data, and discuss its quantitative relationship to earthquake initiation, growth, and arrest processes. The ruptures that initiated along the outer edge of the low-resistivity fluid-rich zones ( 400 °C) fluid-rich zones, whereas shallower low-temperature (200–400 °C) fluid-rich zones either promoted or arrested the ruptures. These results suggest that the distribution of mid-crustal fluids contributes to the initiation, growth, and arrest of crustal earthquakes. The pre-failure pressure/temperature gradient (spatial difference) of the pore fluids may contribute to the rupture initiation, propagation, and arrest.

Subvolcanic hydrothermal systems can lead to hydrothermal eruptions as well as unrest phenomena without an eruptive event. Historical eruptions and recent unrest events, including ground inflation, demagnetization, and a gradual decrease in the plume height, at Mt. Tokachidake, central Hokkaido, Japan, are related to such a subvolcanic hydrothermal system. This study investigates the three-dimensional (3-D) resistivity structure of Mt. Tokachidake to image its subvolcanic hydrothermal system. A 3-D inversion of the magnetotelluric data, acquired at 22 sites around the crater area, was performed while accounting for the topography. Our resistivity model was characterized by a high-resistivity layer at a shallow depth (50–100 m) and two conductors near the active crater and dormant crater. The high-resistivity layer was interpreted to be composed of dense lava, which acts as a caprock surrounding the conductor. The high conductivity beneath the active crater can be explained by the presence of hydrothermal fluid in fractured or leached zones within the low-permeability lava layer, as the sources of ground inflation and demagnetization were identified within the conductive zone immediately beneath the resistive layer. The resistivity structure was used to estimate the volume of hydrothermal fluid within the pore space. The minimum volume of hydrothermal fluid beneath the active crater that can explain the resistivity structure was estimated to be 3 × 106 m3. This estimate is comparable to the water volume that was associated with the long runout and highly fluidized lahar in 1926. The resistivity structure and volume of hydrothermal fluid presented in this study can be used as a reference for further numerical simulations, which aim to reveal the mechanisms of recent unrest events and assess the risk of hazards, such as lahar. [ABSTRACT FROM AUTHOR]

(a) Vertical resistivity section beneath gold deposits in the southern Youjiang Basin. (b) Sketch map of the multiphase Au mineralization. Plate subduction events triggered crustal melting, then the upwelling of magmatic fluids and fluid-rock interaction led to multiphase Au mineralization. [Display omitted] • There are several conductors beneath the northwestern faults in the Youjiang Basin. • There is a spatial relationship between electrical conductors and Au mineralization. • The conductors beneath the northwestern faults are interpreted as paleo-magma chambers. • Preexisting northwestern faults control both magma emplacement and hydrothermal Au mineralization. The Youjiang Basin, located on the southwestern margin of the Yangtze Craton, hosts significant Upper Triassic and Lower Cretaceous gold (Au) deposits. Isotope studies suggest that the polyphase Au mineralization in the Youjiang Basin is derived from Indosinian and Yanshanian magmatic events, but the structural plumbing system beneath the basin remains poorly understood. We collected 32 magnetotelluric (MT) stations extending from the southern margin of the Youjiang Basin which hosts the Zhesang, Badu, and Jinya Au deposits at the western margin of the Jiangnan Orogenic (JNO) belt. The full impedance tensor of MT data is utilized to generate a high-precision lithospheric resistivity model by three-dimensional (3-D) inversion. Importantly, our MT data reveals several crustal conductors (<30 Ω·m) underlying major structures in the northwestern part of the study area. These crustal conductive anomalies spatially overlap with high-magnetic anomalies and polymetallic Au-Sn deposits. We interpret these anomalies as solid conductive materials such as graphite and/or sulfides related to the structurally controlled Mesozoic Au mineralization. Additionally, we interpret these conductive anomalies beneath the known Au deposits as the conduits of magmas and solidified paleo-magma chambers that were emplaced along deep-seated northwestern faults and likely documenting the genetic relationship between Au mineralization and Mesozoic magmatism. The synthesis of the electrical resistivity structure and metallogenic considerations is presented in a schematic sketch model illustrating that the deep-seated northwest-trending structures likely acted as a plumbing system controlling both the migration and emplacement of magmas and related oxidizing fluids that subsequently precipitated the polyphase Au deposits. [ABSTRACT FROM AUTHOR]

Abstract During the past two decades, studies of the Aso volcano in Japan have improved our understanding of the shallow hydrothermal system that exists beneath the active crater of this volcano. Detailed knowledge of the subsurface structure of this volcanic edifice is essential for developing a better understanding of the behavior of the volcanic fluids and of the triggering mechanism of volcanic eruptions. Here, we report a three-dimensional (3-D) electrical resistivity model for the active crater of the Nakadake central cone of Aso volcano using audio-frequency magnetotelluric (AMT) data obtained during 2004–2005. The AMT data were collected at 43 sites on a grid (distance between grid points: ~ 300 m) around the crater. However, as yet, only two-dimensional sectional resistivity models have been generated for this survey area. Using 3-D inversion, we obtain a resistivity model that shows similar characteristics to those of the 2-D models. A highly conductive zone is observed beneath the active crater down to a depth of approximately 300 m. Based on the recent findings regarding the shallow hydrothermal system of the volcano, we interpret this conductive zone to have been formed by highly conductive acidic fluids filling a fractured region. This view modifies the past interpretation made on the 2-D models and promotes understanding of fluid behavior beneath the active crater.

