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Intraplate earthquakes occur more frequently in the Japanese islands than in other regions. Large intraplate earthquakes in the island arc preferentially occur in strain concentration zones detected by geological and geodetic studies. Crustal heterogeneity plays a crucial role in generating large intraplate earthquakes and strain concentrations. Thus, we elucidated the crustal heterogeneities beneath a strain concentration area on the back‐arc side of the northeastern Japan Arc based on electrical resistivity, which is sensitive to weak zones in the crust. By deploying wideband magnetotelluric surveys, we revealed the three‐dimensional electrical resistivity structure in the crust, suggesting the coexistence of two different types of strain‐concentration mechanisms in the strain‐concentration area. The shallow conductive layers and lower‐crustal conductors appear to act as low‐elastic‐modulus and low‐viscosity areas, respectively, and are responsible for the strain concentration. We found shallow and lower‐crustal conductors in the strain concentration zone revealed by geological studies. Those conductive areas are considered to act as mechanically weak zones and cause stress loading on the brittle pars of pre‐existing faults, resulting in large intraplate earthquakes. The resultant earthquakes presumably contribute to strain accumulation on a geological timescale. In addition, a spatial correlation between the epicenters of large intraplate earthquakes and edges of lower‐crustal conductors implies the contribution of the fluids in the lower crust to the generation of large earthquakes. We also identified vertical conductors ranging from the lower crust to Quaternary volcanoes, which may indicate trans‐crustal magmatic systems under these volcanoes. Plain Language Summary: Intraplate earthquakes occur more frequently in the Japanese islands than in other regions worldwide. Large intraplate earthquakes in this region preferentially occur in high‐deformation zones, as detected by geological studies and global positioning system (GPS) measurements. Heterogeneous structure in the crust plays a crucial role in the generation of large intraplate earthquakes and high crustal deformations. Thus, we revealed the three‐dimensional crustal electrical resistivity structure beneath a strain‐concentration area in the northeastern Japan. The resistivity structure suggests the coexistence of two different mechanisms of high deformation. Shallow conductive layers and lower‐crustal conductors appear to act as low‐stiffness and low‐viscosity areas, respectively, leading to high crustal deformation. We found shallow and deep conductors in the high‐deformation zones revealed by geological studies. As both can act as weak zones, those conductors presumably play a key role in loading on faults, resulting in intraplate earthquakes. Because the epicenters of large intraplate earthquakes are positioned near the edges of lower‐crustal conductors, the fluids in those deep conductors possibly contribute to the generation of large earthquakes. We also revealed vertical conductors ranging from the deep crust to Quaternary volcanoes, which may indicate magma‐rich areas under these volcanoes. Key Points: The electrical resistivity structure suggests the coexistence of two strain‐concentration mechanisms in the strain‐concentration areaThe shallow conductive layers and lower‐crustal conductors act as low‐elastic‐modulus and low‐viscosity areas, respectivelyWeak zones in the crust, imaged as conductors, presumably cause stress loading on faults and contribute to large intraplate earthquakes [ABSTRACT FROM AUTHOR]

Along the Hikurangi subduction margin, on the east coast of New Zealand's North Island, the interplate coupling changes from locked in the south to weakly coupled in the north. New magnetotelluric (MT) data from 151 locations linking previous MT surveys into a single contiguous data set that encompasses the weakly coupled part of the margin are analysed. By inverting the combined data we have constructed a 3-D image of the electrical resistivity of the subduction interface shear zone along a 300-km-long segment of the margin. Our results show that the electrical resistivity of the subduction interface shear zone is heterogenous; the degree of heterogeneity decreasing from north to south. The resistivity heterogeneities correlate well with the distribution of near-plate interface seismicity, Vp / Vs values and the pattern of areal strain rate derived from GPS data. These correlations are consistent with variations in the fluid content of the subduction interface shear zone. In the northern part of this segment, conductive areas adjacent to the interface are interpreted to be fluid rich areas where seismicity is sparse, Vp / Vs ratios are high and the areal strain rate is extensional. In contrast, where the areal strain rate is compressional the plate interface is more resistive, and seismicity is more abundant consistent with greater interplate friction. In the south, the resistivity of the plate interface is more homogenous, and the overlying plate is more resistive at shallower levels than in the north. Our results support the hypothesis that the fluid and/or hydrated clay content of the subduction interface shear zone are an important control on interplate coupling. [ABSTRACT FROM AUTHOR]

The Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid‐rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network‐MT survey in addition to a conventional wideband‐MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer‐scale dipoles of the Network‐MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama‐Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower‐crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs. Key Points: Combined inversion method of wideband‐MT and Network‐MT data has been developed to accurately represent long‐dipole measurementsLower‐crustal conductors below active faults and a large conductor above a subducting slab are imaged under a high strain rate zoneLower‐crustal conductors are interpreted as localized ductile shear zones, being responsible for the strain accumulation along the faults [ABSTRACT FROM AUTHOR]

