Estimating non‐linear site response by single period approximation

Site effects characterize the filtering mechanisms within the soil sedimentary layers overlying bedrock. In regions of high seismicity such as California where strong motion records are relatively abundant, site coefficients can be developed by regression of recorded ground shaking parameters. In regions of low‐to‐moderate seismicity or of high seismicity but with a paucity of recorded strong motion data, such empirical models cannot be obtained in the same way. This study describes the theoretical development of a simple, rational manual procedure to calculate site coefficients, based on a single period approximation (SPA), and to construct displacement response spectra (RSD) for soil sites. The proposed simplified model, which takes into account the non‐linear behaviour of soil that is dependent on the level of shaking, impedance contrast at the soil–bedrock interface and the plasticity of soil material, has been verified by comparison with results obtained from non‐linear shear wave analyses and data recorded during the 1994 Northridge earthquake. The proposed model is believed to be a convenient tool for calculating non‐linear site responses and constructing site‐specific response spectra, which has the potential of being incorporated into code provisions. Copyright © 2006 John Wiley & Sons, Ltd.     

Combining Non-local Scalings with a TKE Closure for Mixing in Boundary-layer Clouds

A new approach to the parametrization of the cumulus-capped boundary layer is described. It combines a traditional higher-order turbulence closure, appropriate for boundary layers where the skewness of thermodynamic variable probability distributions is low (typically stratocumulus-capped), with non-local scaled similarity functions. These are introduced in order to represent explicitly that part of the distribution arising from skewed cumulus elements and the scalings are found to work very well against equilibrium shallow cumulus large-eddy simulations. Results from a wide range of single column model simulations, from stratocumulus to shallow cumulus to cumulus rising into stratocumulus, are presented that demonstrate the validity of the approach as a means of parametrizing the cloudy boundary layer. Sensitivity tests show that enhancement of the turbulence length scales and the buoyancy production of TKE are especially important.     

The Spatial Markets of Cities Organized in a Hierarchical System*

Mounting evidence indicates that large, world-class cities have expanded their spatial markets well beyond the regional urban systems in which they are located. This has led many observers to conclude that in the contemporary economy, interdependence, interaction, and connectivity are no longer fundamental characteristics of hierarchical urban systems. Virtually all research, however, deals exclusively with the changing role of large cities, while the spatial markets of medium- and smaller-sized cities comprising an urban system have been ignored. The primary objective of this paper is to investigate whether the spatial markets of medium- and smaller-sized cities in an urban hierarchical system continue to display a systematic relationship with city size, or whether they also reveal the absence of connection, or integration, with the regional system. The analysis uses survey data collected from 480 producer service firms located in an urban hierarchy comprising 15 midwestern cities of the United States. The results indicate that the spatial market of the largest city (Chicago) extends well beyond the Midwest urban hierarchy, while the spatial markets of smaller-sized cities are more integrated within the hierarchical system.     

Characterization of forward-directivity ground motions in the near-fault region

Ground motions close to a ruptured fault resulting from forward-directivity are significantly different than other ground motions. These pulse-type motions can place severe demands on structures in the near-fault region. To aid in the characterization of these special type of ground motions, a simplified parameterization is proposed based on a representative amplitude, pulse period, and number of significant pulses in the velocity–time history. Empirical relationships were developed for estimating the peak ground velocity (PGV) and period of the velocity pulse (T_v) of available forward-directivity motions. PGV in the near-fault region varies significantly with magnitude and distance. Additionally, the PGV for soil sites are systematically larger than those at rocks sites. T_v is a function of moment magnitude and site conditions with most of the energy being concentrated within a narrow-period band centered on the pulse period. Hence, lower magnitude events, which produce lower pulse periods, might produce more damaging ground motions for the stiff structures more common in urban areas.     

An alternative model for positive shifts in shallow-marine carbonate δ13C and δ18O

Abstract Positive shifts in global seawater δ¹³C_DIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ¹⁸O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ¹³C and δ¹⁸O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and ¹³C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate ¹³C and ¹⁸O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.     

Strontium in coral aragonite: 1. Characterization of Sr coordination by extended absorption X-ray fine structure

Aragonite was analyzed from Porites lobata, Pavona gigantea, Pavona clavus, and Montastrea annularis corals by Sr K-edge extended absorption X-ray fine structure (EXAFS) and compared with aragonite, strontianite, and mechanically mixed standards. Bulk analyses were performed and data compared with equivalent micro-EXAFS analyses on small (∼400 μm³) analytical volumes with a microfocused X-ray beam. As a result of the architecture of the coral skeleton, the crystals within the microanalytical volume are not randomly oriented, and the microanalytical X-ray absorption spectra show orientational dependence. However, refinement of bulk and microanalytical data provided indistinguishable interatomic distances and thermal vibration parameters in the third shell (indicative of Sr speciation). The Sr K-edge EXAFS of all the coral samples refine, within error, to an ideally substituted Sr in aragonite, in contrast to previous studies, in which significant strontianite was reported. Some samples from that study were also analyzed here. Strontianite may be less widely distributed in corals than previously thought.     

On knowing what trees to plant: local and expert perspectives in the Western Ghats of Karnataka

This paper investigates the ways in which different groups of people arrive at decisions regarding what tree species to plant. Data is drawn from a case study of afforestation that is taking place under the policy of Joint Forest Planning and Management in the Western Ghats of Karnataka, India. Whilst there is a clear disparity between villager and Forest Department preferences, this difference is not simply a case of Forest Department ‘science’ pitted against villagers’ ‘local knowledge’. On the one hand, both villagers and the Forest Department employ empirical evidence to inform their preferences; on the other hand, decision making goes beyond this, being influenced by a range of institutional and cultural issues. This paper identifies obstacles to a complementary working relationship between local and expert knowledge and considers the resulting management implications.