[1] Lower crustal earthquakes are commonly observed in continental rifts at depths where temperatures should be too high for brittle failure to occur. Here we present accurately located earthquakes in central Ethiopia, covering an incipient oceanic plate boundary in the Main Ethiopian Rift. Seismicity is evaluated using the combination of exceptionally well resolved seismic structure of the crust and upper mantle, electromagnetic properties of the crust, rock geochemistry, and geological data. The combined data sets provide evidence that lower crustal earthquakes are focused in mafic lower crust containing pockets of the largest fraction of partial melt. The pattern of seismicity and distribution of crustal melt also correlates closely with presence of partial melt in the upper mantle, suggesting lower crustal earthquakes are induced by ongoing crustal modification through magma emplacement that is driven by partial melting of the mantle. Our results show that magmatic processes control not only the distribution of shallow seismicity and volcanic activity along the axis of the rift valley but also anomalous earthquakes in the lower crust away from these zones of localized strain.

A previous paper (Parker, 1980) sets out a theory for deciding whether solutions exist to the inverse problem of electromagnetic induction and outlines methods for constructing conductivity profiles when their existence has been demonstrated. The present paper provides practical algorithms to perform the necessary calculations stably and efficiently, concentrating exclusively on the case of imprecise observations. The matter of existence is treated by finding the best fitting solution in a least squares sense; then the size of the misfit is tested statistically to determine the probability that the value would be met or exceeded by chance. We obtain the optimal solution by solving a constrained least squares problem linear in the spectral function of the electric field differential equation. The spectral function is converted into a conductivity profile by transforming its partial fraction representation into a continued fraction, using a stable algorithm due to Rutishauser. In addition to optimal models, which always consist of delta functions, two other types of model are examined. One is composed of a finite stack of uniform layers, constructed so that the product of conductivity and thickness squared is the same in each layer. The numerical techniques developed for the optimal model serve with only minor alteration to find solutions in this class. Models of the second kind are smooth. A special form of the response is chosen so that the kernel functions of the Gel'fand‐Levitan integral equation are degenerate, thus allowing very stable and numerically efficient solution. Unlike previously published methods for finding conductivity models, these algorithms can provide solutions with misfits arbitrarily close to the smallest one possible. The methods are applied to magnetotelluric observations made by Larsen in Hawaii.

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The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow‐moving African continent, which last experienced orogeny during the Pan‐African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas‐rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust and intruded as sills and dikes at shallower depths. In the Main Ethiopian Rift, intrusions comprise 30% of the crust below axial zones of dike‐dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow‐spreading ridges. Our comparisons suggest that transitional crust, including seaward dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15–20 km thick crust. In the 30 Ma Recent CVL, which lacks a hot spot age progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hot spot toward the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology, and contributing significantly to global carbon and water fluxes.

A systematic regular perturbation procedure is developed to account for weak mantle conduction as unsteady electromagnetic fields are extrapolated downward from the Earth's surface to the core-mantle boundary. The mantle is treated as a radially symmetric conductor of highly variable conductivity. The unique poloidal-toroidal decomposition of a magnetic vector potential leads first to three-dimensional and then, after spherical harmonic analysis, to one-dimensional linear diffusion equations for the two defining scalar functions. Emphasis is placed on a regular perturbation solution to the inverse poloidal diffusion problem, for the case where diffusion through the mantle is rapid on the time-scale for changes in forcing at the core-mantle boundary. The perturbation theory is evaluated with reference to a range of proposed conductivity profiles and two geomagnetic field models. It is found that uncertainty in the conductivity is at present less important than errors in the field models, and that the first-order corrections to the main field and secular variation at the core surface are likely to be negligible and small, respectively.

The Main Ethiopian Rift is part of the East African Rift with its unique geological setting as an active continental breakup zone. The Main Ethiopian Rift includes a number of understudied active volcanoes with potentially high risks for this densely populated part of Ethiopia. Using newly recorded (2016) magnetotelluric data along a 110 km long transect crossing the whole rift, we present a regional 2‐D model of electrical resistivity of the crust. The derived model endorses a previous study that drew the surprising conclusion that there was no highly conductive region associated with a magma chamber directly under the central rift volcano Aluto. This has implications for the estimation of the amount of magma present, its water content, and the storage conditions, as the volcano is actively deforming and results from seismicity and CO2 degassing studies all indicate magma storage at about 5 km depth. Additionally, the existence of a strong conductor under the Silti Debre Zeyt Fault Zone approximately 40 km to the northwest of the rift center is confirmed. It is located with a slight offset to the Butajira volcanic field, which hosts a number of scoria cones at the boundary between the NW plateau and the rift. The magnetotelluric model reveals different electrical structures below the eastern and western rift shoulders. The western border is characterized by a sharp lateral contrast between the resistive plateau and the more conductive rift bottom, whereas the eastern flank shows a subhorizontal layered sequence of volcanic deposits and a smooth transition toward the shoulder.

The auroral electrojets (AEJs) are complex and dynamic horizontal ionospheric electric currents which form ovals around Earth's poles, being controlled by the morphology of the main magnetic field and the energy input from the solar wind interaction with the magnetosphere. The strength and location of the AEJ varies with solar wind conditions and the solar cycle but should also be controlled on decadal timescales by main field secular variation. To determine the AEJ climatology, we use data from four polar Low Earth Orbit magnetic satellite missions: POGO, Magsat, CHAMP, and Swarm. A simple estimation of the AEJ strength and latitude is made from each pass of the satellites, from peaks in the along‐track gradient of the magnetic field intensity after subtracting a core and crustal magnetic field model. This measure of the AEJ activity is used to study the response in different sectors of magnetic local time (MLT) during different seasons and directions of the interplanetary magnetic field (IMF). We find a season‐dependent hemispherical asymmetry in the AEJ response to IMF By, with a tendency toward stronger (weaker) AEJ currents in the north than the south during By>0 (By<0) around local winter. This effect disappears during local summer when we find a tendency toward stronger currents in the south than the north. The solar cycle modulation of the AEJ and the long‐term shifting of its position and strength due to the core field variation are presented as challenges to internal field modeling.

The lithospheric contribution to the Earth's magnetic field is concealed in magnetic field data that have now been measured over several decades from ground to satellite altitudes. The lithospheric field results from the superposition of induced and remanent magnetisations. It therefore brings an essential constraint on the magnetic properties of rocks of the Earth's sub-surface that would otherwise be difficult to characterize. Measuring, extracting, interpreting and even defining the magnetic field of the Earth's lithosphere is however challenging. In this paper, we review the difficulties encountered. We briefly summarize the various contributions to the Earth's magnetic field that hamper the correct identification of the lithospheric component. Such difficulties could be partially alleviated with the joint analysis of multi-level magnetic field observations, even though one cannot avoid making compromises in building models and maps of the magnetic field of the Earth's lithosphere E. Thébault ( ) 96 E. Thébault et al.at various altitudes. Keeping in mind these compromises is crucial when lithospheric field models are interpreted and correlated with other geophysical information. We illustrate this discussion with recent advances and results that were exploited to infer statistical properties of the Earth's lithosphere. The lessons learned in measuring and processing Earth's magnetic field data may prove fruitful in planetary exploration, where magnetism is one of the few remotely accessible internal properties.