Monitoring dynamics of volcanic eruptions with geophysics is challenging. In August and November 2012, two small eruptions from Mount Tongariro provided a unique opportunity to image subsurface changes caused by the eruptions. A detailed magnetotelluric survey of the Tongariro volcanic complex completed prior to the eruption (2008–2010) provides the preeruption structure of the magmatic system. A subset of the initial measurement locations was reoccupied in June 2013. Significant changes were observed in phase tensor data at sites close to the eruptive center. Although subsurface electrical resistivity changed, the geometry of the preeruptive reservoir did not. These subsurface resistivity variations are interpreted as being predominantly caused by interaction of partial melt and the overlying brine layer causing volume reduction of the brine layer through phreatic eruption. The ability to detect significant changes associated with the magma reservoir suggests that magnetotellurics can be a valuable volcano monitoring tool. Plain Language Summary: Preeruption and posteruption electromagnetic magnetotelluric measurements are used to determine the variation in subsurface electrical properties resulting from changes in the Tongariro volcanic center magma chamber associated with the 2012 eruptive cycle. The observed electrical property changes are related to the physical eruption properties (e.g., eruptive volume, style, and composition), revealing the state of the magmatic system both prior to and following the eruption. Knowing both the preeruption and posteruption states of the magmatic system and the surface eruptive properties enables reconstruction of the subsurface eruption mechanism. Successful identification of preeruption and posteruptive states of the volcano is evidence for the usefulness of continuous magnetotelluric monitoring of volcanoes to identify variations within magmatic systems that may be indicative of imminent eruption. Key Points: We identify significant temporal change in preeruption and posteruption magnetotelluric soundingsAn eruption mechanism model is developed consistent with both geophysical and geological observationsMagnetotellurics is a viable additional monitoring tool for active volcanic systems [ABSTRACT FROM AUTHOR]

The Transantarctic Mountains (TAM) are the world's longest rift shoulder but the source of their high elevation is enigmatic. To discriminate the importance of mechanical vs. thermal sources of support, a 550 km-long transect of magnetotelluric geophysical soundings spanning the central TAM was acquired. These data reveal a lithosphere of high electrical resistivity to at least 150 km depth, implying a cold stable state well into the upper mantle. Here we find that the central TAM most likely are elevated by a non-thermal, flexural cantilever mechanism which is perhaps the most clearly expressed example anywhere. West Antarctica in this region exhibits a low resistivity, moderately hydrated asthenosphere, and concentrated extension (rift necking) near the central TAM range front but with negligible thermal encroachment into the TAM. Broader scale heat flow of east-central West Antarctica appears moderate, on the order of 60-70mWm-2, lower than that of the U.S. Great Basin. [ABSTRACT FROM AUTHOR]

The two-dimensional electrical resistivity structure beneath Naruko volcano was determined using magnetotelluric soundings. The resulting model shows that a prominent conductor exists through the middle crust to the uppermost mantle beneath the volcano. The location of the conductor agrees closely with a seismic low-velocity zone. Low-frequency microearthquakes occur near the conductor around the Moho depth. The cutoff depth of crustal earthquakes is coincident with the upper boundary of the conductor, implying that the conductor has a temperature appreciably higher than 400?C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed 3He/4He ratios reveals the extent of mantle-derived materials beneath Naruko volcano. Consequently, it is apparent that the conductor determined beneath the volcano reflects the presence of high-temperature mantle-derived materials such as magmas and/or related fluids derived from active magmatism in the northeastern Japan subduction zone. [ABSTRACT FROM AUTHOR]

Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike–slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike–slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models. [ABSTRACT FROM AUTHOR]

