An equivalent linear algorithm with frequency- and pressure-dependent moduli and damping for the seismic analysis of deep sites

The seismic analysis of soil deposits is most often carried out with an iterative computational scheme, proposed by Seed and Idriss, in which inelastic effects are only approximately modeled through soil degradation curves. Laboratory experimental data indicate that for highly confined materials, the standardized reduction curves commonly used overestimate the capacity of soils to dissipate energy. This paper first presents the results obtained with a simple four-parameter constitutive soil model, which when used to simulate cyclic loading, produces results that agree well with available laboratory experiments for soils under arbitrarily large confining pressures. Thereafter, a frequency- and pressure-dependent iterative algorithm for seismic amplification is proposed, which provides time histories that match well the results obtained with a true non-linear model. Finally, the modified linear iterative analysis is successfully used for the seismic analysis of a 1 km deep model for the Mississippi embayment near Memphis, Tennessee, and a class-A prediction of the seismic amplification in Treasure Island during the Loma Prieta earthquake.     

Population biology of the intertidal snailChilina ovalis sowerby (pulmonata) in the Queule River estuary, South-Central Chile

The temporal variability in abundance and population structure of the gastropodChilina ovalis Sowerby was studied in the upper intertidal zone of Queule River estuary, south-central Chile (c. 40°S). Snails were collected monthly (September 1995–December 1997) from haphazardly-located quadrats (50×50 cm, n=5 each time), and counted and measured (shell height) in the laboratory. Water and sediment samples were collected at the same time to study the snail's habitat characteristics. Overall mean abundance was 115 individuals m^?2 (SD=55). Monthly abundance estimates indicated a clear decrease during 1997. This decrease appeared to be related to the annual recruitment success of the species and at least partially to water temperature and sedimentological variability. Overall size range ofC. ovalis was 1.5–27.5 mm shell height. Growth varied seasonally with highest growth rates observed after recruitment (November–February). Slower growth continued throughout the austral winter months. Despite changes in abundance between 1996 and 1997, no differences were detected when population growth estimates were compared between years. A maximum longevity of approximately 4 yr was estimated from the growth curves of the cohorts, and a life cycle with more than one reproductive period is suggested.     

A note on the fast computation of response spectra estimates

An algorithm is presented for fast computation of estimates of response spectra. It is based on the formalism of random vibration theory. Numerical results show that estimates are reliable and useful for practical applications.     

Diagnosing drought using the downstreamness concept: the effect of reservoir networks on drought evolution

ABSTRACT

To effectively manage hydrological drought, there is an urgent need to better understand and evaluate its human drivers. Using the “downstreamness” concept, we assess the role of a reservoir network in the emergence and evolution of droughts in a river basin in Brazil. In our case study, the downstreamness concept shows the effect of a network of reservoirs on the spatial distribution of stored surface water volumes over time. We demonstrate that, as a consequence of meteorological drought and recovery, the distribution of stored volumes became spatially skewed towards upstream locations, which affected the duration and magnitude of hydrological drought both upstream (where drought was alleviated) and downstream (where drought was aggravated). The downstreamness concept thus appears to be a useful entry point for spatiotemporally explicit assessments of hydrological drought and for determining the likelihood of progression from meteorological drought to a human-modified hydrological drought in a basin.     

Changes in magma composition at Arenal volcano,Costa Rica, 1968–1985: Real-time monitoring of open-system differentiation

