Geomagnetic Variation Anomalies Research Articles

Statistical significance of geomagnetic diurnal variation anomalies prior to worldwide earthquakes

Statistical significance of geomagnetic diurnal variation anomalies prior to worldwide earthquakes

Spatiotemporal characteristics of the geomagnetic diurnal variation anomalies prior to the 2011 Tohoku earthquake (Mw 9.0) and the possible coupling of multiple pre-earthquake phenomena

Xu et al. (2013) and Han et al. (2015) have reported unusual behaviors of geomagnetic diurnal variation (GDV) in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To make a better understanding of this phenomenon, temporal-spatial analyses of GDV have been applied in this study. Geomagnetic data of long-term observations at 17 stations in Japan have been analyzed using the same method in Han et al. (2015). Ratios of diurnal variation range between the reference station KAK and the target stations have been computed. After removing seasonal variations, the 15-day backward running mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2months before the mega event at both ESA and MIZ stations in the Tohoku Region. Locations of anomalies in spatial distribution show a good correlation with the epicenter of the Mw 9.0 earthquake. These spatiotemporal results are consistent with those obtained from other independent observations such as groundwater level and GPS displacements. The coupling of multiple pre-earthquake phenomena may help to understand the preparation process of a mega earthquake in the subduction zone.

Mantle upwelling revealed by genetic algorithm inversion of the magnetovariational anomaly around Kyushu island, Japan

The spatial extent of an outstanding mantle upwelling in the west of Kyushu island, Japan, has been estimated by a genetic algorithm inversion. Here we show how the mantle upwelling manifests in modeling of observed anomalies of temporal geomagnetic variations, i.e., magnetovariational anomalies around southwest Japan. Observed ratios of the vertical to horizontal geomagnetic components at short periods are known as indicators of subsurface electrical contrasts. However, they are also sensitive to the presence of oceans and continents because of much more conductive seawater than land rocks. To circumvent the difficulty, we used three‐dimensional nonuniform thin sheet approximation to subtract out the conductive as well as inductive effects of the ocean on the magnetovariational anomalies. The observed anomalies were then inverted by a genetic algorithm inversion for conductance (the product of electrical conductivity and thickness) with the known bathymetry as a priori information. The derived conductance model has particular sensitivity to the northeastern portion of spatial extent of the mantle upwelling probably centered at the East China Sea. The upwelling may result in horizontal flows of the uppermost mantle, which induce the horizontal stress field that can trigger hazardous shallow intraplate earthquakes of large magnitude, such as the earthquake off the west coast of Fukuoka Prefecture that occurred on 20 March 2005. The northeastern rim of the mantle upwelling thus revealed can also be related to an unidentified boundary between the Amurian plate and the Eurasia plate.

Geomagnetic variation anomalies in north-western Italy

A preliminary geomagnetic variation data analysis has been made in the Piedmont-Ligurian area in north-western Italy. Data were collected by means of two instruments: an Askania portable variograph and a LaCour fixed station operated in the Roburent geomagnetic observatory. The portable variograph has been moved to several locations in north-western Italy. Parkinson vectors were drawn for 8, 20 and 60 min respectively, considering bay-like events recorded at single stations. Fourier parameters and anomaly maps for 32 and 64 min have been obtained after selecting similar events in order to simulate a magnetometric array. Results show trends that suggest possible correlations with three main geological features of the investigated area: (1) a coast effect probably also related to the lithospheric thickness of the Ligurian Sea; (2) a possible highly conductive channel connected to the Alpine system that could be analogous to anomalous structures found in Europe; and (3) an effect of the lower crust being obducted in the Ivrea body and subducted in the Southern Alps.

Perturbation of induced electric currents by surface conductivity inhomogeneity with special reference to anomalous behavior of short-period geomagnetic variations in the Kanto Plain, Japan.

It has long been considered that anomalous behavior of short-period geomagnetic variations in the Kanto Plain (the Kanto anomaly) reflects a mantle conductivity anomaly and the sedimentary layer as thick as 3, 000m or more plays a rather minor role, only as local perturbation superposed on a regional anomaly. The complicated surface conductivity structure in this region, characterized by the sea existing to the east and the south and the thick sedimentary layer in the central part, has hampered quantitative discussion on the effects of seawater and sediments on short-period geomagnetic variation anomalies. It is shown, however, that such surface effects can be estimated approximately by considering two different polarization cases separately; one for a northwestward geomagnetic variation and the other for a northeastward geomagnetic variation. In the former case, the sea effect is striking as disclosed by numerical calculations of the peninsula effect. In the latter case, the so-called channeling effect due to the presence of the sedimentary layer seems to be a predominant factor. It is concluded that the Kanto anomaly is primarily accounted for by these surface effects, although no significant information on the crustal and mantle conductivity structure could be derived.

