Inelastic displacement ratios for evaluation of existing structures

Results of a detailed statistical study of constant relative strength inelastic displacement ratios to estimate maximum lateral inelastic displacement demands on existing structures from maximum lateral elastic displacement demands are presented. These ratios were computed for single‐degree‐of‐freedom systems with different levels of lateral strength normalized to the strength required to remain elastic when subjected to a relatively large ensemble of recorded earthquake ground motions. Three groups of soil conditions with shear wave velocities higher than 180m/s are considered. The influence of period of vibration, level of lateral yielding strength, site conditions, earthquake magnitude, distance to the source, and strain‐hardening ratio are evaluated and discussed. Mean inelastic displacement ratios and those associated with various percentiles are presented. A special emphasis is given to the dispersion of these ratios. It is concluded that distance to the source has a negligible influence on constant relative strength inelastic displacement ratios. However, for periods smaller than 1s earthquake magnitude and soil conditions have a moderate influence on these ratios. Strain hardening decreases maximum inelastic displacement at a fairly constant rate depending on the level of relative strength for periods of vibration longer than about 1.0s while it decreases maximum inelastic displacement non‐linearly as the period of vibration shortens and as the relative‐strength ratio increases for periods of vibration shorter than 1.0s. Finally, results from non‐linear regression analyses are presented that provide a simplified expression to be used to approximate mean inelastic displacement ratios during the evaluation of existing structures built on firm sites. Copyright © 2003 John Wiley & Sons, Ltd.     

Tree growth response to the 1913 eruption of Volcán de Fuego de Colima, Mexico

The impact of volcanic eruptions on forest ecosystems can be investigated using dendrochronological records. While long-range effects are usually mediated by decreased air temperatures, resulting in frost rings or reduced maximum latewood density, local effects include abrupt suppression of radial growth, occasionally followed by greater than normal growth rates. Annual rings in Mexican mountain pine (Pinus hartwegii Lindl.) on Nevado de Colima, at the western end of the Mexican Neovolcanic Belt, indicate extremely low growth in 1913 and 1914, following the January 1913 Plinian eruption of Volcán de Fuego, 7.7 km to the south. That event, which is listed among the largest explosive eruptions since A.D. 1500, produced ashflow deposits up to 40 m thick and blanketed our study area on Nevado de Colima with a tephra fallout 15–30 cm deep. Radial growth reduction in 1913–14 was ≥30% in 73% of the sampled trees. We geostatistically investigated the ecological impact of the eruption by mapping the decrease in xylem increment and found no evidence of a spatial structure in growth reduction. Little information has been available to date on forest species as biological archives of past environments in the North American tropics, yet this historical case study suggests that treeline tropical sites hold valuable records of prehistoric phenomena, including volcanic eruptions.     

Sulfur contents of the parental metallic cores of magmatic iron meteorites

Magmatic iron meteorites are thought to be samples of the central metallic cores of asteroid-sized parent bodies. Sulfur is believed to have been an important constituent of these parental cores, but due to the low solubility of S in solid metal, initial S-contents for the magmatic groups cannot be determined through direct measurements of the iron meteorites. However, experimental solid metal-liquid metal partition coefficients show a strong dependence on the S-content of the metallic liquid. Thus, by using the experimental partition coefficients to model the fractional crystallization trends within magmatic iron meteorite groups, the S-contents of the parental cores can be indirectly estimated. Modeling the Au, Ga, Ge, and Ir fractionations in four of the largest magmatic iron meteorite groups leads to best estimates for the S-contents of the parental cores of 12 ± 1.5 wt% for the IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1 ± 1 wt% for the IVB group. The IVA elemental fractionations are not adequately fit by a simple fractional crystallization model with a unique initial S-content. These S-content estimates are much higher than those recently inferred from crystallization models involving trapped melt. The discrepancy is due largely to the different partition coefficients that are used by the two models. When only partition coefficients that are consistent with the experimental data are used, the trapped melt model, and the low S-contents it advocates, cannot match the Ge and Ir fractionations that are observed in IIIAB iron meteorites.     

Curie Point Depths Beneath Precordillera Cuyana and Sierras Pampeanas Obtained from Spectral Analysis of Magnetic Anomalies

A gravity and magnetic survey has been carried out with the purpose of investigating geophysical features of the crusts beneath three geological provinces in western Argentina: Cuyo Precordillera, the Sierras Pampeanas of San Juan and La Rioja, and Famatina System, the results of which are displayed in three maps: Bouguer anomaly, total field magnetic anomaly and total field reduced to the pole.

