Microseismicity, tectonics and seismic potential in southern Caribbean and northern Venezuela

Approximately one thousand microearthquakes with body-wave magnitude mb have been located in northern Venezuela and the southern Caribbean region (9–12° N; 64–70° W) since the installation in 1980 of the Venezuelan Seismological Array, together with forty events of mb 4, one of them with surface-wave magnitude Ms 6. Focal depths are in the range of 0 to <15 km. This geologically complex region is part of the boundary between the Caribbean and the South American Plates. Epicentral locations indicate that this E–W oriented portion of the boundary is formed by two 400 km long subparallel fault zones: San Sebastián fault zone (SSF), 20 km north of Caracas along the coast; and La Victoria fault zone (LVF), 25 km south of the city. They are clearly delineated by the microseismicity. New composite focal mechanism solutions (CFMS) along these faults show right-lateral strike-slip (RLSS) motion on nearly E–W oriented fault planes. NW-striking subsidiary active faults occur in the region and intercept the two main E–W fault zones. These interceptions show high levels of microearthquake activity and seismic moment release when compared to other portions of both, the main and subsidiary faults. New CFMS at those fault crossing sites show NW-striking RLSS motion and normal faulting, in an en-echelon-like structural behavior. Geological data and quantitative comparisons with other transcurrent plate boundaries in the world suggest that the rate of plate motion in this area is on the order of 20 mm/y. Several moderate and large shocks have occurred along the SSF and LVF since 1640, including an Ms 7.6 event in 1900 on SSF. Although the region may be relatively far from a repeat of this earthquake, seismicity data indicate that strong shocks could take place along segments of the seismically active faults identified in this study.     

Levels and distributions of organic source tracers in air and roadside dust particles of Kuala Lumpur,Malaysia

The concentrations of n-alkanes, unresolved complex mixture (UCM), petroleum molecular markers, other tracers of cooking and burning emissions, and natural background in atmospheric particles and roadside dust particles were measured at eight locations in the city center and the suburbs of Kuala Lumpur, Malaysia. Atmospheric particles were collected using high-volume filtration (PM-10, GFF) over 24 h average periods. Road dusts were swept up, dried and sieved. Both types of samples were extracted with dichloromethane/methanol mixture (3:1 v/v) by ultrasonic agitation. The extracts were then fractionated by column chromatography and the alkanes subjected to gas chromatography–mass spectrometry (GC–MS). Total extracts were also analyzed directly by GC–MS after silylation. The molecular distributions of compounds as well as diagnostic geochemical ratios were determined in order to identify the sources of the organic compounds. Samples collected from a rural area and lubricating oils were also analyzed for comparisons. Anthropogenic and biogenic sources such as vehicular emissions, waxes of higher plants, food cooking operations, and biomass and domestic refuse burning processes contributed to the organic matter content of atmospheric and to lesser extent, roadside dust particles.     

Assessment and Measurement Issues Related to Soil Carbon Sequestration in Land-Use, Land-Use Change, and Forestry (LULUCF) Projects under the Kyoto Protocol

Mitigating the potential large negative impacts of a change in the earth's climate will require strong and definite actions in the different economic sectors, particularly within agriculture and forestry. Specifically, soils deserve a close examination due to their large carbon mitigation potential. The Kyoto protocol establishes the possibility for crediting greenhouse gas emission reductions from forestry and agriculture activities. In most circumstances, particularly those regarding developing countries, greenhouse gas mitigation activities will be carried out through projects. These projects will have to meet a series of criteria, for the carbon benefits to be measurable, transparent, verifiable and certified. These criteria include: establishing credible baselines (without-project or reference scenario), additionality, permanence, quantifying and reducing potential leakage of greenhouse gases across project borders, coping with natural or human induced risks, accurately measuring changes in carbon stocks using carbon accounting techniques, and – in the case of the Clean DevelopmentMechanism – resulting in sustainable development benefits. In this paper we describe the methods and approaches that have been developed to cope with the different criteria and discuss their implications for carbon sequestration in soils. Soil carbon represents the largest carbon pool of terrestrial ecosystems, and has been estimated to have one of the largest potentials to sequester carbon worldwide. However, getting credits from soil carbon sequestration through project activities presents several challenges: the need to monitor small incremental changes in soil carbon content relative to large carbon pools, long-time periods to accrue the full carbon benefits, high local variability of soil carbon content, and relatively costly soil carbon measurement procedures. Also, the responses of soil C stocks to forestry and agriculture activities are complex and need careful attention. Specifically, the time dynamics of soil C responses to land use changes, the diversity of soil types, soil-plant interactions, and the availability of accurate soil C inventories, should be considered to successfully implement LULUCF projects.     