Abstract We conducted a broadband magnetotelluric (MT) survey along a north–south transect across Unzen graben, Japan. The MT survey line is located ~ 2 km west of the most recent lava dome and consisted of 27 stations along a 9-km profile. We estimated the 3-D resistivity structure and correlated it with the seismic reflection structure obtained by the same survey line as in the present study. The best-fit resistivity structure shows an upper resistive layer underlain by a moderately conductive layer. The resistive layer, which is interpreted as a cold groundwater zone, is cut by four faults marked by their relatively high conductivity. The underlying layer, which is interpreted as a hydrothermal-water-rich layer, also shows relatively conductive values near the faults. By assuming that the faults are imaged as relatively conductive zones, we infer the dip and depth extent of fracture zones around the faults. Beneath the Chijiwa Fault, which is the longest and most active fault of Unzen graben, the dominant conductor (C1) has a width of 2 km and extends down to below 4 km depth. C1 corresponds to a zone of strong seismic reflection and is located close to one of the pressure sources causing surface deformation. In this study, we interpret C1 as a network of fractures generated by the Chijiwa Fault to which magmatic volatiles are supplied from a deeper pressure source. Given that C1 extends to a greater depth and its resistivity is lower than other conductive zones, it is possible that earthquakes have occurred repeatedly on the Chijiwa Fault. In the center of the study area, we identify a vertically oriented body of high resistivity (R1) that corresponds to a zone of low seismic reflectivity. We interpret R1 as a cooled dyke complex that may have acted as a volcanic conduit.

Since phreatic eruptions often occur near the surface, it is important to understand the shallow subsurface structure of a volcano to clarify the eruption mechanism. The 2015 phreatic eruption at Owakudani, Hakone Volcano, Japan, was preceded by localized uplift and intensified steam activity. However, the cause of these events is unknown due to a poor understanding of the shallow subsurface structure. We report on a three‐dimensional resistivity structure inferred from dense audio‐frequency magnetotelluric surveys conducted in Owakudani that revealed a ∼100‐m thick conductive layer at the surface underlain by a resistive section, at the edge of which eruptions craters were located. This conductive layer is estimated to be a low‐permeability cap layer, because the inflation source prior to the eruption is located beneath it and the ejecta from the eruption was reported to have originated from a depth of 100 m or less. Plain Language Summary: Phreatic eruptions occur as a result of the rapid expansion of water that is heated by a source such as magma. Ejecta from these eruptions does not contain magmatic material, but often contains hydrothermally altered rocks. Understanding the hydrothermal system that develops in shallow volcanic structures is important to understand how phreatic eruptions occur. We imaged the subsurface structure of Owakudani, Hakone Volcano, Japan, where small phreatic eruptions took place in 2015. We found that an ∼100‐m thick layer near the surface is a key structure that may have caused the phreatic eruption. Formation of this characteristic structure is useful for evaluating the potential for phreatic eruptions elsewhere. Key Points: We estimated the 3‐D resistivity structure of Owakudani, Hakone Volcano, where phreatic eruptions occurred in 2015A ∼100‐m thick conductive layer near the surface of Owakudani played an important role in causing the phreatic eruptions [ABSTRACT FROM AUTHOR]

Crustal earthquake ruptures tend to initiate near fluid-rich zones. However, it is relatively unknown whether fluid-rich zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal area of the 2016 Kumamoto earthquake sequence by using 200 sites broadband magnetotelluric data, and discuss its quantitative relationship to earthquake initiation, growth, and arrest processes. The ruptures that initiated along the outer edge of the low-resistivity fluid-rich zones (< 30 Ωm) tended to become large earthquakes, whereas those that initiated either distal to or within the fluid-rich zones did not. The ruptures were arrested by high-temperature (> 400 °C) fluid-rich zones, whereas shallower low-temperature (200–400 °C) fluid-rich zones either promoted or arrested the ruptures. These results suggest that the distribution of mid-crustal fluids contributes to the initiation, growth, and arrest of crustal earthquakes. The pre-failure pressure/temperature gradient (spatial difference) of the pore fluids may contribute to the rupture initiation, propagation, and arrest. [ABSTRACT FROM AUTHOR]

Abstract During the last magmatic eruption period of Aso volcano (November 2014 to May 2015), a controlled-source electromagnetic volcano monitoring experiment (ACTIVE) was conducted. Here, we interpret the temporal variations in the electromagnetic responses. The ACTIVE system installed at the first Nakadake crater, the only active crater of Aso, consisted of a transmitter located northwest of the crater and four (before the eruptions) or three (after the eruptions) vertical induction coil receiver stations. The ACTIVE system succeeded in detecting temporal variations in the resistivity structure during the latest magmatic eruption period. The response amplitude started to increase in November 2014, peaked in February 2015, and decreased slightly in August 2015. An unstructured tetrahedral finite element three-dimensional inversion that accounted for topographic effects was used to interpret temporal variations in the ACTIVE response. The 3-D inversion results revealed that temporal variations in the ACTIVE response are attributed mainly to (1) a broad increase in resistivity at elevations from 750 to 850 m, not only directly beneath the crater bottom but also outside the crater, and (2) a thin layer of decrease in resistivity at the elevation of ~ 1000 m on the western side of the crater. The increase in resistivity can be ascribed to a decrease in the amount of conductive groundwater in the upper part of an aquifer located below the elevation of 800 m, while the decrease in resistivity implies that enhanced fluid temperature and pressure changed the subsurface hydrothermal system and formed a temporal fluid reservoir at the shallow level during the magmatic eruption period.

