On the development of a seismic source zonation model for seismic hazard assessment in western Saudi Arabia

A new seismic source model has been developed for the western part of the Arabian Peninsula, which has experienced considerable earthquake activity in the historical past and in recent times. The data used for the model include an up-to-date seismic catalog, results of recent studies of Cenozoic faulting in the area, aeromagnetic anomaly and gravity maps, geological maps, and miscellaneous information on volcanic activity. The model includes 18 zones ranging along the Red Sea and the Arabian Peninsula from the Gulf of Aqaba and the Dead Sea in the north to the Gulf of Aden in the south. The seismic source model developed in this study may be considered as one of the basic branches in a logic tree approach for seismic hazard assessment in Saudi Arabia and adjacent territories.     

Attempt to identify seismic sources in the eastern Mitidja basin using gravity data and aftershock sequence of the Boumerdes (May 21, 2003; Algeria) earthquake

In order to try to identify the seismogenic sources in the epicentral area, we interpreted data collected from gravity and aftershocks in the eastern part of the Mitidja basin after the occurrence of the 21 May 2003 Boumerdes earthquake (Mw = 6.8). The residual gravity anomaly and the horizontal gradient maps revealed the basement shape and density discontinuities. A seismotectonic model obtained from the aftershocks distribution and gravity data is proposed. This model highlights three active faults: one offshore and two onshore. The offshore reverse fault striking NE-SW, parallel to the coast, is consistent with the USGS focal mechanism of the main event, which is assumed to have the most moment release. The two onshore dipping blind active faults are postulated at crossing angles near the SW tip of the main fault. The interpretation is based mainly on the re-location and distribution of aftershocks, and their focal solutions. It is also supported by the basin structures obtained from the inversion and interpretation of residual gravity anomalies, as well as by additional compiled information such as the pattern of coastal co-seismic uplift. This configuration puts forward the failure mode complexity during the main shock. The topography of the basement obtained from 3D gravity inversion shows that all the onshore located aftershocks occurred in the basement, and the area between the two onshore faults rose as a consequence of their sliding.     

http://www.sciencedirect.com/science/article/pii/S1674987112001247

The Sinai Peninsula has been recognized as a subplate of the African Plate located at the triple junction of the Gulf of Suez rift, the Dead Sea Transform fault, and the Red Sea rift. The upper and lower crustal structures of this tectonically active, rapidly developing region are yet poorly understood because of many limitations. For this reason, a set of P- and S-wave travel times recorded at 14 seismic stations belonging to the Egyptian National Seismographic Network (ENSN) from 111 local and regional events are analyzed to investigate the crustal structures and the locations of the seismogenic zones beneath central and southern Sinai. Because the velocity model used for routine earthquake location by ENSN is one-dimensional, the travel-time residuals will show lateral heterogeneity of the velocity structures and unmodeled vertical structures. Seismic activity is strong along the eastern and southern borders of the study area but low to moderate along the northern boundary and the Gulf of Suez to the west. The crustal V_p/V_s ratio is 1.74 from shallow (depth ≤ 10 km) earthquakes and 1.76 from deeper (depth > 10 km) crustal events. The majority of the regional and local travel-time residuals are positive relative to the Preliminary Reference Earth Model (PREM), implying that the seismic stations are located above widely distributed, tectonically-induced low-velocity zones. These low-velocity zones are mostly related to the local crustal faults affecting the sedimentary section and the basement complex as well as the rifting processes prevailing in the northern Red Sea region and the ascending of hot mantle materials along crustal fractures. The delineation of these low-velocity zones and the locations of big crustal earthquakes enable the identification of areas prone to intense seismotectonic activities, which should be excluded from major future development projects and large constructions in central and southern Sinai.     

