The dispersion of surface waves and crustal structure in African continent

Summary In this study has been interpreted the Love- and Rayleigh-waves of Agadir earthquake on February 29, 1960 recorded at Lwiro and Skopje earthquake on July 26, 1963 recorded at Pretoria and Windhoek by long period Columbia type seismographs, and determined the crustal structure in Africa with dispersion of both Love- and Rayleigh-waves along the paths on two different directions. The obtained results are compared each other and with those founded recently by several investigators for Africa and Europe. Herewith has been showed that the average thickness of the crust in Africa is thinner than in Europe and the thickness in direction from Skopje to Pretoria is more than that from Agadir to Lwiro.

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Zusammenfassung In diesem Artikel wurden die Love- und Rayleigh-Wellen vom Erdbeben in Agadir vom 29. Februar 1960, aufgezeichnet in Lwiro, und vom Erdbeben in Skopje vom 26. Juli 1963, aufgezeichnet in Pretoria und Windhoek von langperiodischen Columbia-Seismographen, untersucht und somit mit Dispersion von beiden Love- und Rayleigh-Wellen der Aufbau der Erdkruste in Afrika bestimmt. Die erhaltenen Resultate wurden miteinander und mit anderen Werten, die bis heute von verschiedenen Autoren für Afrika und Europa bestimmt wurden, verglichen. Somit wurde vorgelegt, dass die Erdkruste in Afrika im allgemeinen dünner ist als in Europa und dass die Schicht in der Richtung von Skopje zu Pretoria dicker ist als in der Richtung von Agadir zu Lwiro.

     

Geochemistry of Proterozoic molybdenite — mineralized aplites in Northern Sweden

The largest known Precambrian molybdenite occurrences in Sweden are of the aplitic type. In order to improve the understanding of their genesis, 67 samples of aplites, aplitic granites and associated granites were analyzed for major elements, Mo, Sn, Be, Ba, Rb, Sr, Cu, Zn, Zr, V, Cr, Ni, Y, Nb, F, Cl and S. Five samples were analyzed for rare earth elements. The following genetic model can be proposed:In the upper parts of granitic intrusions the magma is enriched in lithophile elements and elements such as F and Mo. The aplites represent direct differentiation products of the granites and have solidified without the development of a second mobile aqueous phase, except locally, where small pegmatite segregations have formed. As crystallization proceeds, the concentration of Mo in the magma increases, and when the aplites solidify, the Mo that cannot be incorporated in the rock-forming minerals is precipitated as molybdenite. Biotite seems to be the major Mo-carrier among the rock-forming minerals in the aplites.     

Geosecs North Atlantic radiocarbon and tritium results

Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ¹⁴C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in¹⁴C and³H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both¹⁴C and³H might be associated with the Antarctic intermediate water.     

Seismic hazard studies for Gaziantep city in South Anatolia of Turkey

Seismic hazard studies were conducted for Gaziantep city in the South Anatolia of Turkey. For this purpose, a new attenuation relationship was developed using the data of Zaré and Bard and accelerations were predicted employing this new equation. Deterministic approach, total probability theorem and GIS methodology were all together utilized for the seismic assessments. Seismic hazard maps with 0.25° grid intervals considering the site conditions were produced by the GIS technique. The results indicated that the acceleration values by the GIS hazard modelings were matched with the ones from the deterministic approach, however, they were underestimated comparing with the total probability theorem. In addition, the GIS based seismic hazard maps showed that the current seismic map of Turkey fairly yields conservative acceleration values for the Gaziantep region. Therefore, the constructed GIS hazard models are offered as a base map for a further modification of the current seismic hazard map.     

