Conductive Asthenosphere Research Articles

The Role of Tectonic Plate Thickness and Mantle Conductance in Determining Regional Vulnerability to Extreme Space Weather Events: Possible Enhancement of Magnetic Source Fields by Secondary Induction in the Asthenosphere

AbstractDuring magnetic storms, solar‐magnetosphere‐ionosphere‐Earth interactions give rise to geomagnetically induced currents (GICs) in man‐made technological conductors such as power grids, gas pipelines, and railway networks with potentially damaging outcomes. Generally, electrically conductive regions of the Earth are assumed to be less at risk to GICs than resistive ones, since induced electric fields associated with GICs are linearly related to given magnetic source fields via Earth's impedance. Here, we show that magnetic source fields associated with storms can be enhanced by secondary electromagnetic (EM) induction in Earth's electrically conductive asthenosphere and that this previously neglected effect can give rise to larger electric fields close to the lithosphere‐asthenosphere boundary in regions where the conductance of the asthenosphere is higher. Our analysis of data from the 30 October 2003 “Halloween” and 8 September 2017 storms shows that the magnitudes of electric fields from both storms are affected by lithospheric plate thickness and asthenosphere conductance (conductivity‐thickness product) and that they are 5 times larger in southern Sweden (>5 V/km for the 30 October 2003 “Halloween” storm) than in central Scotland. Our results provide insight into why Sweden experienced a storm‐related power outage in 2003, whereas Scotland did not.

Long-period (10 < T < 20,000 s) Magnetotelluric Studies in Northern Vietnam

Abstract —We consider results of magnetotelluric and magnetovariational soundings in the period range 10 < T ≪ 20 000 s in the North Vietnam area. The simple structure of magnetovariational responses is shown, which generally reflects the electrically conductive quasi-two-dimensional structure of the Earth’s crust. Impedance responses form as a superposition of responses of local complex subsurface and quasi-two-dimensional regional deep-seated structures. Separation of local and regional effects made it possible to construct a geoelectrical depth model of the regional tectonosphere, whose main elements are electrically conductive subvertical trans-lithospheric faults and high-resistivity disturbances of the conductive asthenosphere. These elements, favoring the flow of telluric currents induced in deep-seated electrically conductive systems into the sedimentary cover, form zones of abnormal apparent-resistivity curves. The apparent resistivity monotonously increases with increasing the period up to 20,000 s. We called the combination of such elements of the geoelectrical model a ultradeep fluid–fault system (UDFFS). The modeling has established the location of three orthogonally intersecting UDFFS of NE and SE strikes in the North Vietnam area. It shows that conductive (fluid-saturated) translithospheric faults extending to the base of the sedimentary sequence control the location of petroleum fields and ore deposits. A method for separating local and regional magnetotelluric effects is proposed. It permits one to determine reliably the main strikes of a regional two-dimensional structure and the configuration of the apparent-resistivity curves along them.

Constraints on the resistivity of the oceanic lithosphere and asthenosphere from seafloor ocean tidal electromagnetic measurements