Abstract On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.

Constructing the strata and bedrocks beneath the overburden for three‐dimensional (3D) geological mapping in the Gobi Desert area is challenging but necessary. Rock resistivity is a key parameter to research the underground media. In the Gobi Desert area, the overburden has low resistivity, while the underlying strata and bedrocks have much higher resistivity in general. These features provide the basic conditions for distinguishing the rocks by their resistivities. The audio magnetotelluric (AMT) method is effective for investigating the electrical structure of most upper crust. Due to minor anthropogenic electromagnetic interference, good data can be easily acquired by reducing the ground resistance and by placing a remote reference station in the Gobi Desert area. WAL analysis and phase tensor method, derived from impedance tensors, can indicate the dimension of a structure, which in turn suggests whether a two‐dimensional or 3D inversion needs to be used. The resistivity distribution obtained via inversion can be translated to lithological units associated with rock resistivity measurement. Combined with borehole constraint, the strata and bedrocks beneath the overburden could be explicitly described. By application of AMT, the Quaternary and Tertiary strata could be divided in the Barkol Basin by a low‐resistivity layer, which was attributed to saturation with salty water. The Eocene and Jurassic strata were distinguished in the Hami Basin by 10‐ and 50‐Ωm contour lines, respectively. In both areas, high‐resistivity bedrocks were clearly depicted. [ABSTRACT FROM AUTHOR]

To decipher the mechanism by which the Phanerozoic accretionary terranes collaged with the Tarim Craton, we constructed a three-dimensional (3-D) lithospheric resistivity model using magnetotelluric data and inversion with testing. The model covers the area across the northern Tarim Craton in the south and the southern Altaids in the north and shows five tectonic-related low-resistivity regions. The low resistivity in East Junggar highlights a Permian rifting zone. The southwest Tianshan low-resistivity anomalies mark asthenospheric upwellings and are related to present-day orogenesis. The intra-crustal low-resistivity area in the northern Tarim Craton could be attributed to metamorphic products from the layered underplating coeval within the Tarim large igneous event. The relatively weak low-resistivity anomalies in Central Tianshan may represent fossil suturing zones. Beneath western Junggar and Chinese North Tianshan (NTS), a large low-resistivity anomaly is continuously distributed in the upper mantle, but spatial correspondence was not shown with the overlying resistivity structure, which could be attributed to the movement of the shallow lithosphere decoupled from the lower lithospheric mantle. We argue that the Yili-Central Tianshan terrane moved northeastward to collage with the Junggar terrane, driving the Junggar terrane rotation and shallow buckling of West Junggar, which constructed the thin-skinned Chinese NTS. The closure of the South Tianshan back-arc basin, associated with southward subduction, completed the amalgamation of peripheral accretionary terranes with the Tarim Craton after the Late Permian. The present study supplies a typical example of the amalgamation of accretionary terranes with a core, thus improving the understanding of continental growth. • We constructed the 3-D resistivity model covering > 600,000 km2. • Our model shows low-resistivities are footprints for tracing lithospheric evolution. • The model supports CAOB collaged with Tarim Craton by back-arc basin subduction. • Thickening is likely a prerequisite for young terranes amalgamating with a core. [ABSTRACT FROM AUTHOR]

Abstract The epicentral region of the 26 January 2001 Bhuj earthquake (7.7 M w) in the Kutch region of India has been experiencing a vigorous and unique intraplate aftershock activity, which shows a V-shaped pattern and N/NE migration along the western margin of the Wagad uplift located to the northeast of the mainshock. This study presents 3D resistivity structure of the Wagad aftershock zone, imaged using magnetotelluric (MT) data, to understand the crustal geoelectric structure and subsurface geometry of faults/lineaments, which could provide critical inputs to evaluate and strengthen the earthquake generation mechanism and aftershocks pattern. The obtained 3D conductivity structure showed a thick (~3 km) and homogeneous conductive (1–30 Ωm) layer of Tertiary/Mesozoic sedimentary cover in the area. A highly heterogeneous upper and middle crust, characterized by a mix of conductive (<30 Ωm) and resistive to moderately resistive (>300 Ωm) structures, is imaged below the sedimentary cover. Enhanced conductivity zones showed spatial correlation with the surface trace of various faults/lineaments and are interpreted as fluid enriched zones in the crust, which play a critical role in earthquake triggering. Significantly higher fluid enrichment noticed in the crust below two minor faults, Khadir Transverse Fault (KTF) and Manfara Fault (MF), which are hitherto considered to have an insignificant role in the seismogenesis of the area. The aftershock events seem to occur mostly on the resistive parts of the crust, and its pattern shows good alignment with the anomalous conductive zones associated to the above two faults. The results provide a strong clue to infer that the KTF and MF have a vital role in the observed aftershock activity and control the V-shaped aftershocks pattern. Highlights • 3D resistivity structure of the 2001 Bhuj earthquake aftershock zone presented. • Heterogeneous crustal structure controlled by fluid enrichment in the fault/fracture zones. • Identified major conductive anomalies beneath two minor faults • Study presents strong clues to explain the peculiar V-shaped aftershock pattern in the area. [ABSTRACT FROM AUTHOR]

