Magnetotelluric (MT) data were used to create a three-dimensional electrical resistivity model of the Altiplano-Puna magma body (APMB) in the area surrounding Volcán Uturuncu in southern Bolivia. This volcano is at the center of a zone of surface deformation with a diameter of 150 km and persistent inflation of ~10 mm/yr. Low electrical resistivities (<3 Ωm) at a depth of 14 + 1/-3 km below sea level (16-20 km below surface) are interpreted as being due to the presence of andesite melts in the APMB, and require a mini mum melt fraction of 15%. The upper crustal resistivity structure is characterized by finite-length, dike-shaped conductors, oriented approximately east-west near sea level. A combination of dacite partial melts and aqueous fluids is required to explain the observed low-resistivity values. Geodetic data do not require any deformation in these shallow regions. The geometry of the upper surface of the APMB beneath Volcán Uturuncu is consistent with that predicted by geodynamic models that suggest that the APMB bulges upward directly beneath Volcán Uturuncu, near the measured inflation center (~3 km west of Volcán Uturuncu). Viscosity estimates from the MT-derived resistivity model gives a maximum value of 10 16 Pa•s and is consistent with models that propose diapir-like ascent of magma above the APMB. Resistivity models are compared and quantitatively correlated to seismic velocity models, showing good agreement on the spatial extent and depth of the APMB. A forward modeling study shows that the small differences in the depth to the top of the APMB between the different geophysical methods could be explained by variations in the composition of the magma body.

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The Laguna del Maule (LdM) volcanic field comprises the greatest concentration of postglacial rhyolite in the Andes and includes the products of ~40 km3 of explosive and effusive eruptions. Recent observations at LdM by interferometric synthetic aperture radar and global navigation satellite system geodesy have revealed inflation at rates exceeding 20 cm/year since 2007, capturing an ongoing period of growth of a potentially large upper crustal magma reservoir. Moreover, magnetotelluric and gravity studies indicate the presence of fluids and/or partial melt in the upper crust near the center of inflation. Petrologic observations imply repeated, rapid extraction of rhyolitic melt from crystal mush stored at depths of 4–6 km during at least the past 26 ka. We utilize multiple types of surface‐wave observations to constrain the location and geometry of low‐velocity domains beneath LdM. We present a three‐dimensional shear‐wave velocity model that delineates a ~450‐km3 shallow magma reservoir ~2 to 8 km below surface with an average melt fraction of ~5%. Interpretation of the seismic tomography in light of existing gravity, magnetotelluric, and geodetic observations supports this model and reveals variations in melt content and a deeper magma system feeding the shallow reservoir in greater detail than any of the geophysical methods alone. Geophysical imaging of the LdM magma system today is consistent with the petrologic inferences of the reservoir structure and growth during the past 20–60 kyr. Taken together with the ongoing unrest, a future rhyolite eruption of at least the scale of those common during the Holocene is a reasonable possibility.

The subduction zone of central Chile (36°S) has produced some of the world's largest earthquakes and significant volcanic eruptions. Understanding the fluid fluxes and structure of the subducting slab and overriding plate can provide insight into the tectonic processes responsible for both seismicity and magmatism. Broadband and long‐period magnetotelluric data were collected along a 350‐km profile in central Chile and Argentina and show a regional geoelectric strike of 15 ± 19° east of north. The preferred two‐dimensional inversion model included the geometry of the subducting Nazca plate as a constraint. On the upper surface of the Nazca plate, conductors were interpreted as fluids expelled from the downgoing slab via compaction at shallow depth (C1) and metamorphic reactions at depths of 40–90 km (C2 and C3). At greater depths (130 km), a conductor (C7) is interpreted as a region of partial melt related to deserpentinization in the backarc. A resistor on the slab interface (R1) is coincident with a high‐velocity anomaly which was interpreted as a strong asperity which may affect the coseismic slip behavior of large megathrust earthquakes at this latitude. Correlations with seismicity suggest slab fluids alter the forearc mantle and define the downdip limit of the seismogenic zone. Beneath the volcanic arc, several upper crustal conductors (C4 and C5) represent partial melt beneath the Tatara‐San Pedro Volcano and the Laguna del Maule Volcanic Field. A deeper lower crustal conductor (C6) underlies both volcanoes and suggests a connected network of melt in a thermally mature lower crust.