SUMMARY The electromagnetic (EM) field generated by ocean tidal flow is readily detectable in both satellite magnetic field data, and in ocean-bottom measurements of electric and magnetic fields. The availability of accurate charts of tidal currents, constrained by assimilation of modern satellite altimetry data, opens the possibility of using tidal EM fields as a source to image mantle electrical resistivity beneath the ocean basins, as highlighted by the recent success in defining the globally averaged lithosphere–asthenosphere boundary (LAB) with satellite data. In fact, seafloor EM data would be expected to provide better constraints on the structure of resistive oceanic lithosphere, since the toroidal magnetic mode, which can constrain resistive features, is a significant component of the tidal EM field within the ocean, but is absent above the surface (in particular in satellite data). Here we consider this issue in more detail, using a combination of simplified theoretical analysis and 1-D and 3-D numerical modelling to provide a thorough discussion of the sensitivity of satellite and seafloor data to subsurface electrical structure. As part of this effort, and as a step toward 3-D inversion of seafloor tidal data, we have developed a new flexible 3-D spherical-coordinate finite difference scheme for both global and regional scale modelling, with higher resolution models nested in larger scale solutions. We use the new 3-D model, together with Monte Carlo simulations of errors in tidal current estimates, to provide a quantitative assessment of errors in the computed tidal EM signal caused by uncertainty in the tidal source. Over the open ocean this component of error is below 0.01 nT in Bz at satellite height and 0.05 nT in Bx on the seafloor, well below typical signal levels. However, as coastlines are approached error levels can increase substantially. Both analytical and 3-D modelling demonstrate that the seafloor magnetic field is most sensitive to the lithospheric resistance (the product of resistivity and thickness), and is more weakly influenced (primarily in the phase) by resistivity of the underlying asthenosphere. Satellite data, which contain only the poloidal magnetic mode, are more sensitive to the conductive asthenosphere, but have little sensitivity to lithospheric resistance. For both seafloor and satellite data’s changes due to plausible variations in Earth parameters are well above error levels associated with source uncertainty, at least in the ocean interior. Although the 3-D modelling results are qualitatively consistent with theoretical analysis, the presence of coastlines and bathymetric variations generates a complex response, confirming that quantitative interpretation of ocean tidal EM fields will require a 3-D treatment. As an illustration of the nested 3-D scheme, seafloor data at five magnetic and seven electric stations in the northeastern Pacific (41○N, 165○W) are fit with trial-and-error forward modelling of a local domain. The simulation results indicate that the lithospheric resistance is roughly 7 × 108 Ωm2. The phase of the seafloor data in this region are inconsistent with a sharp transition between the resistive lithosphere and conductive asthenosphere.

Key results on deep electrical conductivity anomalies in the Pannonian Basin (PB), and their geodynamic aspects

In the first part of the paper a brief introduction is given (a) to the magnetotelluric (MT) and magnetovariation (MV) soundings, (b) to electrical resistivity of minerals and rocks, and their dependence on temperature and fluid content. The basic geoelectric model in the Pannonian Basin is a (linear) tectonic zone (“dike”) and a series of dikes. In the second part the main crustal and mantle conductivity anomalies observed in the Pannonian Basin (PB) are summarized: (a) deep conductive crustal fractures and their relation to the occurrence of the earthquakes, boundary of the megablock(s) etc., (b) middle crustal conductors as indicators of decrease in stress and seismic activity depending on thermal regime, (c) the conductive asthenosphere and its lateral change in the PB in connection with the heat flow and tectonics. The general relation between the depth of the conductive asthenosphere and the regional heat flow is also presented, (d) MT anisotropy of the resistivity distribution in the PB and its tectonic explanation based on Haas’s tectonic map (2001, Fig. 1). All these observations and conclusions are considered as results for further investigations.

Indication of meta-anthracite by magnetotellurics in the Kőszeg-Rechnitz Penninic window: a test area

One of the Penninic Nappes is the Kőszeg-Rechnitz (K-R) tectonic window at the Eastern end of the Eastern Alps. It has a complicated metamorphic history from the Jurassic time. The organic material of the Penninic Ocean was transformed to electrically conductive meta-anthracite. Its amount in the chalcophyllite is estimated by geochemists to 0.2 per cent.Taking this conducting structure as a test area pilot deep magnetotelluric (MT) soundings have been carried out and we determined the structure of the conductivity anomaly due to 0.2 per cent meta anthracite in the K-R window and its surroundings the different kinds of MT distortions as lateral (side) effect of the conductor appearing in the crust and mantle the most probable depth of the conductive asthenosphere at the border of the Pannonian Basin (having extreme shallow asthenosphere). The obtained ∼140 km depth is in correlation with value of the asthenospheric map based mainly on seismic data.

Geoelectrical Structure of the Lithosphere and Asthenosphere beneath the Northwestern United States

We have inverted magneto telluric (MT) data collected in nine states of the northwestern United States as a part of the EarthScope project for 3D imaging of electrical resistivity to a depth of 500 km using recent advances in extremely largescale electromagnetic modeling and inversion. The results of our mega-cell 3D inversion reveal multi-scale geo electrical in homogeneities in the upper mantle, which are closely related to major known tectonic features. Our geoelectrical model clearly shows a resistive structure associated with the Juan de Fuca slab subducting beneath the northwestern United States, and the conductive zone of partially melted material above the subducting slab due to the release of fluids from the down going slab. We observe extensive areas of moderate-to-high conductive asthenosphere below 100 to 200 km. The geoelectrical model also shows a prominent conductive feature associated with the partially melted mantle plumelike layer of the Yellowstone hotspot. These results correlate reasonably well with P-wave and S-wave velocity models independently obtained from seismic tomography.