The Central Indian Tectonic Zone (CITZ), a major tectonic feature extending across the Indian sub-continent, was formed by the accretion of the cratonic domains in the north and south blocks of peninsular India during the Proterozoic. It is characterized with prominent shear/fault zones and granulite belts that carry the evolutionary history of the central Indian region. This study presents the two-dimensional resistivity structure of the hitherto least studied eastern segment of the mega lineament in the Indian subcontinent. Magnetotelluric (MT) data along the Rajnandgaon-Bareli profile, which extends from the southern margin (Central Indian Shear-CIS) to the northern limit (Narmada-Son Lineament-NSL) of the CITZ, were used to investigate the resistivity character of the crust and shallow upper mantle. The study imaged conductive to moderately conductive structures, which show correlation to the various shear/faults and granulitic belts mapped in the region, interspersed in a resistive background. An upper crustal isolated conductor was observed below the Bhandra – Balaghat Granulite belt, which is exposed along the CIS. A vertical moderately conductive structure noticed in the middle to upper mantle depths under the Ramakona-Katangi Granulite (RKG) belt and this is interpreted as fault/shear zone that acted as the channel/pathway for the emplacement of parental magma of the RKG rocks. A more interesting feature in the MT model is a north dipping conductive horizon extending from the upper crust to the Moho. This feature shows further extension into the upper mantle as a subvertical moderately conductive zone and can be traced onto the surface mark of the Tan Shear, which suggests suture status to this prominent shear zone within the CITZ. Resistivity section also shows mid-lower crustal conductive/moderately conductive horizons under the Deccan trap and NSL, which are produced due to the large-scale magma intrusions into the crust during the Cretaceous-Tertiary Deccan volcanism through the pre-existing weak zones in the CITZ. The conductive to moderately conductive features of the MT model are indicative of the complex tectonic history of the region that range from accretion processes started in the Paleoproterozoic, followed by multiple metamorphic and deformation cycles in different geological times, to the Late Cretaceous volcanism. [ABSTRACT FROM AUTHOR]

An extensive electromagnetic (EM) survey comprising over 400 sites on the Korosi, Paka and Silali volcanoes in the northern volcanic province in Kenya was conducted in order to assess the occurrence of geothermal resources in the context of the geophysics of the major volcanic centers of the north Kenyan East African Rift System (EARS). This area lies within the (EARS) where active extension is currently taking place. A joint inversion of magnetotelluric (MT) and co-located Transient electromagnetic (TEM) has revealed a resistivity pattern consistent with the existence of several geothermal systems within the study area. Each geothermal system is characterized by a relatively resistive 100 Ω m surficial layer overlying a ∼ 10 Ω m low resistivity zone interpreted as the hydrothermally altered clay cap of the system. The cap overlies a higher resistivity zone of about 60 Ω m with a top at about 1000 m depth, interpreted as a potential high temperature alteration zone. The trend of moderate high resistivity at the depth of the potential reservoir corresponds to the zone of intense faulting and fracturing as imaged on the surface. Similarly the moderate high resistivity at sea level mimics the trend of the rift with a break in between Paka and Silali volcanoes where the resistivity trend is offset to the west. This break and the accompanying offset maps a possible westward shift of the axis of the rift in the rift alignment between Paka and Silali volcanoes. Although the 1000–2000 m thick low resistivity zones imaged on the flanks of the Korosi, Paka and Silali volcanoes have been tentatively interpreted as volcanoclastic sediments, elsewhere in the Kenyan EARS, low resistivity zones adjacent to volcanoes have been interpreted differently. To address this ambiguity, these thick low resistivity zones will be further investigated, principally using gravity surveys. [ABSTRACT FROM AUTHOR]

The M 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50-100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005-3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL. [ABSTRACT FROM AUTHOR]

• Characterization of the Manuguru geothermal system of Telangana State, India and identification the deep geothermal reservoir deploying two complementary geophysical surveys viz; gravity and MT. • A total of 144 gravity and 27 MT sounding data were acquired covering an area of 200 km2. • The geophysical study identified several faults acting as conduits for hot water upflow as well as a distinguished low resistivity zone (<4 Ohm-m) at a depth ≥2.4 km indicating deep hot water storage in the area. Manuguru geothermal field is located on the southeastern margin of the Godavari River valley. A sequence of rocks of the Gondwana group unconformably lies over a quartzite-phyllite basement of the Pakhal Supergroup in the present study area. A geophysical investigation deploying gravity and magneto-telluric (MT) survey was carried out covering an area of 200 km2 to characterize the Manuguru geothermal system and locate the probable reservoir source. Bouguer gravity anomaly shows a noticeable variation of 25 mGal ranging from -72 mGal to -47 mGal. Residual gravity and derivative analysis exhibit that there are several basement faults controlling the deep subsurface hot water flow in the area. In the MT study, a total of 27 MT soundings data were acquired at varied frequency ranges (20 kHz to DC). The MT data were analyzed for 1D as well as 2D modeling using the inversion technique to configure the subsurface resistivity distribution along three selected profiles (P1 to P3). Two exploratory drillings conducted based on our studies validate the inferred low resistivity anomalies. To configure basement and fault systems, a 2D forward gravity inversion modeling was performed along a selected profile which showed that the depth of the Archaean basement underlain by the Gondwana and Pakhal sediments ranges from 1.2–2.2 km. The results inferred a 2D resistivity structure with varying resistivity from <2 Ohm-m to >4000 Ohm-m. The study identified a distinguished low resistivity zone (<4 Ohm-m) at a depth ≥2.4 km indicating deep hot water geothermal storage in the area. [ABSTRACT FROM AUTHOR]