SUMMARY Krafla is an active volcanic field and a high-temperature geothermal system in northeast Iceland. As part of a program to produce more energy from higher temperature wells, the IDDP-1 well was drilled in 2009 to reach supercritical fluid conditions below the Krafla geothermal field. However, drilling ended prematurely when the well unexpectedly encountered rhyolite magma at a depth of 2.1 km. In this paper we re-examine the magnetotelluric (MT) data that were used to model the electrical resistivity structure at Krafla. We present a new 3-D resistivity model that differs from previous inversions due to (1) using the full impedance tensor data and (2) a finely discretized mesh with horizontal cell dimensions of 100 m by 100 m. We obtained similar resistivity models from using two different prior models: a uniform half-space, and a previously published 1-D resistivity model. Our model contains a near-surface resistive layer of unaltered basalt and a low resistivity layer of hydrothermal alteration (C1). A resistive region (R1) at 1 to 2 km depth corresponds to chlorite-epidote alteration minerals that are stable at temperatures of about 220 to 500 °C. A low resistivity feature (C2) coincides with the Hveragil fault system, a zone of increased permeability allowing interaction of aquifer fluids with magmatic fluids and gases. Our model contains a large, low resistivity zone (C3) below the northern half of the Krafla volcanic field that domes upward to a depth of about 1.6 km b.s.l. C3 is partially coincident with reported low S-wave velocity zones which could be due to partial melt or aqueous fluids. The low resistivity could also be attributed to dehydration and decomposition of chlorite and epidote that occurs above 500 °C. As opposed to previously published resistivity models, our resistivity model shows that IDDP-1 encountered rhyolite magma near the upper edge of C3, where it intersects C2. In order to assess the sensitivity of the MT data to melt at the bottom of IDDP-1, we added hypothetical magma bodies with resistivities of 0.1 to 30 Ωm to our resistivity model and compared the synthetic MT data to the original inversion response. We used two methods to compare the MT data fit: (1) the change in r.m.s. misfit and (2) an asymptotic p-value obtained from the Kolmogorov–Smirnov (K–S) statistical test on the two sets of data residuals. We determined that the MT data can only detect sills that are unrealistically large (2.25 km3) with very low resistivities (0.1 or 0.3 Ωm). Smaller magma bodies (0.125 and 1 km3) were not detected; thus the MT data are not sensitive to small rhyolite magma bodies near the bottom of IDDP-1. Our tests gave similar results when evaluating the changes in r.m.s. misfit and the K–S test p-values, but the K–S test is a more objective method than appraising a relative change in r.m.s. misfit. Our resistivity model and resolution tests are consistent with the idea of rhyolite melt forming by re-melting of hydrothermally altered basalt on the edges of a deeper magma body.

A profile of broadband magnetotelluric stations was acquired between 2009 and 2016 at 35°-36°S in the Southern Volcanic Zone of the Chilean Andes to image the subduction zone and its relation with the volcanic arc at this latitude. This transect extends from the Coastal Cordillera across the Central Valley and the volcanic arc of the Principal Cordillera to the Argentine border. Two active volcanic complexes are found along this profile: Tatara-San Pedro is located on the modern volcanic front, and the Laguna del Maule volcanic field is found approximately 30 km to the east. The latter exhibits considerable signs of unrest, such as uplift rates of up to 25 cm/year, and has produced a high concentration of silicic eruptions in the last 25 ky. The data covered the period range from 0.001 to 1000 s. Robust processing techniques were used, including remote reference, and dimensionality was investigated by estimation of geoelectric strike, skew and analysis of the induction arrows. The data were modeled using a 2D inversion algorithm to produce a resistivity model which was consistent with surface geology and seismicity. The final resistivity model shows a generally resistive fore-arc structure, coincident with the tectonic environment, and a wide conductive region from the volcanic front to the east. This suggests a broad region of magmatism throughout the arc, related to three distinct magma bodies, associated with the Tatara-San Pedro and Laguna del Maule volcanic complexes and the Mariposa Geothermal System.

Large geomagnetic disturbances (GMDs) on the surface of the Earth are caused by perturbations to the coupled solar wind-magnetosphere-ionosphere system during geomagnetic storms. Large GMDs can result in geomagnetically induced currents (GICs) which have the potential to cause significant damage to electrical power grids (e.g., Boteler, 2001Boteler, , 2014Kelbert, 2020). These quasi-DC currents develop because

Imaging silicic systems using geophysics is challenging because many interrelated factors (e.g., temperature, melt fraction, melt composition, geometry) can contribute to the measured geophysical anomaly. Joint interpretation of models from multiple geophysical methods can better constrain interpretations of the subsurface structure. Previously published resistivity and shear wave velocity (Vs) models, derived separately from magnetotelluric (MT) and surface wave seismic data, respectively, have been used to model the restless Laguna del Maule Volcanic Field, central Chile. The Vs model contains a 450 km 3 low-velocity zone (LVZ) interpreted as a region with an average melt fraction of 5-6%. The resistivity model contains a conductor (C3) interpreted as a region with a melt fraction >35%. The spatial extents of the LVZ and C3 overlap, but the geometries and interpretations of these features are different. To resolve these discrepancies, this study investigates the resolution of the MT data using hypothesis testing and constrained MT inversions. It is shown that the MT data are best fit with discrete conductors embedded within the larger LVZ. The differences between the MT and seismic models reflect resolution differences between the two data sets as well as varying sensitivities to physical properties. The MT data are sensitive to smaller volumes of extractable mush that contain well-connected crystal-poor melt (C3). The seismic data have lower spatial resolution but image the full extent of the poorly connected crystal-rich magma storage system. The combined images suggest that the LdMVF magma plumbing system is thermally heterogeneous with coexisting zones of warm and cold storage.