Spectral classification methods in monitoring small local events by the Israel seismic network

We use the dense Israel Seismic Network (ISN) to discriminate between low magnitude earthquakes and explosions in the Middle East region. This issue is important for CTBT monitoring, especially when considering small nuclear tests which may be conducted under evasive conditions. We explore the performance of efficient discriminants based on spectral features of seismograms using waveforms of 50 earthquakes and 114 quarry and underwater blasts with magnitudes 1.0–2.8, recorded by ISN short-period stations at distances up to 200 km. The single-station spectral ratio of the low and high-frequency seismic energy shows an overlap between explosions and earthquakes. After averaging over a subnet of stations, the resolving power is enhanced and the two classes of events are separated. Different frequency bands were tested; the (1–3 Hz)/(6–8 Hz) ratio provided the best discriminant performance. We also estimated normalized r.m.s. spectral amplitudes in several sequential equal frequency windows within the 1–12 Hz band and applied multiparametric automatic classification procedures (Linear Discrimination Function and Artificial Neural Network) to the amplitudes averaged over a subnetwork. A leave-one-out test showed a low rate of error for the multiparametric procedures. An innovative multi-station discriminant is proposed, based on spectral modulation associated with ripple-firing in quarry blasts and with the bubbling effect in underwater explosions. It utilizes a distinct azimuth-invariant coherency of spectral shapes for different stations in the frequency range (1–12 Hz). The coherency is measured by semblance statistics commonly used in seismic prospecting for phase correlation in the time domain. After modification, the statistics applied to the network spectra provided event separation. A new feature of all the above mentioned procedures is that they are based on smoothed (0.5 Hz window), instrument-corrected FFT spectra of the whole signal; they are robust to the accuracy of onset time estimation and, thus well suited to automatic event identification.     

Genesis of the Hussainiyat ironstone deposit, Western Desert, Iraq

The Hussainiyat ironstone deposit (Jurassic) is mainly pisolitic, intraclastic and concretionary in texture, associated with kaolinite mudstones and/or with quartzose sandstone. The ironstone consists mainly of goethite, hematite, kaolinite and quartz. The deposits were derived from a variety of parent rocks that included low- and medium-rank metamorphics, intermediate igneous rocks and pre-existing sediments of the Nubio-Arabian Shield. The source rocks suffered deep chemical weathering in the hinterland, and the products (Fe-oxyhydroxides, kaolinite and quartz) were later transported by rivers to the depositional site. Iron was mostly carried in association with the clay fraction and organic matter. Several genetic processes were involved in the ironstone formation. Iron concretions were mostly formed by bacterial build-up in swamps and marshes, and were subsequently embedded in kaolinitic mud. Large-scale development of groundwater laterite blanket (ferricrete) occurred later in the overbanks and floors of wadis, under oxidizing pedogenic conditions. During this stage iron was mobilized from the kaolinitic deposits and migrated upward in dry seasons and, to a lesser extent, downward in wet seasons. Pisolites and oolites grew in situ in the kaolinitic soil at the upper limit of the fluctuating water table. This ferricrete blanket had a wide and continuous extension within an elongated paleodepression. Seasonal heavy rain periods resulted in the flow of ephemeral streams and rivers, where major parts of this ferricrete was reworked and deposited with quartz sand and mud clasts as channel deposits. The original pisolitic-colloform ironstone was reworked continuously to form a semi-continuous sheet. In such wet seasons, additional Fe-enrichment took place as cementing materials or overgrowths. Received: 28 April 1995 / Accepted: 10 July 1997     

“Extreme boiling” model for variable salinity of the Hokko low-sulfidation epithermal Au prospect, southwestern Hokkaido, Japan

The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2 wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150° to 200 °C. The results obtained from stable isotope analysis of  δ¹⁸O in quartz vein material showed a gradual decrease in  δ¹⁸O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ¹⁸O_H2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions. We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid, before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable salinities at the same time, and in close proximity to each other. This is also reflected in the  δ¹⁸O_H2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within the Hokko vein system also occurred at this time as a result of boiling and gas loss. Received: 30 May 1997 / Accepted: 6 January 1998     

A study of transient variations in the Earth’s electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

In the framework of the French-Ivorian participation to the IEEY, a network of 10 electromagnetic stations were installed at African longitudes. The aim of this experiment was twofold: firstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric field variations on the other hand. The first results of the magnetic field investigations were presented by Doumouya and coworkers. Those of the electric field experiment will be discussed in this study. The electromagnetic experiment will be described. The analysis of the electromagnetic transient variations was conducted in accordance with the classical distinction between quiet and disturbed magnetic situations. A morphological analysis of the recordings is given, taking into consideration successively quiet and disturbed magnetic situations, with the results interpreted in terms of the characterization of external and internal sources. Particular attention was paid to the effects of the source characteristics on the induced field of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source effect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two different sources. One of these, the S_R^E source, was responsible for most of the magnetic diurnal variation and corresponded to the well-known magnetic signature of the equatorial electrojet. The other source (the S_R*^E source) was responsible for most of the electric diurnal variation, and was also likely to be an ionospheric source. Electric and magnetic diurnal variations are therefore related to different ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic field diurnal variation is not relevant. Furthermore, the magnetotelluric probing of the upper mantle at dip equator latitudes with the electromagnetic diurnal variation is consequently impossible to perform. In the case of irregular variations, the source effect related to the equatorial electrojet is also discussed. A Gaussian model of equatorial electrojet was considered, and apparent resistivities were computed for two models of stratified Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the profile: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend significantly on both the model used and the distance to the center of the electrojet. These numerical results confirm the existence of a daytime source effect related to the equatorial electrojet. Furthermore, we show that the results account for the observed differences between daytime and night-time apparent resistivity curves. In particular, it was shown that electromagnetic probing of the Earth using the classical Cagniard-Tikhonov magnetotelluric method is impossible with daytime recordings made at dip latitude stations.     