• Candidate supercritical geothermal system identified in the Kuju region. • Potential magmatic heat source identified beneath the Hatchobaru geothermal plant and Kuju volcanoes. • Effects of a deep magmatic intrusion on the shallower geothermal system. We derived the thermal structure of the Kuju region of volcanic activity in central Kyushu, Japan, to assess the potential existence of a supercritical geothermal system. Extrapolation of existing well-logging data suggests the presence of a high-temperature region beneath the area between the Hatchobaru geothermal power plant and Kuju-Iwoyama Volcano, which closely overlaps a low-resistivity body estimated from magnetotelluric data. Based on estimated temperatures exceeding 380 °C at the relatively shallow depth of -3.3 km above sea level, and considering evidence for magmatic fluid transport, this high-temperature region is a potential target for a supercritical geothermal reservoir. [ABSTRACT FROM AUTHOR]

Despite extensive effort over many decades to understand the tectonic evolution of the Tibetan Plateau, the geodynamic processes creating the iconic south-eastward expansion of the plateau at the Eastern Himalayan Syntaxis (EHS) are still unclear and are hotly debated. Two popular (but not necessarily exclusive) geodynamic models, namely crustal flow at mid-to-lower crustal depths and coherent deformation between the crust and lithospheric mantle, are commonly invoked to explain the expansion mechanism. However, neither of these is able to reconcile all of the abundant geological and geophysical data. Here we present a three-dimensional (3D) geo-electrical model, derived from new SINOPROBE magnetotelluric (MT) array data, that reveals the geo-electrical, and by inference rheological, structure of southeast Tibet. Instead of NW–SE conductive channels proposed in prior two-dimensional (2D) MT studies, distinct NNE–SSW directed quasi-linear conductive anomalies are identified in the mid-to-lower crust, which are separated by a large-scale electrically resistive structure that extends from the crust to the upper mantle. This argues against the prior proposed model of south-eastward conductive anomalies, and hence against the southeast lower crust flow of material. To interpret our observations and resultant model, a new mechanism of “extensional extrusion” is proposed to address the lithospheric deformation of the south-eastward expansion of Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• Three-dimensional resistivity structure of the Niseko area is estimated from magnetotelluric survey data. • A dominant sub-vertical conductor was found beneath the central part of the Niseko volcanic group. • The supercritical geothermal fluids possibly exist below an altitude of about -3 km inside and/or edge of the conductor. Supercritical geothermal systems (SGS) have the potential to drastically increase geothermal power generation. The New Energy and Industrial Technology Development Organization (NEDO, Japan) has recently started to research and develop supercritical geothermal power generation. Our team conducted a field survey and data collection at three areas in Japan, to characterize the SGS and evaluate the potential for supercritical geothermal power generation. Here, we present a three-dimensional resistivity structure by observing 112 magnetotelluric and 4 telluric sites in and around the Niseko area, southwestern Hokkaido. The final resistivity structure showed a dominant sub-vertical conductor beneath the central part of the Niseko volcanic group. We inferred that the supercritical geothermal fluids possibly exist below an altitude of about -3 km inside or edge of the conductor. [ABSTRACT FROM AUTHOR]

In this study, 2D surface resistivity survey was undertaken around two power houses in Covenant University Ota in Ogun State, Nigeria to investigate the possible spread within the subsurface, of disposed used hydrocarbon products. The 2D resistivity imaging technique using the Wenner array method was adopted for the survey with the aid of the SAS 1000 Terrameter. Five traverses were occupied with two of them (traverses 1and 4) obtained 3 m away from the two power houses, while the remaining three served as control. The study delineated three subsurface geoelectric layers with resistivity values ranging from 57 Ωm to 1258 Ωm on all the profiles. The inferred lithologies from the 2D resistivity inversion are topsoil, sandy clay and lateritic materials. The high resistivity values (800 Ωm - 1258 Ωm) on traverse 1 near the power house behind College of Science and Technology (CST) building may be as a result of the contamination arising from the disposed hydrocarbon products. Traverses 2 to 5 however showed no evidence of contamination of the soil around the study area. The results of the analysis have shown that no appreciable amount of contamination has been observed as a result of the disposal of used hydrocarbon products around the study area. However, continuous disposal of used diesel and other hydrocarbon products around the power houses in the University pose a lot of danger of contamination to the environment and underground water body with time. [ABSTRACT FROM AUTHOR]