The top and bottom boundaries of the magnetized crust were calculated from power-density spectra of the total-field anomalies from our terrestrial database in 90 2D windows. The depths obtained for the bottom of magnetized crust are assumed to correspond to Curie point depths. The values thus obtained for the Precordillera range between 29 and 40 km, whereas for the Sierras Pampeanas, in the Sierra de Pie de Palo, and other mountain chains along the Bermejo-Desaguadero lineament or Valle Fértil lineament, such depth ranges between 20 and 35 km. These results are consistent with Curie point depths determined on different continental regions in the world.

A map of regional heat flow has been prepared in the present work based on the depth of the Curie point isotherm, which shows that heat flow patterns in Precordillera are different from those found in Sierras Pampeanas.

A significant heat flow extending in Northeast-Southwest direction seems to be directly related to Juan Fernández Ridge trace.

The results of the present investigations also point out a possible relationship between the base of the magnetized crust and the boundary separating the brittle from the ductile crustal regime.     

Vegetation dynamics during the late Pleistocene in the Barreirinhas region, Maranhão State, northeastern Brazil, based on carbon isotopes in soil organic matter

The study place is in the Barreirinhas region, Maranhão State, northeastern Brazil. A vegetation transect of 78 km was studied among four vegetation types: Restinga (coastal vegetation), Cerrado (woody savanna), Cerradão (dense woody savanna), and Forest, as well as three forested sites around Lagoa do Caçó, located approximately 10 km of the transect. Soil profiles in this transect were sampled for δ¹³C analysis, as well as buried charcoal fragments were used for ¹⁴C dating. The data interpretation indicated that approximately between 15,000 and ∼9000 ¹⁴C yr B.P., arboreal vegetation prevailed in the whole transect, probably due to the presence of a humid climate. Approximately between ∼9000 and 4000-3000 ¹⁴C yr B.P., there was the expansion of the savanna, probably related to the presence of drier climate. From ∼4000-3000 ¹⁴C yr B.P. to the present, the results indicated an increase in the arboreal density in the area, due to the return to a more humid and probably similar climate to the present. The presence of buried charcoal fragments in several soil depths suggested the occurrence of palaeofires during the Holocene. The vegetation dynamic inferred in this study for northeastern Brazil is in agreement with the results obtained in areas of Amazon region, based on pollen analysis of lake sediments and carbon isotope analysis of soil organic matter (SOM), implying than similar climatic conditions have affected these areas during the late Pleistocene until the present.     

Detrital thermochronology – a new perspective on hinterland tectonics, an example from the Andean Amazon Basin, Ecuador

In order to understand the significance of detrital grain ages in sedimentary basins, a new approach is presented. Five characteristic paths, identified by the change in age of detrital grain populations combined with the change in lagtime over time, can be related to different geodynamic settings in the source regions. When lagtime and grain age increase over time, a change in source must be invoked – this is usually a direct response to a geological event. A constant cooling age, a vertical path, associated implicitly with increasing lagtime, implies erosion of materials that had passed through the closure temperature rapidly – exhuming sufficient rock to supply detritus over the time of the path. Constant lagtimes, regardless of the lagtime itself, are indicative of thermochronological stability in the source region. This can involve fast or slow cooling. Finally, decreasing lagtimes support the notion of increasing cooling rates in the source regions over time. A test study is presented from sediments of the northern Ecuadorian Sub‐Andean Zone where geological events had previously been identified using alternative methods. The addition of heavy‐mineral studies increased the precision in the interpretation. At 90 Ma, rapidly decreasing lagtimes point to a phase of tectonic activity. From about 85 Ma until about 60 Ma the lagtimes were approximately zero. This represents a phase of rapid exhumation of the source regions correlating with the previously identified Pallatanga event. An associated increase of metamorphic minerals occurs over this time span, pointing to increased erosion from deeper horizons. At about 70 Ma, the oldest source region, the shield to the east, was switched off. This timing correlates with a change from marine to continental conditions in the basin, a change in palaeocurrent directions from the east to the west, as well as an associated influx of material from the growing Cordillera Real. At about 55 Ma, a change in source is identified by a change in slope of the lagtime curve together with a change in heavy minerals. From 50 to 35 Ma a renewed period of tectonism in the source region is correlated with the docking of the Macuchi terrane which clearly had an effect of increased erosion in the Cordillera Real bringing in higher grade metamorphic minerals. From about 32 Ma onwards the lagtime has been somewhat constant at about 30 Myr. This does not imply, however, a steady‐state environment as it is well known from other geological evidence that there have been other events within this time frame. One must be cautious about over‐interpreting the lagtime as a method to determine steady state in any region. It is a matter of scale.     

