Evidence for fault-related directionality and localized site effects from strong motion recordings of the 2003 Boumerdes (Algeria) earthquake: Consequences on damage distribution and the Algerian seismic code

The Algiers–Boumerdes region has been struck by a destructive magnitude 6.8 (M_w) earthquake on May 21, 2003. The study presented in this paper is based on main shock strong motions from 13 stations of the Algerian accelerograph network. A maximum 0.58g peak ground acceleration (PGA) has been recorded at 20 km from the epicenter, only about 150 m away from a PGA of 0.34g, with both a central frequency around 5 Hz, explained by a strong very localized site effect, confirmed by receiver function technique results showing peaks at 5 Hz with amplitudes changing by a factor of 2. Soil amplifications are also evidenced at stations located in the quaternary Mitidja basin, explaining the higher PGA values recorded at these stations than at stations located on firm soil at similar distances from the epicenter. A fault-related directionality effect observed on the strong motion records and confirmed by the study of the seismic movement anisotropy, in agreement with the N65 fault plan direction, explains the SW–NE orientation of the main damage zone. In the near field, strong motions present a high-frequency content starting at 3 Hz with a central frequency around 8 Hz, while in the far field their central frequency is around 3 Hz, explaining the high level of damage in the 3- to 4-story buildings in the epicentral zone. The design spectra overestimate the recorded mean response spectra, and its high corner frequency is less than the recorded one, leading to a re-examination of the seismic design code that should definitively integrate site-related coefficient, to account for the up to now neglected site amplification, as well as a re-modeling of the actual design spectra. Finally, both the proposed Algerian attenuation law and the worldwide laws usually used in Algeria underestimate the recorded accelerations of the 6.8 (M_w) Boumerdes earthquake, clearly showing that it is not possible to extrapolate the proposed Algerian law to major earthquakes.     

Optical studies of natural deformation microstructures in feldspars (gneiss and pegmatites from Occitania,southern France)

Natural deformation microstructures of feldspars have been investigated optically in augen gneisses and pegmatites from southern France that exhibit microstructures due to polyphase deformation. This deformation was produced in both cases under a pressure of about 2 Kb and a temperature decreasing from about 550° to 200–300°C.In microcline, recrystallization, tension gashes and shear planes are the most important microstructures observed. In plagioclase, shear planes and kink-bands prevail. The development of these structures is controlled primarily by three parameters: temperature, influence of the surrounding material and amount of bulk strain. The orientation of the planes of anisotropy of the minerals with respect to the compression axis is also important. Some perthitic patterns are ascribed to deformation.     

La diatomee marine chaetoceros simplex calcitrans paulsen et son environnement,I. Relations avec le milieu de culture; étude de la fraction insaponifiable,des stérols et des acides gras

Detailed analysis of the lipids present in seawater medium before and after growing cultures of the diatom Chaetoceros simplex calcitrans Paulsen in them has been carried out and compared with the lipids found in the diatom itself.Fatty Acids. The seawater contained 1.0 μg fatty acids/ml, C 16:0, C 18:0 and C 18:1 being the predominant components. After removal of 12-day cultures of the diatom, the medium contained 1.1 μg fatty acids/ml and showed 47% decrease of the C 18:0 acid, and decreases of 37% in the C 13:0 and 56% in the C 15:1 contents. The diatom itself contained 10.7% fatty acids of the dry weight, among which C 16:1, C 16:0 and C 14:0 predominated.Sterols. Cholesterol dominated the sterol composition of seawater, which was found to contain 0.15 μg total sterols/ml. The filtered culture medium contained 0.18 μg total sterols/ml, had lost some of its cholesterol content, but contained a significant amount of 24-methylene-cholesterol. Other minor sterols in seawater both prior to and after growing diatoms cultures in it were β-sitosterol, stigmasterol and campesterol.The diatom itself contained 0.47% total sterols of the dry weight with cholesterol and 24-methylene-cholesterol making up ca. 40% of the total each. Minor sterols found were stigmasterol (or fucosterol), campesterol and β-sitosterol.     

Submesoscale vortex structures at the entrance of the Gulf of Lions in the Northwestern Mediterranean Sea

The meanders of a baroclinic coastal current in the Northwestern Mediterranean Sea have already been reported in the literature. These meanders can be surrounded by vortices. Such vortices have been observed in the western part of the Gulf of Lions but the location and the mechanism of their formation are poorly documented. In this paper, we use the current measurements of a one-year experiment, which was conducted in the eastern part of the Gulf of Lions to detect and characterize the vortex activity. A vortex detection algorithm based on few velocity data was developed. Current measurements were available at the sea surface (HF radars) and in the water column from 50 to 140 m depth (four current meter moorings). SST images and hydrologic data were also used. Results focus on observations that are coherent 50 m and at the surface. Vortices are anticyclonic, of submesoscale size and present maximal velocities of 30–50 cm/s. The drift speed of the vortices is comparable to but less than the velocity of the Northern Current. These observations enable to estimate the minimum vortex occurrence in this area. The presence of vortex structures is strongly correlated with a specific sequence of wind patterns.     

A new experimental technique for extracting liquids from peridotite at very low degrees of melting: application to partial melting of depleted peridotite

A new technique that allows extraction of liquids from peridotite at degrees of melting as low as 0.2 wt% is presented. Microfractures that formed in the graphite sample container at the beginning of the experiments were used as traps for the liquid phase. Glass-filled cracks (or 'microdikes') unaffected by quench crystallisation were produced in all experiments and were analysed using standard electron microprobe techniques. Reversal experiments demonstrated that, at moderate degrees of melting (4.4 and 6.5 wt%), the segregated liquid was in equilibrium with the neighbouring peridotite. At very low degrees of melting (0.3 wt%), the liquid in the microdikes failed to fully equilibrate with the peridotite after 5 days and the sandwich technique was used in combination with the microdike technique to approach more closely the equilibrium composition of near-solidus partial melts. The microdike technique was used to study melting of a depleted peridotite at 1 GPa and 1,220 to 1,360 °C.Editorial responsibility: T.L. Grove     

Origine des eaux des émergences karstiques chlorurées du Languedoc-Roussillon

The origin of chloride-rich karstic spring waters representative of the Languedoc-Roussillon region has been investigated with a hydrochemical approach. To this end, the major and trace elements most often used in the study of saline environments have been considered (Cl, SO₄, Br, B, Li). This study allowed distinguishing the different end-members of the various chloride-rich karstic spring waters (evaporitic, marine, geothermal). Associated with the Cl, Br and B contents, the Li/SO₄ ratio appeared as a relevant tracer for the determination of the origin of lithium and by extension of the considered waters. To cite this article: O. Hébrard et al., C. R. Geoscience 338 (2006).     

Extraction of water and solutes from argillaceous rocks for geochemical characterisation: Methods, processes and current understanding

This paper summarises the results of a comprehensive critical review, initiated by the OECD/NEA "Clay Club," of the extraction techniques available to obtain water and solutes from argillaceous rocks. The paper focuses on the mechanisms involved in the extraction processes, the consequences on the isotopic and chemical composition of the extracted pore water and the attempts made to reconstruct its original composition. Finally, it provides some examples of reliable techniques and information, as a function of the purpose of the geochemical study. Electronic Publication