Broadband and long-period magnetotelluric (MT) soundings were collected along a 600-km-long EW-trending profile across the southwestern segment of the Jiangnan Orogenic Belt (JOB) to explore the possible remnants of ancient tectonic processes beneath the central South China Block (SCB). Beneath the JOB, the resistivity model reveals a west-dipping, trans-lithosphere conductor (<20 Ω ∙ m) embedded within the relatively thick (80–120 km) and highly resistive (>1000 Ω ∙ m) lithosphere, possibly associated with relics of the Neoproterozoic subduction zone between the Yangtze and Cathaysia blocks. The crustal portion of this conductor is interpreted as solid conducting materials (e.g., graphite, sulfide) that were emplaced into the crust during subduction and orogenesis, whereas its lithospheric mantle portion is more likely to be volatile-bearing minerals formed by subduction-related metasomatic processes. We attribute the greatly reduced asthenospheric resistivities (1~10 Ω ∙ m) below the JOB to low-degree melting of the early-formed metasomatized mantle. In contrast, the lithosphere of the Cathaysia Block is evidently thinned (60–100 km) and segmented by several subvertical, lithospheric-scale conductors (1~30 Ω ∙ m) that are likely associated with fossil fluid pathways along reactivated shear/fault zones. Notably, the thinnest lithosphere sits above a prominent low-resistivity (10 ~ 30 Ω ∙ m) anomaly in the asthenospheric mantle and is spatially coincident with the location of Late Jurassic basaltic outcrops. We interpret these features as signs of lithospheric extension and asthenospheric upwelling driven by roll-back of the Paleo-Pacific Plate during the Late Mesozoic. The results provide key constraints on subduction–accretion tectonics associated with the Neoproterozoic assembly of the SCB and the subsequent lithospheric evolution during the Phanerozoic. [Display omitted] • Dense magnetotelluric soundings reveal high-resolution lithospheric structure beneath central South China. • Relics of Neoproterozoic subduction preserved beneath the Jiangnan Orogenic Belt. • A notably thinned lithosphere is identified beneath the westernmost Cathaysia Block. [ABSTRACT FROM AUTHOR]

• We have obtained a three-dimensional electrical model of the Kangding geothermal system. • The electrical model reveals the remarkable high conductivity anomalies beneath the Kangding geothermal field. • The conductors are explained as partial melt, which supply heat and fluids to the geothermal fields. The Kangding geothermal field along the Xianshuihe fault is a typical high-temperature geothermal system with an enormous capacity for geothermal power generation. The Xianshuihe fault is a major active sinistral strike-slip fault in eastern Tibet Plateau, where the fault splits into three parallel branches near Kangding. We collected data from 17 magnetotelluric stations in the Kangding geothermal system. After data processing, the dimensionality of the subsurface structure was qualitatively estimated using the phase tensor. The electrical resistivity model inverted with ModEM shows the Kangding geothermal system from the surface to a depth of 20 km, which correlates closely with the near-surface geology. The synthetic tests confirmed that the main resistivity anomalies identified in the electrical resistivity model were robust. There are several significant low resistivity anomalies of less than 10 Ωm beneath the Kangding geothermal system. The cap rock and liquid-dominated reservoir can be interpreted as shallow conductive zones. The conductors beneath the Kangding geothermal field at a depth of 10 km are interpreted as an underlying zone of partial melt, which provides fluids and heat for the geothermal system. The partial melt fractions are in the range of 4–19%, confirming that there is a magmatic heat source beneath the Kangding geothermal system. [ABSTRACT FROM AUTHOR]

Taal Volcano, located in the southwestern part of Luzon Island, Philippines, has frequently experienced catastrophic eruptions from both the Main Crater on Volcano Island and flank eruptions. These eruptions have been magmatic, phreatomagmatic, and hydrothermal, with the latter implying the existence of a large-scale hydrothermal system beneath the volcano. We conducted an electrical resistivity survey using the magnetotelluric method in order to identify the location and geometry of the hydrothermal reservoir and sealing cap rock. Two-dimensional inversion using the observed data indicates four similar resistivity sections. The structure at shallow depths corresponds to volcanic deposits and an aquifer. Below 1 km, the structure features a relatively resistive zone beneath the main crater surrounded by a conductive shell. We interpreted these to be a large hydrothermal reservoir with an impermeable cap rock sealing it. Recent ground deformation detected by GPS measurements suggests that the hydrothermal reservoir is active. The interpreted cap rock thins just beneath the main crater and could easily be destroyed by an imbalance in the hydrothermal system. We conclude that this hydrothermal reservoir plays a significant role in driving catastrophic eruptions that begin with a hydrothermal explosion at the main crater. [ABSTRACT FROM AUTHOR]

We present a case study of applying MT (magnetotellurics) and CSAMT (controlled source audio-frequency magnetotellurics) for geophysical exploration in Jiangxia (江夏), which is located in new industrial developing suburb, where artificial noises are severe. In order to know deep buried structure, fracture status, and characteristics of underground geothermal development about 2 km, we acquired MT and CSAMT data to image subsurface structure through inversion and joint interpretation. The electrical terms of the 2D MT inversion can be divided into three ranges of resistivity values: (1) a highly resistive (>350 Ω·m) layer mainly characteristic of limestone, dolomitic limestone, leuttrite, silicarenite, and packsand; (2) an intermediate resistivity (250-350 Ω·m) layer mainly constituted by siliceous shale, siltstone, battie, and ampelitic limestone; and (3) a low resistivity (20-250 Ω·m) layer, from surface to −100 m, which is related to lacustrine alluvium of Quaternary period; the deep low resistivity layer is interpreted to be representative of the geothermal field. The result of the 2D CSAMT inversion reveals two layers of different electrical resistivities: (1) the first resistive layer (20-250 Ω·m), which is related to lacustrine alluvium of Quaternary period and the heat source, and (2) the second resistive layer (250-3 000 Ω·m). The heat source appears to be bounded within the middle of exploration area and shows the N-S trend. Its depth ranges from more than 1.2 to less than 0.7 km, and its resistivity values range from 20 to 250 Ω·m in the northeast part of Jiangxia. Comparing the results of MT and CSAMT method, the positive anomalies are similar and can be assumed to be generated by the same source. [ABSTRACT FROM AUTHOR]