Inter-plate coupling on the Hikurangi subduction margin along the east coast of New Zealand's North Island changes from weakly coupled in the north to locked in the south. 3-D inverse modeling of magnetotelluric (MT) data across the coupling transition shows that the electrical resistivity structure is correlated with the areal strain rate, which provides a measure of plate coupling. The correlation between strain rate and resistivity is seen parallel to the strike and dip directions of the subduction. In the region where the strain is extensional, the upper plate is more conductive than in the south where contraction is occurring, and the plates are locked. The increased mid-crustal resistivity in the south can be interpreted to be a consequence of reduced fluid interconnectivity due to contraction or reduced fluid content due to decreased upward transport from sources beneath the subduction interface. The increased mid-crustal conductivity in the extensional area is interpreted to be a consequence of fluid released by subducted sediment that are also the cause of the extension. Seismicity in the upper 5 km of the subducting plate shows similar changes, with a greater concentration of near-interface seismicity beneath the conductive (extensional) region. We interpret the concentration of seismicity below the subduction interface in the extensional region to show fluid sourced from dehydration reactions within the subducting plate or from the de-watering of subducted sediments rising into the upper plate and decreasing the resistivity of the middle crust. Decreased near-interface seismicity beneath the contractional region can be interpreted to be a consequence of decreased upper plate permeability preventing upward fluid escape or reduced lower plate fluid availability. • The electrical resistivity of the upper-plate correlates with the areal strain-rate. • Where the plate is conductive, geodetic data suggest a weakly coupled interface. • The reduced resistivity is caused by fluid released from the underlying plate. • The higher resistivity in the locked area reflects decreased upward fluid-transport. [ABSTRACT FROM AUTHOR]

Seismic characteristics of any fault system are affected by the relationship between the relevant stress regime and the fluid content of a region. Under persistent stress accumulation, fluids can facilitate earthquake generation due to their effect on frictional forces. In this study, Armutlu Peninsula serves as a suitable example to investigate the role of fluid-rich environments in seismicity distribution. In the context of electrical resistivity characteristics provided by a fluid-sensitive geophysical method, namely magnetotellurics, seismogenic properties of the western part of the North Anatolian Fault are examined from its mechanical attributes to fault – fluid relationship. Near the epicenter of the catastrophic 1999 İzmit Earthquake, Hersek region appears as a fluid-rich environment between shallow creeping and aseismic strain releasing portions that bound highly resistive Armutlu Peninsula. Conductor related to fluid-rich environment extends until depth of approx. 10 km and precludes earthquake activity along the fault zone. High amount of fluids concentrated on particular conductive regions, may restrain earthquake distribution with the absence of mechanically strong structures around the fault zone. As a potential asymmetric damage zone appears around the NAF, both aftershock distributions of the 1999 İzmit Earthquake in the following three months and long-term earthquake data show a clear tendency for concentration on resistive parts of the study area. Fluid-rich regions possibly indicate the endpoints of the segments in the NAF. The final electrical resistivity model produced in this study reveals that interpreting tectonic, seismic or geological aspects of any fault system can plausibly be dependent on counting spatial extensions of fluid-rich environments. In order to correctly assemble interdisciplinary findings, it may be essential to consider more features than vertical extensions of conductive zones which, mostly indicate fluid-bearing regions. • Three-dimensional electrical resistivity model for western part of North Anatolian Fault was developed. • Relationship between rheology and seismic properties was sought under the light of electrical characteristics. • Spatial distribution of fluid-rich environments was correlated with aftershock activity of 1999 İzmit Earthquake. [ABSTRACT FROM AUTHOR]

The Kusatsu-Shirane volcano is a Quaternary andesitic-to-dacitic active volcano located on the central Honshu Arc. The volcano is known to have had repeated phreatic eruptions in last 200 years. A number of geochemical and geophysical studies have been conducted around the Yugama crater lake, the focus of current volcanic activity. However, the 2018 unexpected phreatic eruption occurred at Mt. Motoshirane, a different pyroclastic cone from that which hosts Yugama. There were also frequent magmatic eruptions until 1500 years ago at this cone and the magma produced at that time has not yet cooled and solidified. A better understanding of the magmatic hydrothermal system of this volcano requires structural information gathered over a larger area, and to greater depths, than has been achieved to date. Here, we describe the three-dimensional (3-D) electrical resistivity structure around Mt. Motoshirane down to a depth of ~10 km using broadband magnetotelluric (MT) data (320–0.0005 Hz) collected in 2015 and 2016. The 3-D resistivity structure model shows an extensive low-resistivity layer at depths of 1–3.5 km beneath the summit. However, structures characteristic of the presence of magma are not observed beneath this layer. It could be too deep or too small to be detected. Combining the new data with results of previous geochemical and geophysical studies, we interpret the conductor as a hydrothermal fluid reservoir that supplies fluids to the crater lake and to hot springs on the eastern and western flanks of the volcano. The existence of a fluid reservoir that extends from the vicinity of the Yugama crater lake to the subsurface beneath an inactive pyroclastic cone that has produced repeated magmatic eruptions in the past suggests that a large-scale magmatic hydrothermal system is developing beneath the volcano. • We conducted broadband magnetotelluric (MT) measurements in Kusatsu-Shirane volcano. • A 3-D electrical resistivity structure was inferred from the MT data. • We found an extensive low-resistivity layer at depths of 1–3 km in the summit area. • Conductor was interpreted as the source of crater-lake water and hot springs. [ABSTRACT FROM AUTHOR]