Tectonic study of Cascade Range and Columbia Plateau in Washington State based upon magnetotelluric soundings

Two magnetotelluric profiles have been recorded in the state of Washington in order to study crustal structures related to the Cascade Range volcanoes. A long magnetotelluric sounding profile was completed from near the Pacific Coast to the eastern part of Washington, covering some 400 km in length. This profile crossed the Cascade Range midway between Mount Rainier and Mount Adams. A shorter east‐west profile of data from southwest of Mount St. Helens to southeast of Mount Adams has also been interpreted. The west end of the long profile from near the Pacific Coast to the vicinity of Mount Rainier can be interpreted to indicate as much as 10 km of conductive Tertiary and possibly pre‐Tertiary sedimentary and volcanic rocks. These rocks have resistivities of 2–10 ohm m as demonstrated on the magnetotelluric soundings and electrical well logs. The interpretations show that these conductive materials are juxtaposed with a pluton in the Goat Rocks Wilderness, southeast of Mount Rainier. This feature and other characteristics on the shorter profile to the south are interpreted to be the cause of geomagnetic variation anomalies observed in the Washington Cascades in other research. The thick conductive units on the west end of the long profile and other assumed sedimentary units detected beneath basalts of the Columbia River region have significance in regional hydrocarbon exploration programs. Displacements in the thick conductive unit occur across the Mount St. Helens seismic zone defined by C. S. Weaver and the general pattern of seismicity in the Puget Lowland and south may be related to the interpreted sedimentary unit. Major differences in crustal electrical structures were observed in the Washington State volcanic Cascades compared to the High Cascades in Oregon and northern California. The main features of the magnetotelluric interpretations can be coupled to offshore seismic and gravity data to provide better constraints on the nature of paleosubduction of the Juan de Fuca plate.

Geomagnetic variation anomalies in northern England: processing and presentation of data from a non-simultaneous array

Summary. The block and basin structure of northern England has been investigated by an array of 35 magnetometer stations over a grid with a spacing of 1Okm. Recordings at these sites were made simultaneously with those at a reference site. Using the common reference data, transfer function analysis enables the three anomalous field components to be estimated at each site, for the period range 10-104s. Using the transfer functions obtained, maps of the anomalous field components may be generated across the array, and used for interpretation of electrical conductivity structure. Maps thus obtained from the non-simultaneous array data in northern England are used to illustrate the advantages of the method, and provide evidence of an east-west current flow concentrated along the southern margin of the Northumberland Trough. The limitations of the procedure, although slight, are considered. The errors which arise in the method are deduced and techniques for their removal are suggested.

Geomagnetic variation anomalies in northern England: processing and presentation of data from a non-simultaneous array

Summary. The entropy S for a continuous distribution p(x) is defined by This expression, however, is a measure of uncertainty relative to the coordinate x so that the probability distribution p(x) generated from the principle of maximum entropy depends on the choice of x. Only when the chosen parameter actually has a uniform prior distribution, can we expect the generated distribution to conform with the empirical data. For a physical system in which the independent variable x is measured to only limited accuracy, the prior distribution m(x) can be shown to be inversely proportional to the measurement error of x. A parameter with uniform prior distribution, then, is one that can be measured with equal accuracy throughout its range. In this context, the magnitude of an earthquake is such a parameter because using this parameter in the principle of maximum entropy leads to the empirically determined Gutenberg-Richter frequency-magnitude relation. Other proposed frequency-magnitude relations can also be generated from the principle of maximum entropy by imposing appropriate constraints. However, it is emphasized that such relations are generated from the principle as null hypotheses, to be tested by the empirical regional or global seismicity data.

Geomagnetic variation anomalies and deflection of telluric currents

We show that most of the abnormal variations of the transient magnetic field have deflection and canalization of telluric currents as main sources. In order to do so, we develop a formalism which is valid in a limited frequency range which satisfies the following hypotheses: (a) the thickness of the laterally inhomogeneous upper layer is small with respect to the penetration depth of the transient field; (b) self-induction of the heterogeneities of conductivity can be neglected. Theoretical results and observations performed at mid-latitudes are in good agreement. The main result we establish is that deviation and canalization of the current responsible for the transient abnormal field do not affect their temporal variations. The study of temporal variations of the abnormal field thus provides us with information about the structure of the crust in the region where the deviated and canalized currents are induced.