Abstract: An extensive study of the resistivity structure of the Hengill area in SW Iceland was carried out by the combined use of TEM and MT soundings. Joint inversion of the collected data can correct for static shifts in the MT data, which can be severe due to large near-surface resistivity contrasts. Joint 1D inversion of 148 TEM/MT sounding pairs and a 3D inversion of a 60 sounding subset of the MT data were performed. The 3D inversion was based on full MT impedance tensors previously corrected for static shift. Both inversion approaches gave qualitatively similar results, and revealed a shallow resistivity layer reflecting conductive alteration minerals at temperatures of 100–240°C. They also delineated a deep conductor at 3–10km depth. The reason for this deep-seated high conductivity is not fully understood. The distribution of the deep conductors correlates with a positive residual Bouguer gravity anomaly, and with transform tectonics inferred from seismicity. One model of the Hengill that is consistent with the well temperature data and the deep conductor that does not attenuate S-waves, is a group of hot, solidified, but still ductile magmatic intrusions that are closely associated with the heat source for the geothermal system. [Copyright &y& Elsevier]

Abstract: We carried out magnetotelluric (MT) surveys in central northeastern Japan. Two-dimensional resistivity profiles along three survey lines show similar features each other. By comparing the resistivity distribution to the distribution of seismic velocities, we inferred the distribution and flow of crustal fluids. Three fluid flow paths were detected based on the distribution of regions of low resistivity. The first path ascends from the top of the upper mantle, passes through the lower crust, and reaches the surface, forming a fluid chamber within the lower or middle crust. This path is related to the volcanic activity in the backbone range. The second path rises from the first fluid chamber and has produced small fluid reservoirs on both sides of the backbone range. These small reservoirs are considered to be related to the seismicity of the region. The third path is located to the east of the volcanic front and represents another fluid flow path from the uppermost mantle to the lower crust that may have formed a small fluid reservoir to the east of the volcanic front. [Copyright &y& Elsevier]

Abstract: Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4km. The surface resistive layer (~several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200°C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200°C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes. [Copyright &y& Elsevier]

The Taiwan Chelungpu-fault drilling project (TCDP) has undertaken scientific drilling and directly sampled the sub-surface rupture of the 1999 Chi-Chi earthquake. Audio-magnetotelluric (AMT) measurements were used to investigate electrical resistivity structure at the TCDP site from 2004 - 2006. These data show a geoelectric strike direction of N15°E to N30°E. Inversion and forward modeling of the AMT data were used to generate a 1-D resistivity model that has a prominent low resistivity zone (< 10 ohm-m) between depths of 1100 and 1500 m. When combined with porosity measurements, the AMT measurements imply that the ground water has a resistivity of 0.55 ohm-m at the depth of the fault zone. Time variations in the measured AMT data were observed from 2004 - 2005 with maximum changes of 43% in apparent resistivity and 18° in phase. The change in apparent resistivity is greatest in the 1000 - 100 Hz frequency band. These frequencies are sensitive to the resistivity structure of the upper 500 m of the hanging wall of the Chelungpu Fault. The decrease in resistivity over time appears to be robust and could be caused by an increase in porosity and a re-distribution of the groundwater. [ABSTRACT FROM AUTHOR]

Data from two deep-penetration electrical resistivity surveys crossing the western margin of the Taupo Volcanic Zone (TVZ) near Tihoi, northwest of lake Taupo, North island, New Zealand, have been used to identify the elements of the margin. In this vicinity the volcanologically defined boundary of the TVZ includes the andesitic cones of Pureora and Titiraupenga. The faulted margin of the TVZ lies east of these cones and is marked in the resistivity data by a dipping interface (faulted margin) between the resistive greywacke rocks to the west and the relatively conductive volcanic rocks that fill the upper few kilometres of the TVZ. The resistivity and gravity signatures of the margin are coincident in this vicinity. To the immediate east of the margin is a low-resistivity zone c. 10 km wide and 2.5 km deep. The low-resistivity material within this region is identified as old (>1 Ma) volcaniclastics that contain clays produced by low-temperature alteration and devitrification. This material is reversely magnetised. we suggest that this zone was formed following the onset of active extension of the TVZ and has since remained relatively undisturbed while the focus of volcanism and deformation has moved to the east. The eastern boundary of this low-resistivity zone coincides with the western edge of the Whakamaru caldera (the boundary of the "young TVZ"). although the resistivity data can be matched using an interface with a dip to either the east or west, an eastward dip would be consistent with the boundary having been formed during the collapse associated with the formation of the Whakamaru caldera. The resistivity signature within the Whakamaru caldera is consistent with >2 km thickness of younger, more resistive volcanic sequences, suggesting the low-resistivity layers found to the west have been disrupted and downfaulted to be replaced with young, resistive caldera infill. [ABSTRACT FROM AUTHOR]

Abstract: Time-domain electromagnetic (TDEM) surveys were carried out around Unzen Volcano, Shimabara Peninsula, South-west Japan in 2001 and 2002 in the eastern part of the peninsula. The surveys were a completion of the previously interpreted TDEM data collected from surveys in 1995 in the western part of the peninsula. Interpretation of the recorded transient magnetic fields resulted in a general feature of the resistivity structure which are composed of three main layers: a resistive surface layer, a conductive layer (below 10 Ωm) from about sea level to 2.5 km below the surface, and a third resistive layer. The conductive layer is considered to be a complex of water-saturated and altered layers which spreads widely beneath Shimabara Peninsula. The spatial conductance distribution shows a W–E trending pattern of high conductance from Tachibana Bay to the summit of Unzen (Fugen–Dake). This evidence indicates that volcanic gas was supplied to shallow layer in the western part of the peninsula, which is in accordance with the path of magma migration derived from seismic and deformation studies. Another high conductance zone was also found near Mayu–Yama in the eastern part of Shimabara Peninsula, indicating magma degassing process due to the possibility of the presence of a magma system beneath it. [Copyright &y& Elsevier]