New contributions to the Upper Senonian stratigraphy in Northern Iraq

The Upper Senonian sediments are well developed in Northern Iraq and represent three types of facies; elastics, reefal, and open-marine. These sediments are classified stratigraphically depending upon similarities in age and lithology. New groups are proposed for the first time; Ruwanduz Reefal Group comprising Aqra Limestone Formation, Bekhme Limestone Formation and Pilsener Limestone Formation, and Zakho Marl Group including Shiranish Formation, Digma Formation and Jib'ab Formation. It is suggested here that Jib'ab Formation is an extension of the Shiranish Formation and should not be treated as a separate lithostratigraphic unit.     

Delineation of potential recharge area using a hybrid model,case of Djelfa Hadjia watershed

Groundwater potential map is important for environmental assessment and water resources management. In this work, a groundwater recharge potential map was established for the watershed of Oued Djelfa Hadjia in Algeria, based on new multiparameters hybrid model. The model has hydroclimatic parameters, geological settings, slope factor, and stream network density factor as inputs. The groundwater recharge estimated by the model range from 0.71 to 14 mm. The model allows delineation of potential area of recharge. The total water abstraction in Djelfa city is about of 14 hm³; however, the calculated groundwater recharge is about 3 mm/year (min 0.71 mm and max 14 mm), which correspond to an average recharge volume of 3.9 hm³ which mean that the aquifer is under over exploitation.     

Reliability-based design of spread footings on fibre-reinforced clay

The potential use of fibres in a number of geotechnical engineering applications is gaining more interest in the geotechnical community. A select application consists of the improvement of soft grounds to mitigate their problematic shear strength characteristics. Extensive experimental work has been reported on the response/behaviour of fibre-reinforced clay (FRC) and was recently complemented by several strength prediction models. The effectiveness of these models has not been thoroughly evaluated yet. The objectives of this study are to (1) quantify the model uncertainty of a newly developed FRC model that is aimed exclusively at predicting the “undrained” shear strength of FRCs, (2) combine the model uncertainty with other conventional sources of uncertainty to assess the reliability levels that are inherent in the ultimate limit state design of spread footings that rest on a top FRC layer underlain by weaker natural soft clay, and (3) recommend factors of safety that would ensure a target reliability level for these footings. Results indicate that the traditional safety factor of 3 should be used with caution as it may not be sufficient to yield the desired level of reliability, particularly for smaller footings, lower applied stresses, larger scales of fluctuation, and larger target reliability indices.     

Les minéralisations ferrifères du Sahara Algerien: le gisement de fer oolithique de Mecheri Abdelaziz (bassin de Tindouf)

Résumé Le gisement de fer oolithique de Mecheri Abdelaziz (Famennien terminal du bassin de Tindouf), présente des caractères morphologiques et sédimentologiques qui permettent de penser que la minéralisation s'est mise en place dans des édifices deltaïques progradants.La minéralisation est complexe et comprend trois paragenèses principales; maghémite-magnétite, bavalite-hématite, bavalite-magnétite et quatre faciès types: faciès microconglomératique (FMC), faciès à oolithes dispersées dans une matrice détritique (FOD), faciès à oolithes dispersées dans une matrice non détritique (FOND) et faciès à oolithes jointives, cimentées (FOC).Les caractères des oolithes et de la mésostase montrent que les corps minéralisés résultent de l'accumulation d'oolithes, développées essentiellement par concrétionnement intrasédimentaire et ayant subi, avant le dépôt définitif, des phases alternantes d'oolithisation et de remise en suspension.Sur l'ensemble du gisement, les teneurs moyennes sont les suivantes: Fe total: 43,49%; SiO₂=13,66%; P₂O₅=2,31%; CaO=5,35%.En ce qui concerne la source du fer, il semble que la mise en place du minerai soit liée aux importantes réserves de quartzites ferrugineux situées dans le Précambrien du socle Réguibat, dont le stock métal initial a subi des remobilisations successives.