Conductivity structure in and around Hokkaido,Japan as revealed by the period dependence of the CA transfer functions.

The anomalies of geomagnetic variations in Hokkaido at a period of 5min are explained by the effect of the induction and the conduction currents within the surface layer such as the conducting sea and the sedimentary layer. However, the behaviour of the anomalies at longer periods than 5min is different in each region. From the model calculations to explain the period dependence of the CA transfer functions for each region, the following conclusions are drawn. The highly conducting layer (0.1S/m) exists at a likely depth range from 30 to 70km beneath the inner part of the Volcanic Front of the northeastern Japanese arc. On the contrary, the shallower part of the upper mantle beneath the outer part of the Volcanic Front seems to be poorly conducting (10-4S/m or so) though a quite thin and highly conducting layer may be present at a depth of about 130km. These results are consistent with other independent geophysical data such as heat flow and attenuation of seismic waves in and around Hokkaido.

Anomalies in geomagnetic variations on peninsular India near Palk Strait

Anomalies in transient geomagnetic variations have been previously studied using data from five permanent observatories in the peninsular India. The results from the three observatories falling under the equatorial electrojet showed the presence of a strong current concentration between India and Sri Lanka Island. The currents seem to flow at the depth of the crust or upper mantle. These findings have been confirmed hereby by collecting data from an array of magnetometers. We associate the channelling to the presence of a geological conductor between India and Sri Lanka Island.

Geomagnetic secular variation anomalies in relation to the recent crustal movement in the southwestern region of Japan : Sumitomo, Norihiko, 1981 Bull. Disaster Prevent. Res. Inst. Kyoto Univ., 30(4)(274):97–130

Geomagnetic secular variation anomalies in relation to the recent crustal movement in the southwestern region of Japan : Sumitomo, Norihiko, 1981 Bull. Disaster Prevent. Res. Inst. Kyoto Univ., 30(4)(274):97–130

Geomagnetic Secular Variation Anomalies in Relation to the Recent Crustal Movement in the Southwestern Region of Japan

Geomagnetic Secular Variation Anomalies in Relation to the Recent Crustal Movement in the Southwestern Region of Japan

The use of equivalent current systems in the interpretation of Geomagnetic Deep Sounding data

Many geomagnetic variation anomalies are probably caused by the channelling, through small-scale bodies, of electric currents induced in much larger conductors elsewhere. Consequently, the direct interpretation of anomalous magnetic fields by modelling the electromagnetic response of conductive structures may give misleading results. It is suggested that, rather than attempting to proceed directly from the electromagnetic fields to conductivity models, we should instead take the intermediate step of determining the distribution of anomalous current flow. Maps of the anomalous fields over a conductive structure can be generated from inter-station transfer functions. If it is assumed that the internal currents are concentrated in a thin sheet at a specified depth, the equivalent current system in the sheet can be computed directly from the vertical magnetic field. The most straightforward method of performing this calculation is to compute the Fast Fourier Transform of the magnetic field data, and then to apply a wavenumber filter. The presence of any vertical currents invalidates the thin sheet model. However, if the spatial distribution of a horizontal component of the anomalous magnetic field is also known, the presence of any vertical currents can be detected directly, and their position determined. The value of the methods is illustrated by applying them to the interpretation of a Geomagnetic Deep Sounding survey of the Kenya rift valley.

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used. Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure. Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.

The Effect of Source Field Polarization on Geomagnetic Variation Anomalies in the British Isles

Summary The published data on geomagnetic temporal variations in the British Isles are re-examined. The normalized complex amplitudes of the vertical component from place to place caused by the local channelling of regional current flow of given polarizations are computed. A contour map of the in-phase vertical component for a period of 40 mins caused by the channelling of regional NS current flow clearly demonstrates the existence of current concentrations in the shallow seas and in the Atlantic Ocean. A second map of the quadrature vertical component, at the same period, caused by the channelling of regional EW current flow shows current concentrations whose axes correlate remarkably well with Caledonian trends in England, Southern Scotland and Ireland. The Eskdalemuir Anomaly is an unusual special case. It is excited both by in-phase NS current flow due to current leaking between the North Sea and the Irish Sea and by quadrature EW current flow due to its location close to a major Caledonian feature. The Caledonian correlation may be caused by unusually low resistivity in rocks presently in the lower crust which formed part of a 'Proto-Atlantic' oceanic crust in the Late Palaeozoic.