Stratigraphy of Pleistocene glaciations in the St Elias Mountains,southwest Yukon,Canada

At least five Middle to Late Pleistocene advances of the northern Cordilleran Ice Sheet are preserved at Silver Creek, on the northeastern edge of the St Elias Mountains in southwest Yukon, Canada. Silver Creek is located 100 km up‐ice of the Marine Isotope Stage (MIS) 2 McConnell glacial limit of the St Elias lobe. This site contains ~3 km of nearly continuous lateral exposure of glacial and non‐glacial sediments, including multiple tills separated by thick gravel, loess and tilted lake beds. Infrared‐stimulated luminescence (IRSL) and AMS radiocarbon dating constrain the glacial deposits to MIS 2, 4, either MIS 6 or mid‐MIS 7, and two older Middle Pleistocene advances. This chronology and the tilt of the lake beds suggest Pleistocene uplift rates of up to 1.9 mm a^?1 along the Denali Fault since MIS 7. The non‐glacial sediment consists of sand, gravel, loess and organic beds from MIS 7, MIS 3 and the early Holocene. The MIS 3 deposits date to between 30–36 ¹⁴C ka BP, making Silver Creek one of the few well‐constrained MIS 3‐aged sites in Yukon. This confirms that ice receded close to modern limits in MIS 3. Pollen and macrofossil analyses show that a meadow‐tundra to steppe‐tundra mosaic with abundant herbs and forbs and few shrubs or trees, dominated the environment at this time. The stratigraphy at Silver Creek provides a palaeoclimatic record since at least MIS 8 and comprises the oldest direct record of Pleistocene glaciation in southwest Yukon.     

Assessment of dissolved nutrients dispersal derived from offshore fish-farm using nitrogen stable isotope ratios (δ15N) in macroalgal bioassays

In this study, the dispersal of wastes from offshore fish farms was evaluated by analyzing nitrogen stable isotope ratios (δ¹⁵N) in macroalgae incubated in the water column at sites located at an increasing distance from the fish cages. Bioassays were performed at three fish farms situated in separate localities with different nutritional conditions (Canary Islands, Murcia and Catalonia) and varying in size, species of fish reared and annual production. Macroalgal bioassays were carried out in two different directions (DI and DII) and they were replicated at each distance in order to evaluate the effect of small-scale variability on the spatial extent of fish farm wastes. The results obtained with δ¹⁵N contribute to a better understanding of the application of nitrogen stable isotopes ratios in macroalgae as an effective bioindicator for tracing the dispersion of offshore fish farm wastes, and demonstrate that fish farm wastes can be traced even over distances of some km from the pollution source. In the Canary Islands, the maximum distance obtained for detection of fish farm wastes was between 450 and 700 m. Of the three installations studied, Murcia presented the greatest distance for detection of fish farm waste influence, ranging from between 1550 and 2450 m, whilst in Catalonia this distance was less than 120 m. In Catalonia, the results were masked by the influence of other sources of nitrogen, and thus fish farm wastes were detected at more reduced distances than expected. These results confirm that fish farm wastes can be traced using the nitrogen stable isotope ratios of macroalgae and that this method can also be useful for identifying areas of potential risk to some sensitive ecosystems, and as an early signal that changes in the community structure might occur.     

Hydrologic and water isotope characterization of a regulated Canadian Shield river basin

Stable water isotope surveys have increasingly been integrated into river basins studies, but fewer have used them to evaluate impact of hydropower regulation. This study applies hydrologic and water isotope survey approaches to a Canadian Shield river basin with both regulated and natural flows. Historical streamflow records were used to evaluate the influence of three hydroelectric reservoirs and unregulated portions of the basin on downstream flows and changes in water level management implemented after an extreme flood year (1979). In 2013, water isotope surveys of surface and source waters (e.g., rainfall, groundwater, snowmelt) were conducted to examine spatial and temporal variation in contributions to river flow. Seasonal changes in relative groundwater contribution were assessed using a water‐isotope mass balance approach. Within the basin, two regulated reservoirs exhibited inverted hydrographs with augmented winter flows, whereas a third exhibited a hydrograph dominated by spring snowmelt. In 2013, spatial variation in rain‐on‐snow and air temperatures resulted in a critical lag in snowmelt initiation in the southern and northern portions of the basin resulting in a dispersed, double peak spring hydrograph, contrasting with 1979 when a combination of rain‐on‐snow and coincident snowmelt led to the highest flood on record. Although eastern basin reservoirs become seasonally enriched in δ¹⁸O and δ²H values, unregulated western basin flows remain less variable due to groundwater driven baseflow with increasing influence downstream. Combined analysis of historical streamflow (e.g., flood of 1979, drought of 2010) and the 2013 water isotope surveys illustrate extreme meteorological conditions that current management activities are unable to prevent. In this study, the influence of evaporative fractionation on large surface water reservoirs provides important evidence of streamflow partitioning, illustrating the value of stable water isotope tracers for study of larger catchments.