Geophysical Facts Research Articles

Hope from Despair*

Hope from Despair^*

Why We Can (and Should!) All Teach Climate History: A Few Brief Ideas from North America

This article reviews some of the reasons why history teachers choose not to teach about climate change. The authors suggest that climate change can be an integral part of a history course and they offer examples of energy, cultural identities, and wildland fire as places to weave climate into traditional history classes. Teaching about climate change does not only have to be about scientific data or geophysical facts; the moral, emotional, and cultural impacts of climate change have a natural fit in history classrooms, environmental and otherwise.

Cambrian tectonic evolution of the northwestern Ordos Terrane, North China: constraints of stratigraphy, sedimentology and zircon U–Pb geochronology

The Helanshan Tectonic Belt and its environs within the northwestern Ordos Terrane, western North China Craton, have been envisaged as aulacogen or plate boundary separating the Alxa Massif from the Ordos Block during Cambrian. However, it conflicts with geological and geophysical facts. This study presents integrated stratigraphic, sedimentary and geochronological data from the Cambrian strata in the northwestern Ordos Terrane to constrain the Cambrian tectonic evolutionary process of the western North China Craton. The Cambrian successions comprise, in ascending order, evaporite platform facies, restricted-open platform facies and evaporite platform facies, featuring a vertical transgression–regression sequence. An eastward-shoaling paleogeography was documented, evidenced by shallow-water dolomite layers enriched in the east and the detritus derived from northeast. Ages of detrital zircons span from 2566 to 1649 Ma with predominant two peaks around 2500 and 1850 Ma, suggesting these Cambrian detritus were sourced from the Archean–Paleoproterozoic rocks in the Yimeng highland. Comparable fossil associations and unified paleogeography system supported that there was no Cambrian plate boundary in the northwestern Ordos Terrane, implying the Alxa Massif was part of the North China Craton during the Cambrian. The westward-deepening facies differentiation with low-angle topography contradicts with the tectonic model of “Helan aulacogen”. In combination with the relatively low subsidence rate and rare magmatic–metamorphic activities, we argue that the Cambrian shallow marine system in the western North China Craton was the continent-ward extension of passive continental margin basin system related to the spreading of the Paleo-Asian Ocean.

Earthquake magnitude prediction in Hindukush region using machine learning techniques

Earthquake magnitude prediction for Hindukush region has been carried out in this research using the temporal sequence of historic seismic activities in combination with the machine learning classifiers. Prediction has been made on the basis of mathematically calculated eight seismic indicators using the earthquake catalog of the region. These parameters are based on the well-known geophysical facts of Gutenberg–Richter’s inverse law, distribution of characteristic earthquake magnitudes and seismic quiescence. In this research, four machine learning techniques including pattern recognition neural network, recurrent neural network, random forest and linear programming boost ensemble classifier are separately applied to model relationships between calculated seismic parameters and future earthquake occurrences. The problem is formulated as a binary classification task and predictions are made for earthquakes of magnitude greater than or equal to 5.5 ( $$M \ge$$ 5.5), for the duration of 1 month. Furthermore, the analysis of earthquake prediction results is carried out for every machine learning classifier in terms of sensitivity, specificity, true and false predictive values. Accuracy is another performance measure considered for analyzing the results. Earthquake magnitude prediction for the Hindukush using these aforementioned techniques show significant and encouraging results, thus constituting a step forward toward the final robust prediction mechanism which is not available so far.

Soil erosion and dam dividends: science facts and rural ‘fiction’ around the Ntabelanga dam, Eastern Cape, South Africa

Large dams can play an important role in rejuvenating economic and social development but are often associated with environmental degradation. The Mzimvubu Water Project will involve the building of two large multi-purpose dams, namely Ntabelanga and Laleni. A key anticipated benefit of the former is the expansion of irrigated agriculture. This article reveals that large areas surrounding the dam are, however, severely affected by soil erosion and that irrigation on these areas is likely to be unsustainable. Focus Group Discussions (FGDs) and interviews were held with members of two communities that will be affected by the dam to gauge community sentiments and anticipations. The FGDs revealed that despite respondents lived experiences of soil erosion, local narratives about it appear irreconcilable with geophysical facts. The one crucial anticipated dam dividend appeared to be support for large-scale irrigated agriculture. Rural anticipations that irrigated agriculture can counteract soil degradation caused by erosion are not supported by scientific evidence, and without appropriate community awareness programmes by the dam developers, the divergence between science facts and local views may in the future engender conflict between the community and other stakeholders.

