104 research outputs found
A Direct Inversion Method for Two-dimensional Modeling in the Geomagnetic Induction Problem
No full textPlane-wave and Flat Earth Approximations in Natural-source Electromagnetic Induction Studies
No full textdoi:10.1016/S0377-0273(03)00119-7
No full textAbstract Time changes in electrical resistivity were observed before the 1986 eruption of Izu^Oshima volcano, Japan. The aim of this paper is to try to interpret these observed changes in terms of the evolution of a conducting body that simulates the ascending magma by finite difference numerical modeling. Before the time changes were examined, it had been shown that the present numerical model well reproduces the spatial characteristics of the observed apparent resistivities. After some trials involving forward calculation, the time changes, observed during a few months before the eruption, were found to correspond to the formation of a small magma reservoir several hundred meters below the summit. The volume of this reservoir was estimated to be 5U10 6 m 3 , which is in good agreement with the volume of magma drained back from the conduit after the eruption, as estimated from repeated gravity surveys. By comparing the modeling results and observations, the mean ascending velocity of the magma head was estimated to be about 100 m per month during the ten months before the eruption.
Anomaly of the geomagnetic Sq variation in Japan: effect from 3-D subterranean structure or the ocean effect?
No full textMany years ago Rikitake et al. described the anomalous behaviour of the vertical component Z of the geomagnetic solar quiet (Sq) daily variation field at observatories in central and northern Japan - namely about 2 hr shift of the local noontime peak towards morning hours. They suggested that this anomaly is associated with the anomalous distribution of electrical conductivity in the mantle beneath central Japan. Although a few works have been done to confirm or argue this explanation, no clear answer has been obtained so far. The goal of this work is to understand the nature of this anomaly using our 3-D forward solution. The conductivity model of the Earth includes oceans of laterally variable conductance and conducting mantle either spherically symmetric or 3-D underneath. Data from six Japanese observatories at four seasons for two different years of the solar cycle are analysed. As an inducing ionospheric (Sq) current system, we use those provided by the Comprehensive Model (CM4) of Sabaka et al. Our analysis clearly demonstrates that 3-D induction in the ocean is responsible for the anomalous behaviour of Z daily variations in this region. We also show that the effects from a suite of 3-D mantle models that include mantle wedge and subducting slab are minor compared with the ocean effec
Resistivity structure of Izu-Oshima volcano revealed by the ELF-VLF magnetotelluric method.
No full textSeismic and Electrical Signatures of the Lithosphere–Asthenosphere System of the Normal Oceanic Mantle
No full text Although plate tectonics started as a theory of the ocean basins nearly 50 years ago, the mechanical details of how it works are still poorly known. Our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere–asthenosphere system (LAS) beneath the ocean. We review the existing observational constraints on the seismic and electrical properties of the LAS, particularly for normal oceanic regions away from mid-oceanic ridges, hot spots, and subduction zones, where plate tectonics is expected to present its simplest form. Whereas a growing volume of seismic data on land has provided remarkable advances in large-scale pictures, seafloor observations have been shedding new light on essential details. By combing through these observational constraints, researchers are unveiling the nature of the enigmatic LAS. Future directions for large-scale seafloor observations are also discussed. </jats:p
Re-evaluation of Linear Trend of Submarine Cable Voltages for the Study of the Toroidal Field Variation at the CMB
No full textComments on “Enormously large tippers observed in southwest China: can realistic 3‑D EM modeling reproduce them?” by S. Xu et al. (2023)
No full textGraphical Abstrac
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