The bearing capacity of surface strip foundations on geogrid-reinforced sand and clay — a comparative study

Summary Laboratory model test results for the ultimate bearing capacity of a strip foundation supported by geogrid-reinforced sand and saturated clay are presented. One type of geogrid only was used for all the tests. On the basis of the model test results, the optimum depth and width of reinforcing layers and the optimum depth of the location of the first layer of the geogrid in sand and saturated clay were determined and compared.     

Seismic Activity and Energy Release of Earthquakes along the Gulf of Aqaba,Egypt

Geotectonics - The Gulf of Aqaba is situated along the southern part of the Dead Sea Rift Area transform (DST), 1000 km (620 miles), the boundary between the African plate and the Arabian plate. It...     

Stellar contributors to the hard X-ray background?

We use simple energetic arguments to estimate the contribution of massive X-ray binaries and supernova remnants to the cosmic X-ray background (XRB) at energies in excess of 2 keV. Recent surveys have shown that active galactic nuclei (AGN) probably account for most of the hard XRB ( E >2 keV), but there have been many suggestions that star-forming galaxies could emerge at fainter fluxes and perhaps account for a significant fraction of the soft and hard X-ray energy density. Assuming that the formation rate of massive X-ray binaries (MXRBs) traces the global star-formation rate, we find that their integrated contribution to the hard XRB can be estimated and is shown to be small (at less than the 1 per cent level). Similarly, the integrated flux of supernovae (SN) is also shown to be insignificant, or at most comparable to MXRBs. AGN therefore remain the most viable candidates for producing the hard XRB, unless additional processes can be shown to dominate the global hard X-ray emission in distant starburst galaxies.     

An examination of groundwater within the Hawara Pyramid,Egypt

The Hawara Pyramid is an outstanding monument. However, the mudbrick structure shows signs of erosion, and the passages and chambers are currently submerged. The problem of water ingress has mainly arisen since the 1880s. In this study, an initial assessment of the pyramid structure was made and causes of water ingress were investigated through analysis of water samples. Stable oxygen isotope measurements indicate that the source of water within the pyramid is the Bahr Selah canal. Water within the pyramid is highly saline compared to the Bahr Selah, and evaporation can only partly account for this high salinity. The composition of dissolved ions suggests that dissolution of salts in soils and from bedrock in the vicinity of the pyramid has enhanced the salinity of water percolating into the pyramid structure. Water ingress and salt deposition are at present the main threat to the integrity of the monument. © 2007 Wiley Periodicals, Inc.     

Out‐of‐plane seismic behaviour of rocking masonry walls

The evaluation of the out‐of‐plane behaviour of unreinforced walls is one of the most debated topics in the seismic assessment of existing masonry buildings. The discontinuous nature of masonry and its interaction with the remainder of the building make the dynamic modelling of out‐of‐plane response troublesome. In this paper, the results of a shaking table laboratory campaign on a tuff masonry, natural scale, U‐shaped assemblage (façade adjacent to transverse walls) are presented. The tests, excited by scaled natural accelerograms, replicate the behaviour of external walls in existing masonry buildings, from the beginning of rocking motion to overturning. Two approaches have been developed for modelling the out‐of‐plane seismic behaviour: the discrete element method and an SDOF analytic model. Both approaches are shown to be capable of reproducing the experimental behaviour in terms of maximum rotation and time history dynamic response. Finally, test results and numerical time history simulations have been compared with the Italian seismic code assessment procedures. Copyright © 2011 John Wiley & Sons, Ltd.     

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ¹³C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ¹⁸O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ¹⁸O values might reflect the contribution of ¹⁸O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca²⁺ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ¹⁸O–δ¹³C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.     

On oligarchic growth of planets in protoplanetary disks

In this paper we present a new semianalytical model of oligarchic growth of planets considering a distribution of planetesimal sizes, fragmentation of planetesimals in mutual collisions, sublimation of ices through the snow line, random velocities out of equilibrium and merging of planetary embryos. We show that the presence of several planetary embryos growing simultaneously at different locations in the protoplanetary disk affects the whole accretion history, specially for the innermost planets. The results presented here clearly indicate the relevance of considering a distribution of planetesimal sizes. Fragmentation occurring during planetesimal-planetesimal collisions represent only a marginal effect in shaping the surface density of solid material in the protoplanetary disc.