Evidences of buried loads in the base of the crust of Borborema Plateau (NE Brazil) from Bouguer admittance estimates

Abstract In the Borborema Province (BP) – northeastern Brazil – two important Cenozoic events occurred at the surface: the Macau magmatism and the Borborema Plateau epeirogenesis. To obtain appropriated-scale geophysical data to explain the deep origins of these two events, different gravimetric/elevation databases were integrated with new surveys. Bouguer admittance estimates reveal that isostatic condition of the BP, especially in the Borborema Plateau, can be explained using elastic models to the lithosphere only if surface and buried loadings are combined. If the buried load is applied in the base of the crust, the ratio between buried and surface weights is circa 15 for a lithosphere with effective elastic thickness around 15 km and crust thickness around 33 km. From an interpretative viewpoint of the buried load, it is assumed that the lower crust under the Borborema Plateau might have an anomalous high value of density. Magmatic underplating might explain this fact as well as the observed surface magmatism and epeirogenesis. Crustal thickening of about 4 km under the Borborema Plateau and intracrustal seismic velocity discontinuity with high Vp/Vs ratio are geophysical facts consistent with magmatic underplating. However, the surface magmatism presents low volume and mainly alkaline composition – facts that are not entirely consistent with the hypothesis of magmatic underplating. Regardless the validity of this hypothesis, Cenozoic-to-present events in BP might be somewhat associated with imbalances in lithosphere-asthenospheric mantle and/or crust-lithospheric mantle systems. The existence of free-air anomalies showing no null integral over area and of an expressive positive geoid anomaly are geophysical evidences of these imbalances. Possibly, the Borborema Plateau is still suffering epeirogenesis. Post-depositional deformation found in Barreiras Formation strata, Late Quaternay fault reactivations, and AFT thermochronology analysis suggesting the existence of a cooling stage between 20 and 0 Ma might be geologic evidences of the continued action of epeirogenesis until the present. In addition, the relatively high level of the present intraplate seismicity recorded in several regions of the BP is another unequivocal geophysical evidence that the crust of the province is still submitted to accommodation processes.

Volcanic geomorphology of northern Viti Levu, Fiji

Analysis of radial, centripetal and gutter drainage in the volcanic region of northern Viti Levu, Fiji, enabled the delineation of several distinct volcanoes, despite the early Pliocene age and considerable later erosion. Three main shields with calderas and several smaller volcanoes are mapped. The results are consistent with some other geological and geophysical facts, but do not support the idea that these volcanoes were dominantly submarine eruptions. The method picks out known centres of gold mineralisation and may be applicable, in conjunction with other techniques, in mineral exploration.

Shaping AGU's contributions to policy debate

In their Forum piece in the April 9 issue of Eos, Kaula and Anderson paint an unrealistically stark choice for the roles AGU might play in policy debates that substantially involve geophysical science. On the one hand is the antiseptic model of AGU‐above‐the‐policy‐fray: the aloof provider of geophysical facts from the literature. On the other hand is the model of AGU‐as‐policy‐advocate: blending geophysical knowledge with value judgements in order to argue for specific policy actions in the political trenches.The problem with the first model is that the form assumed by most geophysical facts in the literature is rather distant from the needs of policy makers. Thus, the facts are easily overlooked in the face of pressing short‐term political agendas. The problem with the second model is that AGU is a professional society comprised of scientists who hold many different value orientations. Any particular set of values adopted in a piece of political advocacy is likely to be at odds with many AGU members.

Magnetohydrodynamic waves in the erath's liquid core

The possibility of generation of magnetohydrodynamic waves in a rotating spherical system under the efect of small disturbances of the medium velocity is investigated. The initial stationary state of the system has been chosen with a view to known geophysical facts, and the effect of the solid inner core is considered. It is proved that two types of waves with distinctly different frequencies can be generated in the liquid region. The behaviour of longperiod magnetic waves is also studied in detail, and it is proved that the direction in which they propagate at a certain point depends on the position of this point in the meridional plane and on the configuration of the initial magnetic field. The phase velocities of the longperiod magnetic waves, propagating towards the west, have been computed within a selected network of points and the region in which they are generated has been defined. It is proved that the phase velocities of these waves are compatible with the observed velocity of the westward drift of the geomagnetic dipole.