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The Mecheri Abdelaziz oolitic iron deposit (late Famennian of Tindouf Basin) shows morphological and sedimentological features of prograding deltaic sequences.The mineralization is complex and allows the recognition of 3 types of paragenesis: maghemite-magnetite, bavalite-hematite, bavalite-magnetite; and 4 types of facies: microconglomerate facies, oolites dispersed in a matrix with detritic elements, oolites dispersed in a matrix without detritic elements and oolites with cement.The comparison of oolites and matrix suggests that the orebodies are the result of oolites deposition, developed by intrasedimentary processes and before the final deposition, by alternating phases of oolitization and suspension.For the whole ore deposit, the main composition is: total Fe=43.49%; SiO₂=13.66%; P₂O₅=2.31%; CaO=5.35%.The major portion of iron was probably provided by continental leaching of the very important iron formations of the Precambrien Reguibat shield.

     

Paleoenvironmental conditions and strontium isotope stratigraphy in the Paleogene Gafsa Basin (Tunisia) deduced from geochemical analyses of phosphatic fossils

Fossil shark teeth and coprolites from three major phosphorite occurrences in the Gafsa Basin (southwestern Tunisia) were investigated for their geochemical compositions to improve local stratigraphy and to better assess paleoenvironmental conditions. ⁸⁷Sr/⁸⁶Sr isotope ratios of shark teeth from the Early Maastrichtian El Haria Formation and from the Early Eocene Métlaoui s.s. Formation yielded Sr isotope ages of 68 ± 1 and 47.9 ± 1.3 Ma, respectively, which accord with the expected stratigraphic positions of these sediments. Conversely, shark teeth from the Paleocene–Eocene Chouabine Formation have large variation in Sr isotope ratios even within individual layers. After statistical treatment and then elimination of certain outlier samples, three age-models are proposed and discussed. The most reasonable solution includes three subsequent Sr ages of 61.8 ± 2.2 Ma, 57.2 ± 1.8 and 54.6 ± 1.6 for layer IX, layers VIII–V and layers IV–0, respectively. Three scenarios are discussed for explanation of the presence of the outliers: (1) diagenesis, (2) re-working and (3) locally controlled seawater Sr isotope ratio. The most plausible account for the higher ⁸⁷Sr/⁸⁶Sr ratios relative to the global ocean in some fossils is enhanced intrabasinal re-working due to low sea level. Conversely, the sample with lower ⁸⁷Sr/⁸⁶Sr than the global seawater may link to diagenesis or to seawater influenced by weathering of Late Cretaceous marine carbonates, which latter is supported by model calculation as well. The ε_Nd values of these fossils are very similar to those reported for Paleogene and Late Cretaceous Tethyan seawater and are compatible with the above interpretations. The relatively low oxygen isotope values in shark teeth from the topmost phosphate bed of the Chouabine Formation, together with the Sr isotope results, point toward recovering better connections with the open sea. These δ¹⁸O data reflect elevated ambient temperature, which may link to the Early Eocene Climatic Optimum.     

Integration of RADARSAT and GIS modelling for estimating future Red River flood risk

A new geomatics-based approach for flood prediction was developed and used to model the magnitude and spatial extent of a future Red River flood in southern Manitoba. This approach combines the statistical modelling capabilities of Markov (non-spatial) analysis and logistic regression (spatial) within a geographic information system (GIS) environment, utilizing modelling inputs derived from remotely sensed RADARSAT imagery and other digital geographic data. The 1997 Red River flood was the second largest in recorded history, and the largest for which accurate data are available. The results indicate: (i) a flood “one time interval-in terms of 3 days time unit measurement- larger in area” than the 1997 flood is expected to affect 17.6% more land (an additional 47.6 km²) within the study area compared to 1997 levels based on Markovian probability derived from observations from the 1997 event; and (ii) the majority of this excess flooding will take place on agricultural land; no additional communities are expected to be at risk. Quantitative assessment verified the capability of this modelling approach for producing statistically significant results. The methodology used in this research would be easily transferable to other areas, and may provide the basis for a viable alternative to conventional hydrologic-based flood prediction approaches This revised version was published online in August 2006 with corrections to the Cover Date.