A finite difference method for earthquake sequences in poroelastic solids

Induced seismicity (earthquakes caused by injection or extraction of fluids in Earth’s subsurface) is a major, new hazard in the USA, the Netherlands, and other countries, with vast economic consequences if not properly managed. Addressing this problem requires development of predictive simulations of how fluid-saturated solids containing frictional faults respond to fluid injection/extraction. Here, we present a finite difference method for 2D linear poroelasticity with rate-and-state friction faults, accounting for spatially variable properties. Semi-discrete stability and accuracy are proven using the summation-by-parts, simultaneous-approximation-term (SBP-SAT) framework for discretization and boundary condition enforcement. Convergence rates are verified using the method of manufactured solutions and comparison to the analytical solution to Mandel’s problem. The method is then applied to study fault slip triggered by fluid injection and diffusion through high-permeability fault damage zones. We demonstrate that in response to the same, gradual forcing, fault slip can occur in either an unstable manner, as short-duration earthquakes that radiate seismic waves, or as stable, aseismic, slow slip that accumulates over much longer time scales. Finally, we use these simulation results to discuss the role of frictional and elastic properties in determining the stability and nature of slip.     

Variation of the solar diameter from solar eclipse observations, 1715–1991

The diameter of the Sun may be measured at the time of a solar eclipse. We have performed an exhaustive search of the astronomical literature to find all existing observations of solar eclipses suitable for this purpose. We have also taken new observations by new techniques. We have undertaken a project to reduce them systematically, and in an automated, self-consistent way. This will produce determinations of the solar radius at the times of solar eclipses from 1715 to the present. Re-reduction, using newer ephemerides, of observations made in 1984 shows that the component of the residuals caused by the ephemeris is substantially reduced. This paper summarizes the research plan, outlines the detailed astronomical features included in the calculations, and presents the results available.     

Metalliferous Sediment and a Silica-Hematite Deposit within the Blanco Fracture Zone,Northeast Pacific

A Tiburon ROV dive within the East Blanco Depression (EBD) increased the mapped extent of a known hydrothermal field by an order of magnitude. In addition, a unique opal-CT (cristobalite-tridymite)-hematite mound was discovered, and mineralized sediments and rock were collected and analyzed. Silica-hematite mounds have not previously been found on the deep ocean floor. The light-weight rock of the porous mound consists predominantly of opal-CT and hematite filaments, rods, and strands, and averages 77.8% SiO₂ and 11.8% Fe₂O₃. The hematite and opal-CT precipitated from a low-temperature (≥ 115° C), strongly oxidized, silica- and iron-rich, sulfur-poor hydrothermal fluid; a bacterial mat provided the framework for precipitation.

Samples collected from a volcaniclastic rock outcrop consist primarily of quartz with lesser plagioclase, smectite, pyroxene, and sulfides; SiO₂ content averages 72.5%. Formation of these quartz-rich samples is best explained by cooling in an up-flow zone of silica-rich hydrothermal fluids within a low permeability system. Opal-A, opal-CT, and quartz mineralization found in different places within the EBD hydrothermal field likely reflects decreasing silica saturation and increasing temperature of the mineralizing fluid with increasing silica crystallinity.

Six push cores recovered gravel, coarse sand, and mud mineralized variously by Fe or Mn oxides, silica, and sulfides. Total rare-earth element concentrations are low for both the rock and push core samples. Ce and Eu anomalies reflect high and low temperature hydrothermal components and detrital phases.

A remarkable variety of types of mineralization occur within the EBD field, yet a consistent suite of elements is enriched (relative to basalt and unmineralized cores) in all samples analyzed: Ag, Au, S, Mo, Hg, As, Sb, Sr, and U; most samples are also enriched in Cu, Pb, Cd, and Zn. On the basis of these element enrichments, the EBD hydrothermal field might best be described as a base- and precious-metal-bearing, silica-Fe-oxide-barite deposit. Such deposits are commonly spatially and temporally associated with volcanogenic massive sulfide (VMS) ores. A plot of data for pathfinder elements shows a large hot spot at the northwestern margin of the field, which may mark a region where moderate to high temperature sulfide deposits are forming at depth; further exploration of the hydrothermal field to the northwest is warranted.     

Permselective Membrane Fouling During Azo Dye Wastewater Treatment

A Na‐montmorillonite membrane wastewater renovation prototype system was developed to specifically treat an ionic azo dye. Efficiency of this prototype system was limited to membrane fouling. Fouling rates were not consistently uniform owing to steric effects and competition for exchange sites. The decrease in solute rejection with time can be attributed to the decrease in the relative permeability of the compacted Na‐montmorillonite membrane to the dye with time due to fouling. This decrease occurs probably as a two‐step nucleation–growth mechanism with the nucleation part dependent in part on solvent flux, number of nucleation sites on the membrane, and sorbed mass part that controls solute flux and organic polymerization. The effect of concentration polarization was significant since the flux was higher than the mass‐transfer coefficient. The low diffusion coefficient of the ionic azo dye resulted in low mass transfer coefficients. The most important macromolecular solution properties to be considered for pilot systems may include high concentration‐dependent viscosity, possible non‐Newtonian fluid behavior, and low and concentration‐dependent self‐diffusivity amongst other factors. For pilot systems, the greater the quantity of large macromolecules in the ambient water, the greater the necessity of reducing the permselectivity of the membrane to prevent significant polarization.     