Abstract: Long-Offset Transient Electromagnetic (LOTEM) data and VIBROTEM data from the south flank of Mount Merapi on Java island, Indonesia, are interpreted with one-dimensional (1D) inversions as well as two-dimensional (2D) forward modelling. One-dimensional joint inversions of several components of the electromagnetic field with Occam''s method reduce the number of equivalent models, which were derived from inversions of single components and fit the data to a similar misfit. The 1D results, together with results from other geophysical measurements, serve as the basic model for further 2D forward modelling. The final model depicts a layering that follows the topography of the strato-volcano. In the depth range of 500 m to 1000 m, the resistivity of the layers decreases rapidly downwards into a good conductor with resistivities below 10 Ωm. The deepest layer has a resistivity of 0.4 Ωm which is quantitatively explained with a combination of saline fluids and hydrothermally altered minerals. Furthermore, the final model supports a hypothesis from the interpretation of central-loop TEM (Transient Electromagnetic) data that there is a fault structure below the southern flank, approximately 7.3 km south of the summit. To the north of the fault, the top of the good conductor is lowered from a depth of 500 m to 1000 m. We propose that the fault structure coincides with an ancient avalanche caldera rim. [Copyright &y& Elsevier]

Abstract: Wide-band (320–0.001Hz) and long period (0.01–0.0001Hz) magnetotelluric (MT) data were acquired along two profiles crossing the western part of the North Anatolian fault zone (NAFZ), Turkey, which consists of two main fault branches. The first profile (İzmit profile) crosses the epicentral area of the 17 August 1999 İzmit earthquake. In fact, the MT measurements along this profile started just a few weeks before the occurrence of the mainshock and four instruments happened to be in operation in the vicinity of the fault when the earthquake took place. The second profile (Adapazarı profile) is located about 30km east of the first profile. Two-dimensional modeling shows the following results. First, the hypocenters of mainshock and aftershocks are located on the highly resistive side near the edge of a conductive zone. Second, the long-period MT data show a low resistivity zone extending down to 50km between the two fault branches. Such a deep conductive zone is interpreted as representing partial melting resulted from the past complex tectonics in this region, and it is related to the non-seismic after-slip in the layer below the seismogenic zone characterized by the highly resistive layer. [Copyright &y& Elsevier]

Inversion of the magnetotelluric data across the southwestern Taurides reveals two subzones of crust with varying thicknesses: conductive lower crust (<75 Ω m), overlain by resistive (>350 Ω m) upper crust, with four resistive cores (>2000 Ω m) separated by three relatively conductive vertical zones. The first and second vertical zones coincide with surface faults interpreted in Anatolia, such as Fethiye Burdur Fault Zone. The third one is the most conductive and lies in continuity with the Strabo Fault Zone in the Mediterranean Sea. A hypocentral cross section of earthquakes along the profile shows more dense seismic activity in the second resistive core where the conductive crust is not present beneath it. The depth of the crust/upper mantle boundary varies between 30 and 50 km and has an undulating character. The resistivity of the upper mantle reaches 500–1000 Ω m. [Copyright &y& Elsevier]

Conclusion: Preconcentration of metals are derived from sulfide-enriched cumulates in the lower arc crust; transcrustal structures imaged by geophysical data shows their roles for time–space window of mineralization in arc. [Display omitted] • Lower-crustal conductor relates sulfide-enriched cumulates beneath Hatu Au-deposit. • No such cumulates present beneath Baogutu porphyry Cu-Au deposit. • Baogutu deposit formed during active delamination/relamination in forearc. • Hatu deposits formed during transition from compression to extension. • Metal preconcentration in arc crust leave footprints on geophysical structures. Previous studies showed that metal preconcentration in fluids and/or magma is critical to the formation of gold deposit in arcs and is closely related to the segregation of sulfide phases. However, the depth range and the magnitude of and associated mechanism by which the deep crust controls the metal preconcentration of arc mineralization remain enigmatic. Here we present geophysical observations of the medium-scale Baogutu porphyry copper–gold deposit and the large-scale Hatu epithermal gold deposit from the late Paleozoic arc in the western Junggar, NW China. During the closure of Paleo-Asian Ocean, Baogutu deposit formed at ∼313 Ma relating to the adakitic intrusions driven by the ridge-related subduction, and Hatu deposit formed at ∼300 Ma just before the subduction termination. The transcrustal structures of resistivity and shear-wave velocity would show the signatures of metal preconcentration by reasoning the cause of low-resistivity anomaly to the persisting sulfide phases and the change of shear-wave velocity to the garnet proportion, respectively. Our results showed that the sulfide phases persisted in the entire deep crust and thus the metal preconcentration could occur in the base cumulates and mush reservoir beneath Hatu deposit, but the signatures were not fully reflected beneath the Baogutu deposit. These observations suggest that the reduced permeability of brittle-ductile transition and the water flux associated with magma likely affect the rising rate of melt, resulting in different efficiency of the metal preconcentration. Thus, the endowments of mineralization in arcs are controlled by crustal structures and contemporaneous arc magmatism during subduction. [ABSTRACT FROM AUTHOR]