Biodegradation effects over different types of coastal rocks

Modern coastal areas have natural and transported rocks (armourstone) on which various types of organisms live. Burrowing, boring and feeding by these organisms can destroy or modify the coastal rocks and hence change the coastal morphology. Two rock types and three dominant types of organisms have been studied in Mersin Bay, Eastern Mediterranean of southern Turkey. In this study area, Plio-Quaternary conglomerates and variously aged limestone armourplates have been affected by Phoronida worms, bivalve Brachidontes pharaonis (Fischer P. 1870) and the limpet Patella sp. Phoronida colonies were found covering the hard substratum as a mat and form tubular endolithes of 35.0 mm depth and 1.5 mm diameter, whilst Brachidontes pharaonis (Fischer P. 1870) form 44 mm deep vase-shaped gastrochaenolites. The bioerosive activity of Limpet Patella sp., found intertidal and within the spray zone, cannot be significantly observed on the rocks over short time periods. The soft sandy matrix of the conglomerates present were found to disintegrate by bioerosional processes, with the released gravels being transported and deposited onto the beach. Within the armourstone limestone blocks, a maximum of 44.0 mm deep holes developed after 50–60 years. However, these biological activities do not threaten the stability of the blocks due to their hard and homogeneous internal structure. Furthermore, the organism colonies that cover these rocks as a strong mat (maximum 29.0 mm) act to protect their surfaces from further biological attack and wave action.     

Spectral determination of source parameters in the Marmara Region

Ever since the 1999 Kocaeli earthquake, in which the Kandilli Observatory and Earthquake Research Institute (KOERI) was not able to correctly reflect the magnitude size in its preliminary report because of the saturation effect, a rapid and accurate determination of the earthquake becomes a very important issue. Therefore, in the framework of this study, an automatic determination of the moment magnitude was performed by using the displacement spectra of selected earthquakes in the Marmara Region. For this purpose, 39 three-component broadband stations from KOERI seismic network which recorded 174 earthquakes with magnitudes 2.5?≤?M?≤?5.0 in between 2006–2009 were used. Due to the importance of quality factor in determination of the moment magnitude with spectral analysis method, the quality factor was calculated for the whole region in the beginning. Source spectrum which was obtained by converting the velocity records to displacement spectra and moment magnitudes of earthquakes were determined by fitting this spectrum to classical Brune model. For this aim, an automatic procedure was utilized which based on minimizing the differences between observed and synthetic source spectra identified by the S waves. Besides moment magnitude and location parameters, some source parameters such as seismic moment, spectral level, corner frequency and stress drop were also calculated. Application of the method proves that determining the seismic moment from the source spectra is applicable not only for earthquakes with small magnitude but also moderate earthquakes as well.     

Fidelity and repeatability of wave fields reconstructed from multicomponent streamer data

Wave field reconstruction – the estimation of a three‐dimensional (3D) wave field representing upgoing, downgoing or the combined total pressure at an arbitrary point within a marine streamer array – is enabled by simultaneous measurements of the crossline and vertical components of particle acceleration in addition to pressure in a multicomponent marine streamer. We examine a repeated sail line of North Sea data acquired by a prototype multicomponent towed‐streamer array for both wave field reconstruction fidelity (or accuracy) and reconstruction repeatability. Data from six cables, finely sampled in‐line but spaced at 75 m crossline, are reconstructed and placed on a rectangular data grid uniformly spaced at 6.25 m in‐line and crossline. Benchmarks are generated using recorded pressure data and compared with wave fields reconstructed from pressure alone, and from combinations of pressure, crossline acceleration and vertical acceleration. We find that reconstruction using pressure and both crossline and vertical acceleration has excellent fidelity, recapturing highly aliased diffractions that are lost by interpolation of pressure‐only data. We model wave field reconstruction error as a linear function of distance from the nearest physical sensor and find, for this data set with some mismatched shot positions, that the reconstructed wave field error sensitivity to sensor mispositioning is one‐third that of the recorded wave field sensitivity. Multicomponent reconstruction is also more repeatable, outperforming single‐component reconstruction in which wave field mismatch correlates with geometry mismatch. We find that adequate repeatability may mask poor reconstruction fidelity and that aliased reconstructions will repeat if the survey geometry repeats. Although the multicomponent 3D data have only 500 m in‐line aperture, limiting the attenuation of non‐repeating multiples, the level of repeatability achieved is extremely encouraging compared to full‐aperture, pressure‐only, time‐lapse data sets at an equivalent stage of processing.     

Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

Abstract— Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45–55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic‐ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of ³He, ⁴He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of ⁴He into cosmogenic and radiogenic components was performed using the lowest ⁴He/³He ratio we measured on mineral separates (⁴He/³He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic ⁴He. Systematics of cosmogenic ³He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic ⁴He loss, ranging between ?20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant ⁴He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of ⁴He diffusion in the main U, Th carrier phase apatite showed that post‐shock temperatures of ?300 °C are necessary to explain observed losses. This temperature corresponds to the post‐shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for ⁴He loss.