Electromagnetic Imaging of the Thrace Basin and Intra-Pontide Subduction Zone, Northwestern Turkey

Magnetotelluric data were acquired at nine stations along an ˜80 km profile crossing the Thrace Basin and the Istranca Massif. These data were modeled using two-dimensional inverse techniques. The main findings are: (1) In the northernmost part of the profile, a wide (˜21 km) zone of the Istranca Massif with very high resistivity (>2000 ohm m) was imaged at a depth of 2.5 km, extending to ˜35 km depth. (2) Sedimentary rocks of the ˜63 km wide Thrace Basin (<75 ohm m) extend to a maximum depth of about 8 km along the profile. (3) Below the Thrace Basin, an undulating ˜10 km thick zone of conductive lower crust with low resistivity (<75 ohm m) is present. (4) The depth to the lithospheric upper mantle (˜250 ohm m) is about 45 km beneath the Istranca Massif in the northernmost part of the profile, and rises to a lowest value of 17 km toward the southeast part of the profile, possibly indicating mantle uplift. (5) The location and existence of the Intra-Pontide subduction zone in the Thrace Basin is the subject of debate. We imaged for the first time a slab subducting to the north beneath the Istranca massif in the Thrace Basin. (6) Our georesistivity model also indicates that, in contrast to western Turkey, an electrically conductive asthenosphere is not present in northwestern Turkey.

The magnetotelluric (MT) investigation of the Diósjenő dislocation belt

Firstly the authors give an overview on the geological, geophysical and tectonical features of the Diósjenő dislocation belt (or zone, according to some authors) around the river Ipoly near the Hungarian-Slovak border among great structural units: Vepor, Gemericum and formations of the Mid Hungarian Mts. The longest magnetic anomaly of the Pannonian Basin lies in this belt. It is assumed that it is due to ultrabasic magmatite of greenschist facies. The near-surface geoelectric soundings did not find any conductivity increase near Diósjenő (western part of the zone), but there are graphitic micaschists in the boreholes around Szécsény. There is some earthquake activitiy in the region with hypothetical depth of 7-8 km. Two deep magnetotelluric (MT) profiles cross the dislocation zone. The resistivity distribution from the surface to the conductive asthenosphere along these profiles was obtained by using instruments, operating in two different period ranges. After processing the measured data by 1D/2D inversion, it became obvious that the dislocation zone includes electrically conducting roots at a depth of 7-11 km. This result hints at the presence of fluid in the broken rocks having increased porosity in the dislocation zone. Another component that can increase the conductivity could be the graphite (carbon) originating from the Paleozoic crystalline rocks of the Gemericum (or Vepor). The ductile phase (fluid/graphite) observed by high conductivity in the centre of the dislocation zone can play an important role in the generation of the earthquakes according to the most recent statements of the international literature.

Phase paradox in the Békés basin and a possibility for its resolution by 2D/3D modelling

In the area of the typical 3D Bekes basin (“graben”) high anisotropy (difference) appears in the extreme values of the magnetotelluric impedance phases in the period interval corresponding to the indication of the conductive asthenosphere. As proved by the forward 2D/3D modelling, this anisotropy can only be explained by a strong 2D effect. This fact may confirm the validity of the results of the 2D inversion of PGT1 magnetotelluric data indicating the upwelling of the asthenosphere below the Bekes basin.

Is the asthenosphere electrically anisotropic?