Annual and seasonal variations in the low-latitude topside ionosphere

Annual and seasonal variations in the low-latitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range ±25° are higher at the December solstice than at the June solstice by about 100% during daytime and 30% during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the differences in the modelled electron densities at the two solstices are only about 30% of that seen in the observed values. The model calculations suggest that while the differences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a significant contribution to the daytime annual anomaly, the E × B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6% difference in the solar EUV fluxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations confirm that the seasonal behaviour results from effects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere.     

Environmental risk factors for diseases transmitted by vectors: a case study in North-Argentina

The study quantifies the environmental risk factors for two diseases with different vectors and cycles of transmission: malaria and Chagas' disease in N.W. Argentina near the Bolivian border. This is the area within Argentina where malaria is still a serious health problem. Chagas' disease is to some extent present in many parts of the country. The field work for the study concerned 5903 people in 1466 dwellings. The study resulted in detailed maps of risk factors: particularly water quality and contacts with migrants from Bolivia in the case of malaria; and thatched roofs and dogs in the case of Chagas' disease. This revised version was published online in August 2006 with corrections to the Cover Date.     

Petrogenesis of impure dolomitic marble from the Dabie Mountains, central China

Petrogeneses of impure dolomitic marble and enclosed eclogite from the Xinyan area, Dabie ultrahigh-pressure (UHP) metamorphic terrane, central China were investigated with a special focus on fluid characteristics. Identified carbonate-bearing UHP assemblages are Dol + Coe ± Arg (or Mgs) ± Ap, Dol + Omp ± Coe ± Ap ± Arg (or Mgs), Phen + Omp + Coe + Dol ± Arg and Dol + Coe + Phen + Rt ± Omp ± Arg ± Ap. Retrograde assemblages are characterized by symplectitic replacement of Tr–Ab and Di–Ab after omphacite, and Phl–Pl symplectite after phengite. The P–T conditions of UHP metamorphism were estimated to be P > 2.7 GPa and T > 670 °C by the occurrence of coesite inclusions in garnet in enclosed eclogite and garnet–clinopyroxene geothermometer. The P–T conditions of initial amphibolitization were estimated to be 620 < T < 670 °C and 1.1 < P < 1.3 GPa by calcite–dolomite solvus thermometer and mineral parageneses. Phase relations in P–T– X CO ₂ space in the systems NaAl–CMSCH and KCMASCH were calculated in order to constrain fluid compositions. Compositions and parageneses of UHP-stage minerals suggest the presence of fluid in UHP and exhumation stages. Occurrence of retrograde low-variance assemblages indicates that fluid composition during amphibolitization was buffered. A metastable persistence of magnesite and very restricted occurrence of calcite, magnesite and dolomite suggest a low fluid content in the post-amphibolitization stage.     

Gamma-ray bursts: post-burst evolution of fireballs

The post-burst evolution of fireballs that produce γ-ray bursts (GRBs) is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Owing to the adoption of a new relation between t R and γ, our results differ markedly from previous studies. Synchrotron radiation from the shocked interstellar medium is carefully calculated, using a conventional set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory agreement with observations. We also find that the expansion will no longer be highly relativistic about 4 d after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.     

Sur quelques concentrations plombo-zinciferes du devonien inferieur du massif schisteux rhenan