Arenal volcano in Costa Rica has been erupting nearly continuously, but at a diminishing rate, since 1968, producing approximately 0.35 km³ of lavas and tephras that have shown consistent variations in chemistry and mineralogy. From the beginning of the eruption in July 1968 to early 1970 (stage 1, vol.=0.12 km³) tephras and lavas became richer in Ca, Mg, Ni, Cr, Fe, Ti, V, and Sc and poorer in Al₂O₃ and SiO₂. Concentrations of incompatible trace elements (including Sr) decreased by 5%–20%. Phenocryst contents increased 20–50 vol%. During stage 2 (1970–1973, vol. = 0.13 km³) concentrations of compatible trace elements rose, and concentrations of incompatible trace elements either remained constant or also rose. Al₂O₃ contents decreased by 1 wt%. Phenocryst content increased slightly, principally due to increased orthopyroxene. During stage 3 (mid-1974 to the present, vol.= 0.10 km³) concentrations of SiO₂ increased by 1 wt%, compatible trace elements decreased slightly, and incompatible trace element concentrations increased by 5% to 10%. Although crystals increased in size during stage 3, their overall abundance stayed roughly constant.Our modeling suggests that early stage-1 magmas were produced by boundary layer fractionation under high-p H₂O conditions of an unseen basaltic andesitic magma that intruded into the Arenal system after approximately 500 B.P. Changes in composition during stage 2 resulted from mixing of this more mafic original magma with new magma that had a similar SiO₂ content, but higher compatible and incompatible element concentrations. The changes during stage 3 resulted from continued influx of the same magma plus crystal removal.We conclude that the eruption proceeded in the following way. Before 1968 zoned stage-1 magma resided in the deep crust below Arenal. A new magma intruded into this chamber in July 1968 causing ejection of the stage-1 magmas. The intruding magma mixed with mafic portions of the original chamber producing the mixed lavas of stage 2. Continued mixing plus crystal fractionation along the chamber and conduit walls produced stage-3 lavas. The time scales of crustal level magmatic processes at Arenal range 10⁰–10³ years, which are 3–6 orders of magnitude shorter than those of larger, more silicic systems.     

Application of formaldoxime colorimetric method for the determination of manganese in the pore water of anoxic estuarine sediments

The formaldoxime colorimetric method for manganese, previously utilized for freshwater and seawater samples of large volume, has been modified for the analysis of small volume, iron-rich sediment pore water samples. Using as little as 3 ml of sample and the addition of EDTA to destroy the Fe-formaldoxime interference (at concentrations of 9 μM Mnl^?1), a precision of 3% is obtained.     

Growth of the Bay Scallop,Argopecten irradians,fes an artificial diet rich in protein

Bay scallops had minimal shell growth and lost weight on an artificial diet rich in protein (Purina’ Trout Chow’) even though they ingested the food. Two concentrations of food were used, one simulating natural levels of particulate organic carbon and another approximately 20 times the first concentration. These results and those of others indicate that high protein, artificial diets cannot be assimilated by bivalves even though phytoplankton, their natural food, is high in protein. The data suggest caution in establishing estuarien food chains based only upon food quality, quantity, and ingestion without regard to assimilation or growth.     

Studies of the effects of outliers and data transformation on variogram estimates for a base metal and a gold ore body

Variograms for gold and lead values from the Loraine and Prieska mines, respectively, indicate that data outliers can seriously distort and/or mask the real variogram patterns. Studies show that this problem is best overcome for these mines by logarithmic transformation of the data, and/or a suitable screening out of such outliers, and/or more robust variogram estimation procedures; the benefits are particularly significant when the basic data is limited.     

In-ground environmental radioactivity monitoring in the Culpeper Basin,Virginia, using LiF thermoluminescence dosimeters

A new environmental in-ground radioactivity monitoring technique using LiF thermoluminescence dosimeters was tested in the Culpeper Basin, Northern Virginia. The dosimeters were buried at a depth of 0.45 m (∼ 18 in.) for approximately four months. There was a significant positive correlation (at the 99.9% confidence level) between the total accumulated radioactivity signal from the dosimeters and the on-site 100 second gamma-ray spectrometer measurements. The minimum-maximum dose rate from the buried thermoluminescence dosimeter measurements was 0.06 to 1.08 mR per day (or 2.5 to 44.5μR per hour). There are two factors which permit better background levels of radioactivity to be established by thermoluminescence dosimeters compared with other methods for environmental monitoring programs. First is the great sensitivity of thermoluminescence dosimeters in terms of minimum dose rate that can be registered (mR per month orμR per hour). Second is the fact that accumulation of radioactivity signal over a long period of time tends to eliminate short-term environmental changes that affect measurements with gamma-ray spectrometers and scintillation counters.