Observation and modelling of fault-zone fracture seismic anisotropy – II. P-wave polarization anomalies

Summary. In Part I of this paper we modelled shear-wave splitting observed in crystalline rock bordering an active, normal fault-zone at Oroville, California, with Červený's ray-tracing system applied to anisotropic heterogeneous media using Hudson's formulation of elastic constants for a medium containing aligned cracks. In Part II we use the ray-tracing results of Part I to quantitatively interpret P -wave polarization anomalies observed in the three-component seismograms recorded in the Oroville fault zone. We show that the eigenvectors of the first-order Christoffel tensor defined by the ray-tracing slowness vector and Hudson's first-order anisotropic corrections to the isotropic elastic tensor correctly account for P -wave first motion that deviates from the ray vector.     

Continental impact on marine boundary layer coarse particles over the Atlantic Ocean between Europe and Antarctica

Aerosol samples were collected in the Atlantic marine boundary layer between the English Channel and Antarctica during November–December 1999. The composition of coarse (aerodynamic diameter 1–3 μm) individual aerosol particles was studied using the SEM/EDX method. The major particle types observed were fresh sea salt, sea-salt particles reacted partly or totally with sulphuric acid or nitric acid, Mg-sulphate, Ca-sulphate, mixed aluminosilicates and sea salt, aluminosilicates, Ca-rich particles and Fe-rich particles. The relative fractions of sea-salt particles with moderate or strong Cl depletion were high near the coasts of Europe (65–74%) and Northern Africa (44–87%), low far from the coast of Western Africa (10–20%) and very low in remote sea areas between Africa and Antarctica (1%). The Cl depletion was strongest when air masses arrived from the direction of anthropogenic pollution sources. The fractions of Mg-sulphate particles were high (18–25%) in 2 samples near Europe. The Mg-sulphate particles were probably formed as a result of fractional recrystallization of sea-salt particles in which Cl was substituted by sulphate. It remained unclear whether these particles were formed in the atmosphere or during and after sampling. The relative fractions of particles from continental sources were quite low (10–15%) near Europe, very high (25–78%) near the coast of Northwestern Africa and very low in the remote sea areas (0–2%). Most of the continental particles were aluminosilicates and some of them were internally mixed with sea salt. Near the coast of Northwestern Africa, the main source of aluminosilicates was Saharan dust, and near the Gulf of Guinea, emissions from biomass burning were also mixed with aluminosilicates and sea salt.     

Multiple scattering and energy transfer of seismic waves—Separation of scattering effect from intrinsic attenuation II. Application of the theory to Hindu Kush region

In order to separate the scattering effect from intrinsic attenuation, we need a multiple scattering model for seismic wave propagation in random heterogeneous media. In paper I (Wu, 1985), radiative transfer theory is applied to seismic wave propagation and the energy density distribution (or the average intensity) in space for a point source is formulated in the frequency domain. It is possible to separate the scattering effect and the absorption based on the measured energy density distribution curves. In this paper, the data from digital recordings in the Hindu Kush region are used as an example of application of the theory. We also discuss two approximate solutions of coda envelope in the time domain: the single scattering approximation and the diffusion approximation and discuss the relation with the frequency domain solution. We point out that in only two cases can the apparent attenuation be expressed as an exponential decay form. One is thedark medium case, i.e., whenB ₀0.5, whereB ₀ = _s /( _s + _a ) is the seismic albedo, _s is the scattering coefficient, _a is the absorption coefficient. In this case the absorption is dominant, the apparent attenuationb can be approximated by the coherent wave attenuationb = _s + _a . The other case is thediffuse scattering regime, i.e., whenB ₀0.5 (bright medium) andRL _s ,t _s , whereR andt are the propagation distance and lapse time,L _s and _s are the scattering lengths (mean free path) and scattering time (mean free time), respectively. However, in this case the envelope decays with a rate close to the intrinsic attenuation, while the intensity decreases with distance with a coefficientb d ₀( _s + _a ) d _{s s} , whered ₀ andd _s are the diffusion multipliers (0<d ₀,d _s <1).For the Hindu Kush region, by comparing the theory with data from two digital stations of 53 events distributed up to depths of 350 km, we find that the scattering is not the dominant factor for the measured apparent attenuation ofS waves in the frequency range 2–20 Hz. From the observation on high frequency (f>20 Hz) seismograms, we suggest the existence of a stron-scattering surface layer with fine scale heterogeneities in the crust, at least for this region.     

Effects of multiple scattering on coda waves in three-dimensional medium

The contribution of multiple scattering to the coda waves for three-dimensional elastic medium is investigated by extending the computational procedures developed earlier for the two-dimensional medium. It is shown that the effects of multiple scattering start to become important at a shorter lapse time,t _c = 0.65(n ₀σu)^?1, than in the two-dimensional case (t _c = 0.8(n ₀σu)^?1).     

Effect of slip rate on stress drop

Recent observations made by Kanamori and Allen about earthquake recurrence time and average stress drop revealed a very interesting relation: earthquakes with longer recurrence times have higher average stress drops. They attributed the difference in stress drop to the difference in long-term average slip rate. To interpret their result in terms of the healing effect, we simulated earthquake recurrence with a one-dimensional mass-spring model, incorporating a recently developed rate-and-state dependent friction law for different loading rates and heterogeneous strength distributions. We first calculated the stress drop and recurrence time as functions of loading rate for a homogenous fault model. We found that the stress drop increases up to 30% when the loading rate decreases from 10 cm/yr to 0.01 mm/yr. Thus, the observed great variability of stress drop, from a few bars to a few hundred bars, which is obtained by replotting the data of Kanamori and Allen in the form of stress drop versus long-term slip rate, may not be attributable to the healing effect alone. Our numerical simulation shows that the variability may be due primarily to the spatial heterogeneity of strength on the fault. Our simulation also suggests that of the two empirical laws that were inferred from the same laboratory friction data, called the power law and the logarithmic law by Shimamoto and Logan, the former can explain the observed relation between stress drop and slip rate better than can the latter, at least for strike-slip fault. The logarithmic law is an earlier and simpler version of the rate-state-dependent friction law.     

Microstructural Characterization of Granular Cast Iron for Permeable Reactive Barriers

Although intensive research on Fe(0) permeable reactive barriers (PRB) for in situ groundwater remediation has been conducted and multiple applications have been installed in the past two decades, some properties of reactive materials in use have not been fully considered and discussed yet. In the present investigation, a typical granular cast iron has been characterized with different techniques. The grain size distribution not only has an influence on the resulting pore geometry and the surface area but material properties significantly differ between fine and coarse grains. Metallographic analyses revealed large differences in both graphite inclusions and microstructures that likely influence the reactivity. Both graphite and cementite proved to be more resistant toward acidic dissolution compared to Fe⁰. The intrinsic material characteristics described here have not been covered in the existing PRB literature.