Zusammenfassung The Lower Devonian of the Rhine Schist Massif is characterized by a fine detrital sedimentation which reflects a coastal plain environment located between the Old Red Sandstone continent in the N and the marine Bohemian Facies in the SE. Mineralisations are located within these coastal-plain sediments, especially in fractured horizons topped by pelitic and floored by sandy sediments of Sieg Emsian age. The mineralised localities occur in three zones: the districts of Bensberg, Eitorf and Mayen. The Lüderich locality (Bensberg) is the most important having approximately one million tons of Zn and Pb metal. It is localised within a zone having marked sedimentary characteristics since the Siegenian. Mineralisation occurs as penecontemporaneous lenses, fracture fillings etc, formed at abnormal contacts between lithological units having different competance during Siegenian and Lower Emsian deformation. However, locally there is lateral transition between the upper units of the sandy channel series and the lower units of the pelitic swamp facies. Vertical extension of mineralisations is strictly limited to the tectonic contact zone between the Odenspiel Sandstone and the Bensberg pelites. Study of mineralisations at various levels (mapping, morphology, structure, paragenetic and geochemical) leads to the formulation of a genetic model requiring complex fracturation at the contact between contrasting lithologies and preferential drainage through these fractured zones; metals are trapped on the sandstone floor, the pelitic roof trapping the vadose hydrothermal solutions. These basic controls seem to apply throughout the Bensberg, Eitorf and Mayen districts. On a more regional scale the age variations of the gangue sediments reflects a lateral displacement of red/grey facies limits due to coastal evolution. Thus one may demonstrate a type of mineral occurrence whose model may be that of epigenetic reconcentration within fractures affecting syngenetic geochemical anomalies.Zusammenfassung Das Unterdevon des Rheinischen Schiefergebirges wird durch eine feinkörnige detritische Sedimentation gekennzeichnet. Die Verteilung dieser detritischen Sedimente widerspiegelt eine flache Küstenlandschaft zwischen dem Kontinent des Unterrotliegenden im Norden und des Meeres mit der Böhmischen Fazies im Südosten. In diesen Schichten befinden sich gangförmige Vererzungen, die in Bruchzonen auftreten, die ein pelitisches Hangendes und ein sandsteinreiches Liegendes vom Siegen-Ems Alter haben. Die Lagerstätten kommen in drei Bezirken vor: Der Bensberger Bezirk, der Bezirk Eitorf und der Bezirk Mayen. Die Lagerstätte Lüderich (Bensberger Revier) ist die größte (ungefähr eine Million Tonnen Metall Zink+Blei). Sie befindet sich in einer Zone, in der schon in der Siegen-Stufe bemerkenswerte sedimentäre Strukturen auftreten. Die Vererzung tritt als penekonkordante Linsen, Gänge, Stockwerke in tektonischen Kontaktzonen auf. Diese Kontaktzonen befinden sich zwischen 2 verschiedenen sedimentären Einheiten, deren Gesteinseigenschaften sehr verschieden sind. Es handelt sich um Pelite mit Sandstein-Einschaltungen einerseits, die zur unteren Emsstufe gehören sollen und um fluviatile Sandsteine andererseits, die zur oberen Siegenstufe gehören sollen. Lokal ist ein lateraler Übergang durch Faziesänderung von den oberen Sandsteinen in die unteren Pelitschichten jedoch nicht unmöglich. Die Sandsteine vertreten einen fluviatilen Sedimentationsbereich, die Pelite einen sumpfigen Sedimentationsbereich. Die Vererzung (mit einer vertikalen Ausdehnung ungefähr von 300 m) tritt nur im Bereich der Kontaktzone zwischen den Odenspieler Sandsteinen und den Peliten der Unteren Bensberger Schichten auf. Die Resultate der Kartographie, der morphologischen Studie der Erzkörper, der Struktur und der Paragenese führen zu einem genetischen Modell. Dabei spielt die Anwesenheit einer Bruchstruktur an der Grenze zwischen 2 Bereichen mit verschiedenen lithologischen Eigenschaften eine Hauptrolle für die Konzentration der im Hangenden fein verteilten Metalle. Die hydrothermalen Lösungen kommen demnach nicht von der Teufe, sondern aus dem Nebengestein. Im ganzen Bensberger Erzrevier wie in der Gegend von Eitorf und in der Gegend von Mayen finden wir dieselben Leitfäden für die Vererzung in den Lagerstätten und Vorkommen. Beobachtet man die ganze Provinz, dann merkt man, daß die Altersverschiedenheiten in den Nebengesteinen der verschiedenen Lagerstätten eine geographische Wanderung der Faziesgrenze zwischen rot und grau in dieser küstennahen Ebene Widerspiegeln. Die verschiedenen Vorkommen und Lagerstätten treten immer in unmittelbarer Nähe dieser Grenze auf. Die Eigenschaften dieser Lagerstätten weisen auf eine Bildung durch eine epigenetische Umlagerung in eine gangförmige Bruchstruktur hin.     

Holocene evolution of the Canet-St Nazaire lagoon (Golfe du Lion, France) as determined from a study of sediment properties

The sedimentation in the lagoons of the Golfe du Lion coast (Mediterranean Sea, France) records their post-Holocene evolution. The Canet-St Nazaire lagoon provides an example of such an evolution. Eight cores (ranging from 13.5 to 19.7 m in length) from a transect of the lagoon provide a cross-section of the late Quaternary stratigraphy. The sedimentary sequence comprises mud and sand layers. The two sand formations are identified as parts of two distinct shelf-sedimentary prisms. Stratigraphic analysis defines three stages in the recent, muddy infilling, corresponding to the last divisions of the Holocene. Sedimentation rates of the muds, related to climatic and palaeogeomorphological events, largely determine their geotechnical and geochemical properties. It appears, indeed, to be the most important factor in determining the compressibility index, the saturated bulk density and the organic and water contents of the deposits. Subboreal muds, poorly consolidated and rich in organic matter, correspond to a high rate of deposition. In the Subatlantic muds, in contrast, the sedimentation rate is lower whilst there is a decrease in the organic matter and an increase in the degree of cohesion.