In December 2019, the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group (V-MOD) adopted the thirteenth generation of the International Geomagnetic Reference Field (IGRF). This IGRF updates the previous generation with a definitive main field model for epoch 2015.0, a main field model for epoch 2020.0, and a predictive linear secular variation for 2020.0 to 2025.0. This letter provides the equations defining the IGRF, the spherical harmonic coefficients for this thirteenth generation model, maps of magnetic declination, inclination and total field intensity for the epoch 2020.0, and maps of their predicted rate of change for the 2020.0 to 2025.0 time period.

The magnetotelluric component of the Mantle Electromagnetic and Tomography (MELT) Experiment measured the electrical resistivity structure of the mantle beneath the fast-spreading southern East Pacific Rise (EPR). The data reveal an asymmetric resistivity structure, with lower resistivity to the west of the ridge. The uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles. Mantle resistivities to the west of the ridge are consistent with a low-melt fraction (about 1 to 2 percent interconnected melt) distributed over a broad region and extending to depths of about 150 kilometers. The asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates.

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A seafloor geomagnetic observatory in the northwest Pacific detected clear electromagnetic (EM) variations associated with tsunami passage from two earthquakes that occurred along the Kuril Trench. Previous seismological analyses indicated that the M8.3 earthquake on 15 November 2006 was an underthrust type on the landward slope of the trench, while the M8.1 earthquake on 13 January 2007 was a normal fault type on the seaward side. The EM measurements enabled precise monitoring of the tsunami propagation direction as well as particle motion of the seawater. The estimated horizontal water velocity differs significantly for the 2006 and 2007 tsunamis, in terms of initial motion and dispersive characters, being consistent with the hydrodynamic simulation results of the tsunamis. Namely, the tsunami‐induced horizontal geomagnetic components showed opposite signs for the rise and retreat waves as expected from the “electric current wall hypothesis.” The dispersion effect is more remarkable in the 2007 event with a smaller source region of its tsunamigenic earthquake. The 2007 tsunamis, therefore, tend to violate the long‐wave approximation. The Boussinesq approximation was required to reproduce the dispersive character of the 2007 event in our numerical simulation. In terms of tsunami forecast, an important advantage of EM sensors over conventional tsunami sensors, such as seafloor pressure gauges, is their capability of vector measurements: in addition to their ability to monitor particle motions, the first peak of the downward magnetic component always precedes the tsunami peak, suggesting a significant improvement in global tsunami warning systems if vector EM sensors are integrated into the existing systems.

Tsunami flow coupled with the geomagnetic field generates electric currents and associated magnetic fields. Although electromagnetic (EM) tsunami signals can be used for analysis and even forecasting tsunami propagation, the dynamically self‐consistent effect of shoaling water depth on the fluid + electrodynamics has not been adequately clarified. In this study, we classify tsunami EM phenomena into three cases based on the ocean depth and find that the deeper ocean results in stronger self‐induction due to the increase in both tsunami phase velocity and ocean conductance. In this deep‐ocean case, the phase lead of the vertical magnetic variation relative to the sea surface elevation is smaller, while an initial rise in the horizontal magnetic component becomes observable prior to tsunami arrival. Furthermore, we confirm that the enhancement of tsunami height in shallower oceans shifts the ocean depth supplying maximum amplitudes of tsunami magnetic fields from approximately 2.0 km to 1.5 km.

Various types of electrodes designed for the measurement of the electric field in the soil or in sea water at periods larger than one minute have been compared in a one-year experiment in Garchy, France. The experiment included more than fifty electrode pairs with liquid or absorbed electrolytes and Pb/PbCl2, Ag/AgCl, Cu/CuSO4 and Cd/CdC12 metal-ion couples. The electrode parameters were systematically measured in the laboratory and the electrodes were installed in the field to constitute 50-meter long parallel dipoles separated by 2 meters. Pairs of electrodes used for sea measurements were monitored in a salted water vessel. Fourtytwo potential differences were recorded with a sampling interval of 1 minute between May 1995 and April 1996. When electrodes are compared, large differences are observed in the long term stability as well as in the sensitivity to diurnal variations, rainfall and soil saturation. For measurements in soil, the installation method of the electrodes plays an important role. In salted water, the best performing electrode pair has a drift of the order of 0.1 mV per year. In soil, typical drifts for the best sensors are of the order of 0.2 mV per month in dry soil and 0.5 mV per month in soaked soil. Preferred electrode designs and installation methods, depending on the external conditions or the type of geophysical measurement, emerge from this experiment. In addition to the magneto-telluric field, potential variations which are not electrode or installation effects are observed and attributed to electrical sources in the soil.

We obtained an electrical transect image of the Niigata‐Kobe Tectonic Zone (NKTZ). Several major active faults are located in this zone of concentrated deformation. The main features of the final two‐dimensional model are a thick resistive block in the upper crust, with a thinned‐out portion beneath the Atotsugawa Fault, and a strong conductor in the lower crust that intrudes upward into the upper resistor. The upper crustal resistive zone corresponds well to the spatiality of the NKTZ, and relatively conductive zones sandwiching this resistor may contribute to observed changes in displacement rates. The overlapping locations of the conductor and the low‐velocity body in the lower crust indicate that the conductor represents a zone that was weakened by fluids. Given that microearthquakes are localized in the regions between the resistive and conductive zones, we suggest that the distribution of earthquakes is influenced by intrusions of fluid derived from the conductor.

Motional induction in the ocean by tides has long been observed by both land and satellite measurements of magnetic fields. While these signals are weak (∼10 nT) when compared to the main magnetic field, their persistent nature makes them important for consideration during geomagnetic field modelling. Previous studies have reported several discrepancies between observations and numerical predictions of the tidal magnetic signals and those studies were inconclusive of the source of the error. We address this issue by (1) analysing magnetometer data from ocean-bottom stations, where the low-noise and high-signal environment is most suitable for detecting the weak tidal magnetic signals, (2) by numerically predicting the magnetic field with a spatial resolution that is 16 times higher than the previous studies and (3) by using four different models of upper-mantle conductivity. We use vector magnetic data from six ocean-bottom electromagnetic (OBEM) stations located in the Northwestern Pacific Ocean. The OBEM tidal amplitudes were derived using an iteratively re-weighted leastsquares (IRLS) method and by limiting the analysis of lunar semidiurnal (M2), lunar elliptic semidinurnal (N2) and diurnal (O1) tidal modes to the night-time. Using a 3-D electromagnetic induction solver and the TPX07.2 tidal model, we predict the tidal magnetic signal. We use earth models with non-uniform oceans and four 1-D mantle sections underneath taken from Kuvshinov and Olsen, Shimizu et al. and Baba et al. to compare the effect of upper-mantle conductivity. We find that in general, the predictions and observations match within 10-70 per cent across all the stations for each of the tidal modes. The median normalized percent difference (NPD) between observed and predicted amplitudes for the tidal modes M2, N2 and O1 were 15 per cent, 47 per cent and 98 per cent, respectively, for all the stations and models. At the majority of stations, and for each of the tidal modes, the higher resolution (0.25 • × 0.25 • ) modelling gave amplitudes consistently closer to the observations than the lower resolution (1 • × 1 • ) modelling. The difference in lithospheric resistance east and west of the Izu-Bonin trench system seems to be affecting the model response and observations in the O1 tidal mode. This response is not seen in the M2 and N2 modes, thereby indicating that the O1 mode is more sensitive to lithospheric resistance.