Orogenic zones in central Australia: intraplate tectonics?

THE plate tectonics model1,2 has been successful in explaining the orogenic features of the ocean basins and continents associated with the dispersal of Pangaea in post-Triassic time. Dewey and Bird3 extended these concepts, suggesting that there is a unique relationship between orogenesis and plate-margin tectonism, and various authors have, accordingly, interpreted certain Precambrian and Palaeozoic orogenic zones or ‘mobile belts’ as plate sutures4–5. We argue here that plate-margin tectonism is only one of several possible conditions for the development of an orogenic belt. We suggest that the plate tectonic model of orogenesis needs to be extended and modified, and this suggestion is discussed for Gondwanide Australia. Recent attempts to impose a rift (aulacogene)6 or subduction7 model for orogenesis in central Australia are confounded by the observed geological and geophysical facts pertinent to this region.

Geologic Preconditions for Oil and Gas Possibilities in Soviet Arctic: ABSTRACT

The Soviet Arctic, including the continental shelf, occupies an area of about a third of the entire Soviet Union. Within this area, major platform structures with a thick sedimentary mantle are developed. These are the West Siberian and Pechora plates and the Siberian, Barents-Kara, and Hyperborean platforms. All of the discovered oil and gas areas and the most prospective territories and shelves are related to these platforms. Areas favorable for petroleum exploration comprise more than 60-70% of this area. The largest of these is the West Siberian oil and gas basin, east of the Urals. In the Arctic part of the West End_Page 2482------------------------------ Siberian province the largest gas field in the world was discovered (Urengoy). Productive and prospective strata are known to range through the whole Jurassic and Upper Cretaceous sections. Prospective oil and gas structures extend offshore into the southern part of the Kara Sea shelf. In the Timan-Pechora area west of the Polar Urals-Pay-Khoy, several oil and gas fields have been discovered. Productive and prospective zones are in the middle and upper Paleozoic, and possibly in the Mesozoic. Structures favorable for oil and gas accumulation extend offshore into the southern part of the Barents Sea shelf. The great petroleum possibilities of the northern part of the Siberian platform and of the bordering Mesozoic troughs are confirmed by the discovery of gas fields in the western part of the Yenisey-Khatanga trough and of oil fields in the eastern part of the trough. In addition a large bitumen (tar) field is near the southern border of the Lena-Anabar trough, and abundant oil and bitumen shows occur through the whole section from the upper Precambrian through the Lower Cretaceous. The Mesozoic-Cenozoic troughs and depressions of the Verkhoyansk-Kolyma, Koryak-Kamchatka, and Chukotsk areas in the northeastern part of the USSR also are prospective for oil and gas. The high estimate of the oil and gas possibilities for the shelf seas is based on the favorable geologic and geophysical facts from the Soviet Arctic islands and shelves. The most prospective are structures within the Barents-Kara platform, the offshore part of the West Siberian basin, the Chukotsk-East Siberian troughs, the southern Chukotsk and southern Laptev troughs, and the Wrangel rise. End_of_Article - Last_Page 2483------------

Intraoceanic islands, east pacific ridge, island arcs: Volcanism and upper mantle

Two classes of volcanic rocks can be distinguished with respect to petrochemistry, namely oceanic and continental classes. Lavas of island arcs belong to the continental type. Lavas of oceanic ridges are close not to the oceanic, but to the continental type. On petrochemical and geophysical bases the author has drawn the following conclusions: 1. (1) the foci of volcano feeding lie beyond the limits of the earth's crust—in the upper mantle. 2. (2) the composition of the upper mantle under continents, oceans, island arcs and oceanic ridges is somewhat different. 3. (3) the role of assimilation on the passage of magma to the surface is very limited as a rule. According to these conclusions, volcanism may be considered as a certain indicator of the composition and state of the upper mantle matter. A scheme for the evolution of volcanism is suggested on this basis (as a reflection of the evolution of the upper mantle) which is in an agreement with geological and geophysical facts.