Partial melting and electrical conductivity anomalies beneath the Japan and Philippine seas

Geomagnetic variation anomalies observed in the central part of Japan can be accounted for by highly conducting layers having a conductivity of 0.5 S/m or thereabouts beneath the Japan and Philippine seas. High temperature may be a possible cause of such layers if we take into account that temperature amounts to 1,000°C at a depth of 40 km beneath the areas of high heat flow in the Japan and Philippine seas. It seems difficult, however, to account for a conductivity of 0.5 S/m which is unusually high at a depth of 50 km or so by means of high temperature only. We can interpret such a high conductivity in terms of partial melting of peridotite in the upper mantle, provided highly conducting molten material is interconnected. This is in good agreement with the interpretation of low-velocity zones beneath the Japan and Philippine seas in terms of a partial melting of 4–6%.

Geomagnetic variation anomalies in the Canadian arctic. I. Ellesmere Island and Lincoln Sea.

The geomagnetic variation anomaly on Ellesmere Island has been attributed to an elongated conductor located deep in the crust or in the upper mantle and striking northeast across the northern portion of the island. Recently observations have been made on the ice of the Lincoln Sea lying to the north of Ellesmere and Greenland, and the anomaly has been found to extend beyond the coast for a distance of 100km or more beneath the continental shelf. Fitting the data with a uniform current model suggests that the anomalous conductor is about 100km wide, 10km thick, and located in the crust at depths between 10 and 20km.The anomaly lies at the northeastern end of the Innuitian Province. It follows approximately the axis of the Franklinian Geosyncline (Ordovician to mid-Silurian) and parallels the structural trend of the lower paleozoic strata which were folded during the Ellesmerian Orogeny of Devonian-Mississippian times. There is no geophysical evidence suggesting a contemporary or recent origin for the anomaly. The high conductivity zone seems more likely to be caused by hydration and structural changes deep in the crust; the geological evidence suggests that it may be a relic of ancient tectonic activity.

Geomagnetic variation anomalies in the Canadian arctic. II. Mould Bay anomaly.

In 1963, 1964, and 1970 geomagnetic variations in 3 components were measured at 14 sites in the Western Arctic Islands of the Canadian Archipelago. The suppression of Z amplitudes, first observed at Mould Bay, is now confirmed to extend from Mould Bay south to Holman Island, a distance of 620km, and from McCormick Inlet to Houghton Head, an east-west distance of 292km.Spectral decomposition of many component time series gave mean Z/X energy density ratios which fall into two groups. One group (anomalous) is characterized by slopes of 2, with attenuations of 10-50 between the 60 (or 100) and 10min energy ratio estimates. The other group (non-anomalous) do not display such behaviour in the component ratio plots. At periods near 10min the energy ratios of the two groups differ by factors of 5 to 10. Distributions of Parkinson vectors for various period intervals support in part a deep conducting body and in part electric currents in salt water channels or deep ocean.Coherency analysis of horizontal components between stations supports the assumption that external source dimensions are proportional to the 1/3 power of period of disturbance. However, the electromagnetic theory of induction for horizontal layered models of infinite extent has failed to explain the observed reduction of Z amplitudes near 10min periods. Long period (100-300min) energy ratio estimates are explained as the effect of a conducting (1 S/m) layer or half-space in the upper mantle underlying most of the Western Arctic. Love-wave model studies have detected a low-velocity zone in the upper mantle between Mould Bay and Coppermine.

Geomagnetic variation anomalies in the Koyna and the Bhadrachalam seismic areas in Peninsular India.

Special magnetic observations carried out at twenty temporary stations close to the latitude of Hyderabad in peninsular India, along the Alibag-Hyderabad and the Hyderabad-Bhadrachalam-Kalingapatnam profiles including two in the Koyna region, are discussed and the salient features in the seismically active areas of Koyna and Bhadrachalam are brought out.It is found that while the Z-amplitudes of both short-period and long-period geomagnetic variations show the normal coastal effect at Koyna, they get enhanced at Malharpeth, 35km east of Koyna. The amplitudes of H-variations, however, remain unaffected. At Bhadrachalam, the ranges of both H and Z long-period daily variations are observed to be enhanced.The tectonics and the deeper conducting structures of the Koyna and the Bhadrachalam regions as inferred from the anomalous geomagnetic variations, appear to be similar except for the directions of the conductors.