Two techniques for resolving the conductance of an electrically conductive asthenosphere are presented. The first technique combines observatory electromagnetic data in the period range of the daily variation and longer, which provides penetration depths of 400 km and deeper, with laboratory data of the conductivity of upper mantle materials. The conductance can be estimated by this technique, but the geometry - e.g. the exact depth - is not resolved. The second technique extends the classical magnetotelluric (MT) tensor estimation and decomposition to the period range 1000–30 000 s. If an electrical asthenosphere exists, then in this period range the electromagnetic field penetrates into it (1000 s) but also through it (30 000 s). The concept of regional strike directions which has been successfully used to obtain crustal anisotropy directions is again employed at these long periods. The surprising result is that in the depth range of the asthenosphere a strong directional dependence of the conductance again occurs. Although large arrays will be necessary in order to prove that this is really conductivity anisotropy in the asthenosphere, first field results from arrays 100–200 km in extent are encouraging. The concept of a superposition of crustal and upper mantle anisotropy explains to some extent why earlier magnetotelluric determinations of the asthenosphere had little success: only in the fortunate case where a MT experiment was performed not parallel to the direction of the crustal conductor, but parallel to the asthenospheric one, is the second structure resolved. Electrical anisotropy in the uppermost mantle supports the concept of intracrystalline water, which allows for a contribution of hydrogen diffusity to the conductivity of olivine: this diffusity is highly anisotropic with respect to the crystal axis, and if the crystals are partly aligned, the conductivity measured with MT is also direction-dependent.

Electromagnetic induction in the New Zealand South Island

Laboratory magnetic field measurements for a scaled analogue model of New Zealand and the surrounding oceans are used to provide induction arrows for simulated geomagnetic variations of 5–120 min periods for a series of traverses over the South Island. As the island is relatively narrow, the field responses for all on-land locations show significant effects of the surrounding oceans. The model simulates a uniformly resistive island, the oceans, and a conductive asthenosphere at depth, and thus discrepancies between the model and field results for sites on the island are attributed to anomalous geological structures not simulated in the model. Removing the coast effects before interpretation, by subtracting the model induction arrows from the field site arrows at Eyrewell permits an obvious reinterpretation that attributes a significant component of the field responses to the nearby Alpine and Marlborough Faults, rather than to the channelled current in Cook Strait.

Comparison of analogue model and field station EM responses on Southern Vancouver Island

Abstract Measurements of geomagnetic induction responses for a laboratory analogue model, that includes a simulation of the subducting Juan de Fuca Plate, are compared with those obtained at three sites on Vancouver Island. Good agreement between model and field responses at the central Vancouver Island site is observed over the period range 3–60 min, while at the east and west coastal sites, good agreement is achieved only for periods greater than 20 min. At shorter periods, departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation, and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model. Nevertheless, the analogue model results are consistent with the premise of a Juan de Fuca plate, underlain by the conductive asthenosphere, subducting at a shallow depth beneath Vancouver Island.

A Two-Dimensional Conductivity Model across Central Japan

Horizontal profiles of geomagnetic transfer function were obtained at the periods of 15, 30, 60 and 120 minutes by analysing 3-component geomagnetic variation data at 21 sites in the central part of Japan and 7 sites on the Philippine Sea floor. A two-dimensional conductivity model was constructed, by use of the finite element method, to represent the spatial and period dependence of these transfer functions.Characteristics of the observed transfer functions on the Pacific coast and continental shelf of the Philippine Sea were well explained by introducing a highly conductive layer beneath the Philippine Sea plate at a depth of about 30km in addition to a large conductivity contrast between land and seawater. In order to account for the anomalous phase variation observed at the northern half of the central Japan sites, conductive lower crust was required beneath this region. This lower crustal conductor also explained the reduction of the coast effect on the Japan sea coast, and the existence of free water in the lower crust was considered as its likely cause. Our result did not require the conductive upper mantle beneath the Japan Sea, at least 200km away from the coast, to explain the data, which may suggest that the plate motion is the possible cause of highly conductive asthenosphere as well as the partial melting.

Depth of crustal conducting layer and asthenosphere in the Pannonian basin determined by magnetotellurics

On the basis of numerical modelling and statistical investigations of the horizontal electrical inhomogeneities (narrow fractures, change of conductance of the surface sediments) ρ min magnetotelluric (MT)-sounding curves were found to be less distorted and, therefore, they can be used for the determination of deep conducting formations in the crust and upper mantle. Carefully selected MT curves indicate the crustal conductive layer at depths of 7–13 km and the conductive asthenosphere between 50 and 80 km. The relatively shallow position of the asthenosphere corresponds to the high heat flow ( q > 80 mW m −1) in the Pannonian basin according to the empirical relation derived by Ádám (1978).