Irregular cruśtal but regular mantle movements

It is a problem whether the mantle produces mid-oceanic ridges indepedently of crustal conditions or whether inversely regional crustal conditions induce the processes in the mantle that cause the ridges. The author proposes the hypothesis of a mutual influence from both sides. The low-velocity zone is assumed to be a zone of currents which on the one hand depend upon crustal conditions and upon the irregular crustal pattern and on the other hand are causally connected, fed and widely influenced by a regular current system in the mantle. This system is assumed to consist of twelve equisized cells in the lower mantle which are mechanically connected with an inverse system of twenty cells in the upper mantle. The author first published a map of such a system in 1952 (Havemann, 1952), and proposed a dynamics of it (Havemann, 1955). But a satisfactory dynamic connection of crust and system was lacking. The progress of research in the last decade enables the author to propose a solution concerning the contact of near crust and system movements and of the generation of mid-oceanic ridges. The currents of the system are assumed to be moved by chemical as well as by thermal impulses and, therefore, as partly irreversible partly reversible. Geological and geophysical facts and arguments in favour of the theory are discussed. They comprise position and dynamics of large crustal arcs and of ridges causing continental drift; a seismic great circle coinciding with the median circle of a system girdle and its causal connection with Mediterranean movements; satellite-observed anomalies in accordance with the system pattern. The problem of an axis-symmetric system position and of polar wandering is discussed.

Recent advances in geodynamics

A survey is given of recent developments in geodynamics. The first two sections describe briefly the recently accumulated pertinent geological and geophysical facts. Then, the new developments in the physical basis of geodynamics, in the theory of the earth's rotation, in our knowledge of epeirogenesis, in the proposed geotectonic hypotheses, in the theory of faulting, earthquake origination, folding and various miscellaneous phenomena, are described.

Some points of geophysical interest revealed in the mass‐analysis of teleseismic records

Mass‐analysis of seismograms may at first glance sound uninteresting. As a matter of fact to the casual observer of the subject this is no doubt quite true. When one is in continuous contact with the interpretation of seismograms the reverse is very apt to be true. The picture that one gets under these circumstances might well be compared to the old picture‐puzzle with hidden faces to be found. By careful observation one can eventually find all of them. The same is true in seismology though the faces are exchanged for geophysical facts.

Summary of the year's work, Department of Terrestrial Magnetism, Carnegie Institution of Washington

The year ended June 30, 1938 has been marked by energetic prosecution of experimental investigation and continued coordination and integration of various researches paving the way for more general formulation of geophysical facts. Good progress has been made in two major projects which open new fields of investigation and promise important advances. One of these is the completion of the Atomic‐Physics Observatory (see Fig. 1) and the considerable progress in the installation of the electrical equipment. The electrostatic generator and tube are designed to operate at potentials exceeding five million volts and offer unique opportunity for further studies in the expanding field of nuclear physics—a field to which we must look for additional understanding of magnetic phenomena.

Progress‐Report on researches in terrestrial magnetism and electricity at Department of Terrestrial Magnetism, Carnegie Institution of Washington, for year April 1937 to March 1938

The year has been marked by energetic prosecution of experimental investigation and continued coordination and integration of various researches paving the way for more general formulation of geophysical facts. Good progress has been made in two major projects which open new fields of investigation and promise important advances.Atomic‐Physics Observatory—One of these is the completion of the Atomic‐Physics Observatory and the considerable progress in the installation of the electrical equipment. The electrostatic generator and tube are designed to operate at potentials exceeding five million volts and offer unique opportunity for further studies in the expanding field of nuclear physics—a field to which we must look for additional understanding of magnetic phenomena.

Isostasy and the Size and Shape of the Earth

IT is the speaker's desire, in this paper, to touch briefly on some phases of geodesy which should be of general interest to the physicist. Geodesy is one of the oldest sciences; probably second in this respect to astronomy alone. What is the extent of the earth in the horizontal plane, to what depth does it extend, and what is on the other side are questions which must have been asked by men of the earliest times. The history of geodesy will have to be passed over, for lack of time, although it would be interesting to follow the accumulation of data as to the shape and size of the earth. Before one can study efficiently the great geophysical facts and comprehend them, he must know the shape and size of the earth with considerable accuracy. For some time before the end of the seventeenth century, it was taken for granted that the earth was a true sphere. The announcement by Newton in 1687 of his theory that the earth was an oblate spheroid added much interest to the subject of its exact shape. A meridional arc of about 8i degrees, extending north and south of Paris, was measured by J. and D. Cassini between the years 1683 and 1716. The results of the first computations of this arc indicated that the length of the degree was less at the northern than at the southern end. This gave to the earth the shape of a prolate spheroid. A great controversy arose over