Structure of Saturn's Mesosphere from the 28 Sgr Occultations

We analyze an extensive data set of immersion and emersion lightcurves of the occultation of 28 Sgr by Saturn's atmosphere on 3 July 1989. The data give profiles of number density as a function of altitude at a variety of latitudes, at pressures ranging from about 0.5 to about 20 μbar. The atmosphere is essentially isothermal in this range, with a temperature close to 140 K for an assumed mean molecular weight of 2.135. Owing to favorable ring geometry, an accurate radial scale is available for all observations, and we confirm the substantial equatorial bulge produced by zonal winds of ∼450 m/s first observed in the Voyager radio-occultation experiments. The fact that the bulge is still present at microbar pressures suggests that the equatorial winds persist to high altitudes. According to our radial scale, the 2.4-μbar level, which corresponds to half-flux in the stellar occultations, is at an equatorial radius of 60,960 km. This radial scale is in good agreement with the Voyager radio-occultation data at mbar pressures and allows smooth interpolation of the isothermal structure between the stellar-occultation and radio-occultation regions. We do not have such a smooth interpolation between our data and Voyager ultraviolet occultation data, unless we discard the lowest 200 km of Voyager ultraviolet data. When this is done, we obtain a complete atmospheric model from an equatorial radius of 61,500 km down to an equatorial radius of 60,500 km. This model gives excellent agreement between all 28 Sgr, Voyager, and Pioneer 11 data.     

Book Reviews

Books reviewed:

Pyne, Stephen J., How The Canyon Became Grand: A Short History

Gibson, Clark C., Politicians and Poachers: The Political Economy of Wildlife Policy in Africa

Voeks, Robert A., Sacred Leaves of Candomblé: African Magic, Medicine, and Religion in Brazil

Grossman, Lawrence S., The Political Ecology of Bananas: Contract Farming, Peasants and Agrarian Change in the Eastern Caribbean

Fennell, David A., Ecotourism: An Introduction

Soffer, Arnon, Rivers of Fire: The Conflict Over Water in the Middle East

Light, Andrew and Smith, Jonathan M., Philosophy and Geography II: The Production of Public Space

Davies, Richard O., Main Street Blues: The Decline of Small-Town America

Driver, Felix and Gilbert, David, Imperial Cities: Landscape, Display and Identity

Baldassare, Mark, California in the New Millennium: The Changing Social and Political Landscape

Herzog, Lawrence A., From Aztec to High Tech: Architecture and Landscape Across the Mexico-United States Border

Cowie, Jefferson, Capital Moves: RCA’s Seventy-Year Quest for Cheap Labor

Aiken, Charles, The Cotton Plantation South Since the Civil War

Smith, Steven D., Crass, David, and Zierden, Martha A., The Southern Colonial Backcountry: Interdisciplinary Perspectives on Frontier Communities

Hoelscher, Steven D., Heritage of Stage: The Invention of Ethnic Place in America’s Little Switzerland

Bradley, Betsy Hunter, The Works: The Industrial Architecture of the United States     

Fe-Mg order-disorder in orthopyroxenes

Published data on the distribution of Fe and Mg between the two distinct octahedral sites (M1 and M2) in orthopyroxenes show that the equilibrium Fe-Mg distribution coefficient (K_D) is roughly constant at intermediate ferrosilite content (X_Fs of 0.2 to 0.6) but at low X_Fs (<0.2) is either anomalously high (Mössbauer data) or low (XRD data) compared to K_D at intermediate X_Fs. We report an experimental study on the equilibrium K_D values for two natural single crystals of orthopyroxene (with X_Fs of 0.011 and 0.160) using a high-sensitivity Mössbauer spectrometer. The K_D values were determined at 1 bar and temperatures between 600 and 1000°C, and reversed at 600 to 900°C. The values are found to be independent of orthopyroxene composition at a given temperature within analytical uncertainties, and in agreement with K_D values at intermediate X_Fs. A least-squares fit to the K_D data yields: lnK_D = 0.391(±0.131) − 2205(±141)/T, where T is temperature in K, and uncertainties are at the 2σ level. This equation is valid for X_Fs values up to at least 0.6. We suggest that previous Mössbauer and XRD data at low X_Fs were compromised by analytical difficulties. The new result that K_D is roughly independent of X_Fs greatly simplifies treatment of equilibrium and speedometry based on Fe-Mg order-disorder in orthopyroxene.     

Economic potential of the ultramafic rocks of Jamaica and Tobago: two contrasting geological settings in the Caribbean

Ultramafic rocks in Jamaica are dunites with minor lherzolite, often serpentinised, and are part of a dismembered ophiolite complex. In Tobago, dunites, wehrlites, pyroxenites and hornblendites form the lower part of a plutonic complex of island arc affinity. The mineral assemblages and chemistry reflect these differences. Chromite in Jamaica is high in Al and Mg, whereas in Tobago it is rich in Fe, as in Alaskan-type intrusives. Ni-Cu-PGE assemblages in Jamaica are pentlandite, with later low temperature heazlewoodite, awaruite and native copper, the latter with Pt and Pd. In Tobago an assemblage of pentlandite, pyrrhotite, pyrite and chalcopyrite is much less affected by later alteration. PGE phases also occur. The dunites in Jamaica have sufficient MgO to be a potential source of olivine. The higher Fe in olivine from Tobago indicates that olivine cumulates in plutonics from island arc settings are a less suitable source of the mineral. Ni-laterites in Jamaica are unlikely because of high topographic relief. The prospect for Ni-laterites in Tobago is low as there is little Ni in the olivines. Chrysotile asbestos, talc and magnesite are absent in both islands. This is probably a consequence of the lack of secondary serpentine recrystallisation to form fibrous chrysotile veins, the deep tectonic level and lack of hydrothermal circulation for magnesite to form, and the absence of metamorphic/metasomatic events and/or late stage extension tectonics which might have yielded talc. Received: 14 September 1998 / Accepted: 4 March 1999